Biology SME copy 1

Key unit competence: Describe the structure, characteristics of microorganisms and explain the process of culturing                                                                   microorganisms and the factors affecting their population growth.

Introductory activity 10

It was Saturday when Gakwaya and his friends ate bread in the breakfast. Some bread was left in the cupboard and when Gakwaya woke up on Monday to eat the bread before going to school, he found that the bread had turned into a black and whitish color with a fuzzy appearance and its odor was changed.

Photo: Normal bread and bread with lack and whitish color. (Source: https:// www.kingarthurflour.com/recipes/walter-sands-basic-white-bread-recipe and http://craves.everybodyshops.com/is-it-safe-to-tear-off-the-mold-and-eat-the-restof-the-bread/ )

a. What do you observe from the pictures? 

b. Look up in a dictionary the meaning of the words: Micro, organism, micro-organism, yeast, germ, virus, bacteria, mould, fungi, algae, microbe, microbiologist, nucleus, parasite, viral, decomposer.

 c. What do you think may have happened on the bread? 

d. What is the nature of those substances on the bread? Are these living or nonliving thigs? 

e. What do you think may happen to Gakwaya if he eats such a bread?

10.1 Types of microorganisms 

Activity 10.1

1. what do you understand about?

a. Microbiology

b. Prokaryotic

2. State any two beneficial effects of microbes.

3. What are the main differences between archaebacterial and eubacteria?

The term “microbiology” comes Greek words: ‘micros’ which means small, ‘bios’ which means life and ‘logos’ which means science. Microbiology is the study of microorganisms or microbes which are too small organisms that can be only seen under microscope. They include bacteria, fungi, algae, protozoa and viruses. They play positive role in the life of living organisms and are also harmful to the other living organisms. 

 Micro-organisms are everywhere: in the air, water soil, in intestine and the skin of animals, on plants, on rock surfaces in very hot and cold places (ice). Before the invention of the microscope, microbes were unknown and thousands of people died in devastating epidemics because, vaccines and antibiotics were not available to fight against infectious diseases. Nowadays, microorganisms can be grown in the laboratory and studied. 

 Microorganisms can be classified on the basis of cell structure (morphology), cellular metabolism, or on differences in cell molecular components such as DNA, fatty acids, pigments, antigens, and quinones. In general, the microbes are categorized into five groups which are bacteria, archaea, fungi, algae, protozoa and viruses.

Prokaryotes include several kinds of microorganisms, such as bacteria and cyanobacteria while eukaryotes include such microorganisms as fungi, protozoa, and simple algae. Viruses are considered neither prokaryotes nor eukaryotes because they lack the characteristics of living things, except the ability to replicate (which they accomplish only in living cells) and studied under microscope.

10.2 Effects of microorganisms on environment and human activities

Microbes are everywhere in the biosphere, and their presence invariably affects the environment that they are growing in. The effects of microorganisms on their environment can be beneficial or harmful or inapparent with regard to human measure or observation where they can perform a variety of functions, such as photosynthesis, breaking down waste, and infecting (agents of disease) other organisms.

 The beneficial effects of microbes are derived from their metabolic activities in the environment, their associations with plants and animals, and from their use in food production and biotechnological processes. The beneficial effects are:

1. Nutrient cycling and the cycles of elements that make up living systems Microorganisms have the ability to recycle the primary elements that make up all living systems, especially carbon (C), oxygen (O) and nitrogen (N) which are in different molecular forms that must be shared among all types of life. 

In primary production, the photosynthetic microorganisms such as algae and cyanobacteria are involving in CO2 fixation.

Decomposition or biodegradation results in the breakdown of complex organic materials to forms of carbon that can be used by other organisms. 

Nitrogen fixation is a process found only in some bacteria which removes N2 from the atmosphere and converts it to ammonia (NH3 ), for use by plants and animals. Nitrogen fixation also results in replenishment of soil nitrogen removed by agricultural processes. Some bacteria fix nitrogen in symbiotic associations in plants. 

Oxygenic photosynthesis occurs in plants, algae and cyanobacteria. It is the type of photosynthesis that results in the production of O2 in the atmosphere.

2. Associations with animals and plants 

Microbes invariably enter into beneficial associations with all higher forms of organisms. For example, bacteria and other microbes in the intestines of animals and insects digest nutrients and produce vitamins and growth factors. In the plants, leguminous plants (peas, beans, clover, alfalfa, etc.) live in intimate associations with bacteria that extract nitrogen from the atmosphere and supply it to the plant for growth. The microbes that normally live in associations with humans on the various surfaces of the body (called the normal flora)

3. Production of foods and fuels

 In the home and in industry, microbes are used in the production of fermented foods. Yeasts (like Saccharomyces) are used in the manufacture of beer and wine and for the leavening of breads, while some bacteria (lactic acid bacteria) are used to make yogurt, cheese, sour cream, buttermilk and other fermented milk products. Vinegars are produced by bacterial acetic acid fermentation. Other fermented foods include soy sauce, sauerkraut, dill pickles, olives, salami, cocoa and black teas. Bacteria are the agents of most other food fermentations.

4. Medical, pharmaceutical and biotechnological applications

 In human and veterinary medicine, for the treatment and prevention of infectious diseases, microbes are a source of antibiotics and vaccines.

