Microbiology: Bacteria, Viruses, Fungi, Protozoa, Disease and Useful Microbes
Microbiology is the branch of biology that studies microorganisms, that is, living beings that are too small to be seen with the naked eye. These include bacteria, viruses, fungi, protozoa and some algae. Microorganisms are found everywhere, in soil, water, air, food and even inside the human body. Though we often think of them only as the cause of disease, in reality a very large number of microbes are useful to us. They help in making curd, bread, cheese and alcohol, in producing antibiotics, in fixing nitrogen in the soil, and in cleaning the environment. For candidates preparing for APSC, ADRE, DHS, SSC, Railway and police examinations in Assam, questions on microbes, diseases, and fermentation appear regularly in the general-science paper.
This article gives a complete deep-study revision of microbiology. It first fixes the basic vocabulary, then explains the classification and structure of microbes, the diseases caused by them, the useful role of microbes in daily life and industry, the meaning of immunity and vaccination, and the methods of preventing disease. Finally, the examination trends, common mistakes and a practical preparation plan are discussed. The scientific names of the important pathogens and processes are given so that objective questions can be answered accurately.
Basic Terms
- Microbiology: the study of microorganisms.
- Microorganism: a living being too small to be seen with the naked eye.
- Pathogen: a microorganism that causes disease.
- Bacteria: single-celled, prokaryotic microorganisms.
- Virus: a tiny infectious agent that can multiply only inside a living cell.
- Fungus: a eukaryotic organism that feeds on dead or living matter, such as mould and yeast.
- Protozoa: single-celled eukaryotic organisms, some of which cause disease.
- Fermentation: the breakdown of sugar into alcohol or acid by microbes without oxygen.
- Antibiotic: a substance produced by a microbe that kills or stops the growth of other microbes.
- Vaccine: a preparation that gives immunity against a disease.
- Immunity: the ability of the body to resist a disease.
- Vector: an organism that carries a pathogen from one host to another.
- Carrier: a person who carries a pathogen without showing symptoms.
- Sterilisation: the process of killing all microorganisms from an object.
Core Concepts and Figures
- Bacteria are prokaryotes and lack a true nucleus; viruses are acellular and have no cell at all.
- The study of bacteria is called bacteriology, of viruses virology, and of fungi mycology.
- The bacterium that fixes nitrogen in the root nodules of legumes is Rhizobium.
- The antibiotic penicillin is obtained from the fungus Penicillium notatum.
- Yeast is used in fermentation to make alcohol and in baking to make bread rise.
- The disease tuberculosis is caused by Mycobacterium tuberculosis, and malaria by Plasmodium.
- Dengue and chikungunya are spread by the Aedes mosquito, and malaria by the Anopheles mosquito.
- The father of microbiology is Anton van Leeuwenhoek, who first observed bacteria; Robert Koch identified the causes of tuberculosis and cholera; and Edward Jenner developed the first vaccine, against smallpox.
Classification and Structure of Microbes
Microorganisms are grouped into several major kinds. Bacteria are single-celled prokaryotes, that is, they have no true nucleus and no membrane-bound organelles. A bacterial cell has a cell wall, a cell membrane, cytoplasm, ribosomes and a circular DNA, and some have a capsule and flagella for movement. Bacteria are of different shapes: cocci are round, bacilli are rod-shaped