Antibiotics are substances produced by microorganisms that kill or inhibit other microbes which are used in the treatment of infectious disease. Antibiotics are produced in nature by molds such as Penicillium and bacteria such as Streptomyces and Bacillus.

Vaccines are substances derived from microorganisms used to immunize against disease. The microbes that are the cause of infectious disease are usually the ultimate source of vaccines.

Biotechnology 

Microbiology makes an important contribution to biotechnology, an area of science that applies microbial genetics to biological processes for the production of useful substances. Important tools of biotechnology are microbial cells, microbial genes and microbial enzymes.

5. Basic research 

 Microorganisms, in particular the bacterium, Escherichia coli and the yeast, Saccharomyces, have been used as model organisms for basic research and the study of cellular life. Because of cell theory and the unity of biological processes in all organisms, this information provides us with insight and understanding of life at all levels, including human.

Application activity 10.2

1. What are the basic shapes of bacteria?

2. Name any two examples of archaebacteria.

3. What are the similarities and differences between protozoa and animals?

10.3 Characteristics of microorganisms

10.3.1 Characteristics and structure of viruses 

Activity 10.3

The figure below shows two types of viruses (HIV virus and a bacteriophage)

1. Use the figure to identify the main parts of a virus?

2. Analyze the figure and identify the main differences between avirus and a eukaryotic cell.

3. The HIV virus on the figure has an enzyme called reverse transcriptase or RT. What is its 

The term “virus” was first used in the 1890s to describe agents smaller than bacteria that cause diseases. The existence of viruses was established in 1892, when, Russian scientist, Dmitry Ivanovsky discovered later microscopic particles known as the tobacco mosaic virus.

There are at least 3,600 types of virus. Hundreds of which are known to cause diseases in animals, bacteria, and plants. Viruses consist of an inner core of either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) plus a protein protective coat called capsid made of protein or of protein combined with lipid or carbohydrate components.

The core confers infectivity, and the capsid provides specificity to the virus. In some virions, the capsid is further enveloped by a fatty membrane. The later may cause virion inactivation by exposure to fat solvents such as ether and chloroform.

- Viruses are complex biochemical molecules having the following characteristics:

- Viruses are not visible under light microscope because they are very small than bacteria. 

- They possess a single type of nucleic acid either DNA or RNA enclosed ina protein coat. 

- They can reproduce and grow inside the host cell. 

- They have no cell and no cell organelles. They are accellular

- They are obligate parasite i.e. cannot survive outside a host cell.

- They do not feed, respire and excrete.

- They are neither prokaryotes nor eukaryotes

10.3.2 Virus types 

DNA and RNA viruses differ in the way they use the host cell’s mechanisms to produce new viruses. For example, a DNA virus may act in one of the two ways: 

The virus may directly produce RNA that is used to make more viral proteins or it may join with the host cell’s DNA to direct the synthesis of new viruses. RNA viruses replicate differently from DNA viruses. Upon entering the host cell, a viral RNA is released into the host cell’s cytoplasm. There, it uses the host cell’s ribosomes. Some RNA viruses known as retroviruses contain an enzyme called reverse transcriptase in addition to RNA. Reverse transcriptase uses RNA as a template to make DNA. The DNA then makes an RNA transcript of itself. This RNA is then translated into proteins that become part of new viruses. Reverse transcriptase is so named because it reverses the normal process of transcription, in which DNA serves as a template for producing RNA.

Application activity 10.3

1. Explain why viruses are called obligate intracellular parasites?

2. What are the main differences between viruses and cells?

3. What is a temperate virus?

10.4 Protozoans, algae, molds and eubacteria.

Activity 10.4: Research activity

Using library textbook or search engine,

a. identify the characteristics of each of the following organisms

i. Protozoa

ii. Molds

iii.Eubacteria

b. Explain the life cycle of Plasmodium

10.4.1 Moulds 

Moulds pervade our world, living wherever moisture is present. Some are of great benefit to humans, providing antibiotics, acting as decomposers so that nutrients can be recycled, or taking part industrial processes. Other moulds cause diseases which lead to serious damage. 

Moulds have cells arranged in long thread-like filaments, the hyphae, that form a mass called Mycelium. Moulds are usually considered as fungi, but mould may also be formed by filamentous bacteria, slime moulds, and water moulds. Therefore, there are two main types of moulds: fungal moulds and non-fungal moulds.

10.4.2 Fungal moulds 

All fungi that produce mycelia can be called moulds, but the term is usually used for an organism in which the mycelium forms the main body of the fungus. In the black bread mould Rhizopus and the pin mould Mucor, the mycelium consists of a tangled mass of hyphae with many nuclei. These hyphae are called coenocytic because the fungal tissue is not separated by cell walls. 

Fungal hyphae have an outer cell wall made of chitin and inner lumen which contains the cytoplasm and organelles. A cell surface membrane surrounds the cytoplasm and sticks tightly to the cell wall. Rhizopus and Mucor are Saprotrophic, obtaining their nutrients from dead organic material. 

Rhizopus nigricans and Mucor mucedo can live on bread but some species of Rhizopus feed on living plants, and Mucor commonly grows on rotting fruits and vegetables, in the soil or on dung. 

Rhizopus and Mucor secrete hydrolytic enzymes onto their food source and digest the food outside the organism and then absorb the soluble digestion products and assimilate them.

10.4.3 Penicillium and saccharomyces 

a. Penicillium and antibiotics 

Penicillium is highly known for producing penicillin, the first antibiotic discovered in 1928 by a scientist Alexander Fleming when he was culturing some Staphylococcus bacteria during his medical research.

After leaving some Petri dishes for many days, he found a mouldy growth of Penicillium notatum contaminating a corner of one of dishes. Then Fleming 368368 realized that Staphylococcus next to the mould has been destroyed. After studying Staphylococcus closely, Fleming concluded that the Penicillium mould was producing a substance that killed the Staphylococcus. He carried on with finding out if the broth of Penicillium mould contained penicillin which could destroy pathogenic bacteria.

In 1931, Fleming dropped his research. Howard Florey and Ernst Chain went on to produce purified penicillin. A successful work was reported 1940, and penicillin has been used to treat wounded soldiers in Second World War. In 1945, Fleming, Florey and Chain received the Nobel Prize for the discovery of penicillin.

b. Saccharomyces 

Definition and characteristics

 - Saccharomyces is a genus of yeasts which include all unicellular fungi that reproduce asexually by budding. 

 - They occur commonly on feces, in the soil, and on the surfaces of plants and animals. 

- The most familiar and industrial important yeast is Saccharomyces cerevisiae.

 - The tiny cells of this yeast are very active metabolically. They are usually aerobic but in the absence of oxygen they use anaerobic metabolism, producing carbon dioxide and ethanol (alcohol) as waste products which are industrially useful - Each cell of Saccharomyces cerevisiae has a single nucleus and is usually egg shaped. 

- Cells contain most of organelles of a typical eukaryote.

10.4.4 Protozoa 

 Protozoa are single celled microscopic organisms that are noted for their ability to move independently. Protists live in many different environments; they can drift in the ocean, creep across vegetation in fresh water rivers and ponds, crawl in deep soil and even reproduce in the bodies of other organisms. Most protozoans are heterotrophic obtaining their nutrients by ingesting small molecules of cells. These particles are usually broken in food vacuoles, membrane-bound chambers that contain digestive enzymes.

Many species of protozoa are free-living, while others are parasitic. Free-living protozoa live in any habitat where water is available at some time during the year. Many species make up the zooplankton, a population of organisms that 369 constitutes one of the primary sources of energy in aquatic ecosystems. Other free-living protozoa live in the soil. Parasitic protozoa usually have complex life cycles that take place in the cells, tissues, and blood stream of their hosts. Several species cause a variety of serious human diseases, including malaria, amebic dysentery and giardiasis. Protozoa that cause diseases

Entamoeba histolytica

a. Characteristics of Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite responsible for a disease called amoebiasis. It occurs usually in the large intestine and causes internal inflammation as its name suggests (histo which means tissue, lytic which means destroying). 50 million people are infected worldwide, mostly in tropical countries in areas of poor sanitation. Inside humans Entamoeba histolytica lives and multiplies as Trophozoites. Trophozoites are oblong and about 15–20 µm in length. In order to infect other humans, they encyst and exit the body.

b. Life cycle Entamoeba histolytica 

Entamoeba histolytica life cycle does not require any intermediate host. Mature cysts (spherical, 12–15 µm in diameter) are passed in the feces of an infected human. Another human can get infected by ingesting them in fecally contaminated water and food. If the cysts survive the acidic stomach, they transform back into trophozoites in the small intestine. Trophozoites migrate to the large intestine where they live and multiply by binary fission. 

Both cysts and Trophozoites are sometimes present in the feces. Cysts are usually found in firm stool, whereas Trophozoites are found in loose stool. Only cysts can survive longer periods (up too many weeks outside the host) and infect other humans. If trophozoites are ingested, they are killed by the gastric acid of the stomach. Occasionally trophozoites might be transmitted during sexual intercourse.

c. Symptoms

Many Entamoeba histolytica infections are asymptomatic and Trophozoites remain in the intestinal lumen feeding on surrounding nutrients. About 10–20% of the infections develop into amoebiasis which causes 70 000 deaths eachyear. Minor infections (luminal amoebiasis) can cause symptoms that include:

- Gas (flatulence) intermittent

- constipation loose stools 

- stomach ache 

- Stomach cramping.

Severe infections inflame the mucosa of the large intestine causing amoebic dysentery. The parasites can also penetrate the intestinal wall and travel to organs such as the liver via bloodstream causing extra-intestinal amoebiasis. Symptoms of these more severe infections include: Anemia, Appendicitis (inflammation of the appendix), bloody diarrhea, fatigue, fever, gas (flatulence), genital and skin lesions, intermittent constipation, liver abscesses (can lead to death, if not treated), malnutrition, painful defecation (passage of the stool), peritonitis (inflammation of the peritoneum which is the thin membrane that lines the abdominal wall), pleurapulmonary abscesses, stomach ache, stomach cramping, toxic mega-colon (dilated colon), Weight loss.

d. Prevention 

To prevent spreading the infection to others, one should take care of personal hygiene. Always wash your hands with soap and water after using the toilet and before eating or preparing food. Amoebiasis is common in developing countries. Some good practices, when visiting areas of poor sanitation: 

- Wash your hands often. - Avoid eating raw food. 

- Avoid eating raw vegetables or fruit that you did not wash and peel.

 - Avoid consuming milk or other dairy products that have not been pasteurized. 

- Drink only bottled or boiled water or carbonated (bubbly) drinks in cans or bottles. Natural water can be made safe by filtering it through an “absolute 1 micron or less” filter and dissolving iodine tablets in the filtered water.

Plasmodium spp.

 a. Characteristics

 - Plasmodium is the genus of the class of Sporozoa that includes the parasite that causes malaria. Plasmodium is a type of protozoa, a singlecelled organism that is able to divide only within a host cell.

 - The main types of Plasmodium spp are P.falciparum, the species that causes falciparum malaria, the most dangerous type of malaria; P. malariae, the species that causes quartan malaria; P. ovale, a species found primarily in east and central Africa that causes ovale malaria; and P. vivax, the species that causes vivax malaria, which tends to be milder than falciparum malaria.

b. Life cycle of Plasmodium

 Plasmodium species exhibit three life-cycle stages gametocytes, sporozoites, and merozoites. Gametocytes within a mosquito develop into sporozoites. 

The sporozoites are transmitted via the saliva of a feeding mosquito to the human blood stream. From there, they enter liver parenchyma cells, where they divide and form merozoites. Inside the host’s liver cell, the Plasmodium cell undergoes asexual replication. The products of this replication, called merozoites, are released into the circulatory system. The merozoites invade erythrocytes and become enlarged ring-shaped Trophozoites.

More erythrocytes are invaded, and the cycle is reinitiated. The merozoites are released into the bloodstream and infect red blood cells. Rapid division of the merozoites results in the destruction of the red blood cells, and the newly multiplied merozoites then infect new red blood cells. Some merozoites may develop into gametocytes, which can be ingested by a feeding mosquito, starting the life cycle over again. 

 The red blood cells destroyed by the merozoites liberate toxins that cause the periodic chill-and-fever cycles that are the typical symptoms of malaria. P. vivax, P. ovale, and P. falciparum repeat this chill-fever cycle every 48 hours (tertian malaria), and P. malariae repeats it every 72 hours (quartan malaria). P. knowlesi has a 24-hour life cycle and thus can cause daily spikes in fever.

10.4.5. Trypanosoma spp. 

a. Characteristics 

- Trypanosoma is the genus containing a large number of parasitic species which infect wild and domesticated animals and humans in Africa. 

 - Commonly known as African sleeping sickness, human trypanosomiasis is caused by the species Trypanosoma brucei and is transmitted to humans through either a vector or the blood of ingested animals. 

 - The most common vector of Trypanosoma brucei is the tsetse fly, which may spread the parasite to humans and animals through bites.

 - Through a process called antigenic variation, some trypanosomes are able to evade the host’s immune system by modifying their surface membrane, essentially multiplying with every surface change. Trypanosoma brucei gradually infiltrates the host’s central nervous system.

b. Symptoms 

Symptoms include: Headache, weakness, and joint pain in the initial stages; anaemia, cardiovascular problems, and kidney disorders as the disease progresses; in its final stages, the disease may lead to extreme exhaustion and fatigue during the day, insomnia at night, coma, and ultimately death.

 c. Occurrence 

 Human trypanosomiasis affects as many as 66 million people in sub-Saharan Africa. Trypanosomes are also found in the Americas in the form of Trypanosoma cruzi, which causes American human trypanosomiasis, or Chagas’ disease. This disease is found in humans in two forms: as an amastigote in the cells, and as a trymastigote in the blood. 

 d. Mode of transmission 

- The vectors for Trypanosoma cruzi include members of the order Hemiptera, such as assassin flies, which ingest the amastigote or trymastigote and carry them to animals or humans.

 - The parasites enter the human host through mucus membranes in the nose, eye, or mouth upon release from the insect vectors. Left untreated, Chagas’ disease may cause dementia, megacolon and damage to the heart muscle, and may result in death.

e. Life cycle of Trypanosoma 

Trypanosoma’s cell structure plays a vital role in allowing the cell to morph into three forms (trypomastigote, epimastigote, and amastigote) during its life cycle, depending on where the cell is located in the host’s anatomy. The location of the kinetoplast in relation to the nucleus and the flagellum emergence dictate in which stage the trypanosome cell is found.

10.4.6. Eubacteria 

 They occur in many shapes and sizes and have distinct biochemical and genetic characteristics. Eubacteria that are rod-shaped are called bacilli, sphereshaped are called cocci (sing. Coccus) and spiral-shaped are called spirilla (sing. Spirillum).

 1. Bbacilli: bacteria with rod-shape. Ex: Clostridium tetani, Bacillus subtilis 

2. Vibrios: comma-shaped with a single flagellum. eg: Vibrio cholera 

3. Cocci: group of bacteria with spherical shape such as Streptococci. Cocci that occur in chains are Staphylococci which are grapelike clusters of cocci and Diplococci which is sphere shaped that are grouped two by two. 

4. Spirilla: bacteria with spiral shape. e.g.: Spirillum volutans.

10.4.7. Algae 

Algae are plantlike organisms that belong to the kingdom protista. Although most of algae are unicellular, some such as the macrocysts are large multicellular organisms. Algae differ from protozoa, which are also classified in the kingdom protista in that they manufacture their food through the process of photosynthesis.

Characteristics of algae

 Algae are a diverse group of protists. They range in size from microscopic single-celled organisms to large seaweeds that may be many meters long. Unlike protozoa which are heterotrophic, algae are autotrophic protists; they have chloroplasts and produce their own carbohydrates by photosynthesis.

 In the past, some classification systems placed the algae in the plant kingdom. However, algae lack tissue differentiation and thus have no true roots, stems or leaves. The reproductive structures of algae also differ from those of plants; they produce gametes in single-celled gametangia or gamete chambers. Plants by contrast, form gametes in multicellular gametangia. For these reasons, algae are classified as protists. 

Despite their diversity, different kinds of algae have several features in common. For example, most algae are aquatic and have flagella at some stages of their life cycle.

In addition, algae cells often contain pyrenoids, organelles that synthesize and store starch.

Structure

 The body portion of an alga is called a thallus. The thallus of an alga is usually haploid. A variety of thallus formats characterize algae. In some species, the thallus consists of a single cell. In other species, it is made of many cells in varying arrangements. Four types of algae are recognized based on the following body structures: unicellular, colonial, filamentous and multicellular. 

• Unicellur algae: have a structure that consists of a single cell. Most unicellularalgaeareaquaticorganismsthatcomposethephytoplankton, a population of photosynthetic organisms that forms the foundation of aquatic food chains. Through photosynthesis, phytoplankton produces almost half of the world’s carbohydrates, thereby providing important nutrients for numerous aquatic organisms. Such unicellular algae are also among the major producers of oxygen in the atmosphere. The chlamydomonas is an example of a unicellular alga. 

• Colonial algae: such as volvoxes have a structure that consists of a group of cells acting in a coordinated manner. Some of these cells become specialized. This division of labor allows colonial algae to move, feed and reproduce efficiently.

• Filamentous algae: such as spirogyra, have a slender, rod shaped thallus composed of rows of cells joined end to end. Other species of filamentous algae have specialized structures that anchor the thallus to the ocean bottom. This adaptation secures the alga in one place as it grows toward the sunlight at the water’s surface. 

• Multicellular algae: often have a large complex thallus. For instance, Ulva has a leaf like thallus that may be several centimeters wide but two cells thick.

Application activity 10.4

1. Complete the table below

2. List any three symptoms of amoebiasis. 

3. What is the cause of anemia for people suffering from malaria?

10.5 E. coli, food poisoning and evolution of harmful strains

ACTIVITY 10.5

You may have understood different people who have had problems after eating different kinds of food. These situations may happen after eating food at home, in a restaurant and even in the hotel. Make a search to know the possible reasons of such cases. What can be done in order to prevent these problems?

10.5.1 E. coli and food poisoning 

E. coli is a rod-shaped bacterium measuring about 2.5µm by 0.5µm. I t is mainly found in guts of vertebrates. It is chemoheterotrophic, capable of thriving on a variety of the organic molecules. Its presence in water indicates contamination by feces. E. coli reproduces asexually by binary fission. It can also take part in a primitive form of sexual activity called conjugation where genetic material is passed in one direction from bacterium to another through a pilus. Although conjugation does not in itself produce new offspring, after the process has finished, the bacteria reproduce asexually, passing on their new genetic makeup to their offspring.

10.5.2 Evolution of harmful strains 

E. coli was thought to be a relatively harmless resident of the human gut which might linked to the occasional upset stomach and mild diarrhea. When massive colonies of mutualistic bacteria are present in the gut, including most strains of E. coli, they help to keep harmful bacteria away from starving them of food. They also help make vitamin K. But in 1982, it became clear that a new strain of E. coli had evolved into a much more troublesome organism. The strain had acquired a gene that enabled it to produce a powerful toxin which damages the intestinal wall, causing severe diarrhoea and internal bleeding. 

This may lead to internal serious dehydration in young children and elderly people, and may result into death. In majority of the cases, infections of pathogenic strain of E. coli are not fatal and the disease clears without treatment.

Application activity 10.5 

Nowadays we are taking drugs to treat malaria. These drugs are different from those that people have been taking in the 1990s. Make a search to know the possible reasons of this. Do you know any other drug that it is not treating any disease that it used to treat? Why?

10.6 Food conservation and water purification 

Activity 10.6

The figure below shows different ways of conserving and preserving food.

a. Name any three food seen on the photo and show how they canbe conserved.

b. Which one of the methods stated above on the figure can be usedto conserve food for long periods of time?

10.6.1 Food conservation 

The term food conservation refers to any one of a number of techniques used to prevent food from spoiling. It includes methods such as canning, pickling, drying and freeze-drying, irradiation, pasteurization, smoking, and the addition of chemical additives. 

The optimum storage conditions differ; raw meat and poultry are kept at around 00c, meat products at 1oC - 40oC. 

Canned foods and many vegetables in dry conditions at 10oc - 150oc, and dried foods such as flour are stored, in air tight containers at10oc – 150oc. For long term storage, meat and fish are vacuum-sealed or can be vacuum packed in laminated plastic containers. For pasteurization, food and drinks such as milk are heated to a temperature that kills disease causing microorganisms. Example: Mycobacterium tuberculosis. (https://www.britannica.com/topic/ food-preservation)

The most frequent and traditional conservation systems used are:

 • By cooling: refrigeration (Between 0º and 5ºC), freezing (less than -18ºC). It consists of maintaining the cold (refrigeration or freezing) throughout the entire process through which the food passes: production, transport, reception, storage, sale to the consumer. If the correct temperature is not maintained during the whole process, the food will suffer IRREVERSIBLE consequences.

 • By heating: microorganisms are destroyed by heat. The most popular methods are: pasteurization, cooking, sterilization and ultrapasteurization.

 • Removing part of the water from the food: drying, salting, sugaring, smoking.

 • Others: pickling, use of preservatives.

10.6.2 Water purification 

 Water purification is the process by which undesired chemical compounds, organic and inorganic materials, and biological contaminants are removed from water. That process also includes distillation (the conversion of a liquid into vapor to condense it back to liquid form) and deionization (ion removal through the extraction of dissolved salts). One major purpose of water purification is to provide clean drinking water. Water purification also meets the needs of medical, pharmacological, chemical, and industrial applications for clean and potable water. The purification procedure reduces the concentration of contaminants such as suspended particles, parasites, bacteria, algae, viruses, and fungi. Water purification takes place on scales from the large (e.g., for an entire city) to the small (e.g., for individual households). 

Most drinking water in Rwanda is obtained is obtained from the lakes and rivers where pollution levels are low. The process of water treatment involves both filtration and chlorination. The process of water purification is summarized below:

• Impure water is first passed through screens to filter out floating debris.

 • Filtration through coarse sand traps larger and insoluble particles. The sand also contains specially grown microbes which remove some of the bacteria. 

 • A sedimentation tank has chemicals known as flocculants, for example alum, added to it to make the smaller particles, which remain in the water, stick together and sink to the bottom of the tank. 380380 

• These particles are removed by further filtration through fine sand.

 • Finally. A little chlorine gas is added, which kills any remaining bacteria. This sterilizes the water.

Application activity 10.6 

Place pieces of meat in air-tight storage jars (or plastic storage bags), making sure to fully cover the meat with salt. Alternate layers of meat and salt to ensure all parts of the meat are covered in salt. Keep the jars/bags in a cool place for two weeks. Do not allow to freeze.

10.7 Fermentation

Activity 10.7

Banana beer can be produced from banana juice.

a. How can banana juice be converted to banana beer?

b. What are the ingredients necessary to produce banana juice?

Fermentation refers to the metabolic process by which organic molecules (normally glucose) are converted into acids, gases, or alcohol in the absence of oxygen. 

Types of fermentation 

There are many types of fermentation that are distinguished by the end products formed from pyruvate or its derivatives. The two fermentations most commonly used by humans to produce commercial foods are ethanol fermentation (used in beer and bread) and lactic acid fermentation (used to flavor and preserve dairy and vegetables). 

a. Alcoholic fermentation

 Yeast releases digestive enzymes which allow the transformation of glucose into ethanol as result of anaerobic fermentation. The presence of bubbles is the evidence that carbon dioxide is released as waste product of the alcoholic fermentation. Making Beer depends on a process called malting. You soak and keep barley grains in water. As germination begins, enzymes break down the starch in the barley grains into a sugary solution. You then extract a solution produced by malting and use it as an energy source for the yeast. The mixture of yeast and sugar solution is then fermented to produce alcohol. Hops are added at this stage to give flavor. The beer is given time to clear and develops its flavor before putting it in bottles or to be sold. Interestingly, alcohol in large quantities is toxic to yeast as well as to people. This is why the alcohol content of wine is rarely more than 14%. Once it gets much higher, it kills all the yeast and stops fermentation.

b. Lactic acid fermentation

 Lactic acid fermentation is the production of lactic acid. Fermentation occurs in mammalian muscle during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid. This type of fermentation is used routinely in mammalian red blood cells and in skeletal muscle that has an insufficient oxygen supply to allow aerobic respiration to continue (that is, in muscles used to the point of fatigue). In muscles, lactic acid accumulation must be removed by the blood circulation and the lactate brought to the liver for further metabolism.

Application activity 10.7

Make a research on the use of fermentation in the production of bread.

10.8 Measuring population growth of bacteria and fungi 

 Activity 10.8 

The figure below shows the growth curve of bacteria. Observe it and answer the questions that follow:

a. The population of bacteria is increasing

b. The population of bacteria is decreasing

c. There is no population growth

d. What are the causes of the increase of the bacteria population?

When bacteria or any other microorganisms are incubated in a suitable culturing medium, they reproduce by binary fissions and the number of individuals increases. The ordinary growth of population is described as sigmoid curve or S-shaped curve made of 4 main phases: 

- The lag phase: period of adaptation of microorganisms to the new habitat (new environment)

 - The log or exponential phase: period of high rate of reproduction. Bacteria are sensitive to the limiting factors of the growth or anti-microbial agents 

- The stationary phase: Stationary phase of plateau-growth slows down. The population remains constant because the rate of dividing/growth is equal to the rate of death within the population. The maximum number that a habitat can accommodate for a long period is known as the carrying capacity.

 - The decline or death phase: period of high rate of death than the rate of dividing/growth due to the scarcity of food, the abundance of metabolic waste products, presence of antibiotics or any other drugs killing the germs. 

Figure 19.5 shows the phases explained above.

10.8.1 Measuring population growth of bacteria 

 The typical growth curve of a population of bacteria is similar to the growth curve expected for yeast, a unicellular fungus or the growth of any population. When measuring the growth of a population of bacteria or yeast, we can carry out direct counting of the numbers of cells or indirectly by measuring some indication of the number of cells such as the coldness of a solution, or production of a gas.

It is usual to inoculate a small sample of the microorganisms in a sterilized nutrient medium and to place the culture in an incubator at the optimum temperature for growth. Other conditions are pH, oxygen concentration and ionic and osmotic balance. Growth can be measured from the time of inoculation. Two types of cell count are possible, namely viable count and total count. The viable count is the total of living cells only and total count is the total number of both living and dead cells and is easier to measure.

Application activity 10.8

1. The graph shows a population growth graph for bacteria.

b. Which of the four phases, labelled A, B, C and D, represents the stationary phase and which the lag phase? 

c. During which phases will some of the bacteria die? 

d. State two factors that could affect the rate of population growth during phase C. 

10.9 Culturing microorganisms

Activity 10.9: Research activity

Make a search in different books or on the internet on the following elements.

a. What is a culture medium?

b. What is the importance of culturing microorganism?

c. What are the ingredients necessary to make a culture medium?

d. What are the possible apparatus necessary to make a culture medium?

Many microorganisms can be grown in the laboratory. This allows scientists to learn a lot about them. We can find out which nutrients they need to survive and which chemicals will kill them. We can also discover which microorganisms can be useful to us and which cause deadly disease.

To find out more about microorganisms, you need to culture them. Culturing microorganisms involves growing very large numbers of them so that you can see the colony as a whole. 

To culture microorganisms, you must provide them with everything they need. This usually involves providing a culture medium containing carbohydrates to act as an energy source. 

 A long with this, various mineral ions some supplement of proteins and vitamins are included. 

The nutrients are often contained in an agar medium. Agar is a substance which dissolves in hot water and sets to form a jelly. You pour hot agar containing all the necessary nutrients into a Petri dish. Microorganisms are living organisms. Therefore, they have requirements for their growth, maintenance and multiplication. These include:

- Optimum temperature (30-40ᵒC) for enzymes to work better. 

- Source of energy such as glucose, maltose, juice.

 - Source of other nutrients (minerals such are as potassium, sodium, iron, magnesium and calcium, vitamins, proteins 

- Air for aerobic microbes or complete absence of air for anaerobic microorganisms.

The medium for culture of microbes can be the dead organic matters (food, fruits, remaining of organism, juice, milk) or a prepared medium such as Agar agar (universal medium for any germ)… Different types of media are used culture microorganisms.

10.9.1 Types of media 

 There are many different types of media described by their components or ingredients. 

Universal media: this allow the growth of every type of bacteria e.g. agar-agar Differential/selective media: are specific to some types of bacteria for example Lowenstein for tuberculosis bacteria. Their ingredients will favour growth of certain types of bacteria.

 A pure culture: this contains only one kind of microorganism. The pure cultures are important for scientific method as they are free from other types of microorganisms.

10.9.2 Principles of sterile culturing 

 - Wash hands before touching a sterile Petri-dish 

- Open the Petri-dish as little as possible, and replace the lid quickly 

- Never cough or sneeze near the dish 

- Never touch the infected jiffy with fingers

 - When culturing is no longer required, they should be flooded with strong disinfectant 

- After cleaning out the nutrient from Petri-dish, they should be washed and disinfected, and then if they are glass, heat sterilize.

 - Wash your hands thoroughly after all operation by using soap. 

- Never push hands near the mouth during experimental work.

Safety precautions 

Bacteria grow and reproduce more quickly when they are warm than when they are cold. It would be dangerous to incubate cultures at temperatures close to body temperature (37°C) because doing so might allow the growth of pathogens harmful to health. So the maximum temperature used in school and college labs is 25°C. However, higher temperatures can be used industrially, and these produce faster growth.

10.9.3 Culture media 

A medium is a solid or liquid preparation containing nutrients for the culture of microorganisms. A pure microbial culture undergoes the following steps namely:

-Choice of the culture medium. 

- Sterilization of the culture medium.

 - A culture with a collection of microbial cells growing on or, in a medium.

 - Selection of a pure colony from a collection of microbial cells growing 

 – Introduction of a microorganism into a suitable growth medium 

– Streaking to carrying out a pure culture.

Microorganisms may be cultured in a solid medium or a liquid medium or broth. When there is not a culture with a collection of microbial cells growing on or, in a medium. A source of microorganisms is spread on the surface of an agar to produce individual colonies. Once individual colonies are obtained, this collection of microorganisms can then use to carry out a pure microbial culture.

a. Solid medium 

Solid media are particularly suitable for bacteria and fungi and are prepared by mixing the liquid nutrient solution with a gelling agent, usually agar, at a concentration of about 1-5%, thus, producing nutrient agar that allows the growth of colonies.

b. Liquid media 

The liquid media are water – based solutions that are generally termed as broths, milks and infusions.

 Liquid media are often useful for measuring population growth. They may be placed in a test tube, stopped by a plug of cotton wool or a metal cap, or in a glass, screwcrapped bottle such as a universal bottle which holds about 25cm2 enough for one agar plate.

 The medium must be sterilized and after, adding a small quantity of cells to the medium is called inoculation.

c. Enrichment media 

An enrichment medium is a medium in which substances are added to meet the requirements of certain microorganisms in preference to others. As a result, certain microorganisms grow better than others. 

 d. A selective medium 

It is a medium in which one or more substances are added to favor the grown of specific microorganisms and to inhibit the growth of others. Example, the addition of penicillin to a culture to select for those organisms resisting to it, or the selection of hybridizes cells during the production of monoclonal antibodies.

10.9.4 Aseptic technique

 Aseptic technique is using sterilized equipment and solutions and preventing their contamination. Sterilization is the removal or destruction of all living microorganisms, including spores (inactive structures that enable some microorganisms to survive unfavorable periods). Bacterial and fungal spores are abundant in most environments including laboratories. A range of special techniques and apparatus are designed to prevent contamination of nutrients media. Autoclaves are used to sterilize equipment and culture media before experiments and also to sterilize equipment and specimens before disposal. In addition, after sterilization, a great care is taken during experiments to ensure that there is no infection.

Application activity 10.9

Make a list of materials and products necessary to make a culture medium and the importance of each one.

10.10 Staining bacteria and growing viruses

Activity 10.10

Discuss the following questions

a. What is the importance of staining bacteria?

b. How is the Gram stain performed?

c. Which colors do bacteria take after Gram staining?

10.10.1 Staining bacteria 

Bacteria have a peptidoglycan or murein cell wall that maintains cell shape, provides protection and prevents the cell from lysis. Based on the composition of the cell wall, bacteria can be classified as Gram-positive and Gram-negative. During the process of Gram staining , some bacteria without a lipid layer along with their peptidoglycan cell wall take the gram stain and appear violet (purple) and are therefore called gram positive. Example streptococcus and staphylococcus. Bacteria having a lipid layer along with their peptidoglycan cell wall do not take up the gram stain and are therefore called gram negative.

Example: Escherichia coli, Azotobacter, Salmonella. 

Gram-positive bacteria retain the gram stain and appear purple under the technique. Gram- negative bacteria do not retain the purple stain and take up a second pink stain instead. Because gram-positive bacteria have a thicker layer of peptidoglycan in their cell wall than gram-negative bacteria do, they are able to retain the gram stain. 

Gram-positive and gram-negative bacteria also differ in several other ways: for example, they have different susceptibilities to antibacterial drugs, they produce different toxic materials and they react differently to disinfectants. For these reasons the gram stain is useful for identifying and grouping bacteria.

Procedure of the gram stain

• In the gram stain procedure, bacteria that have been placed on a slide are stained with a purple dye solution called crystal violet.

• The purple dye is washed off with water, and then a solution of iodine is added to the slide.

• The bacteria are rinsed with alcohol.

• And then re-stained with a pink dye solution called safranin.

• Gram-positive bacteria will retain the purple dye and appear purple, while gram-negative bacteria will appear pink.

10.10.2 Growing viruses 

The culture of viruses is made more difficult than the culture of bacteria or fungi because viruses can only grow and multiply inside living cells. This can be done by infecting whole organisms such as plants or animals but, where possible, cell, tissue cultures are now used. An early technique was to grow certain viruses in chick embryos while the embryo was still growing inside the egg.

Application activity 10.10

Identifying bacteria by using the Gram stain technique The Gram s 

The Gram stain technique

• Place bacteria on a slide

• Stained the bacteria with a purple dye solution called crystal violet.

• The purple dye is washed off with water, 

• Add a solution of iodine to the slide.

• The bacteria are rinsed with alcohol.

• And then re-stained with a pink dye solution called safranin.

• Gram-positive bacteria will retain the purple dye and appear purple, while gram-negative bacteria will appear pink.

Skills lab 10 

Production of alcohol using banana juice 

Knead the bananas until they are soft and pulpy. Use a stiff grass to help knead and squash the banana pulp and to extract the juice. The pulp residue will remain in the grass. Pour off the extracted juice into a large clean bucket or similar container. This banana juice is non-alcoholic and can be diluted and drunk at this stage if desired. Add clean boiled water to the extracted juice (one volume of water for three volumes of banana juice). It is necessary to dilute the banana juice so that the concentration of soluble solids is low enough for the yeast to ferment the juice. Grind the cereal (sorghum or millet) and lightly roast it over an open fire. Add the roast cereal (1 part cereal to 12 parts juice) to the diluted banana juice. Cover the bucket with a clean lid and leave in a warm place to ferment for 18 to 24 hours. The ground cereal improves the color and flavor of the beer. After fermentation the beer is filtered through a sterilized cotton cloth.

End unit assessment 10

1. State any TWO diseases caused by:

a. Bacteria

b. Protozoa

c. Microscopic fungi

2. What is the main feature of moulds?

3. Why viruses are not generally considered to be living things?

4. The diagram below represents the structure of the human immuno deficiency virus (HIV/AIDS).

i. Name A, B, C, and D. 

ii. HIV/AIDS is under retroviruses. What is meant by retroviruses?

 iii.What type of leucocytes (white blood cells) are destroyed by HIV/ AIDS?

 5. What are different types of media used in the laboratories for culturing microorganisms? 

 6. How do biologists differentiate between Gram –positive and Gram – negative bacteria? 

7. Describe the three methods of preventing bacterial growth in food. 

8. How does temperature affect the growth of bacteria in culture media?

 9. Assuming that you have a bacterial infection, would you ask for vaccination against the bacteria? Why or why not? 

10. How do bacteria maintain the balance in the environment? 

11. Identify the following groups of bacteria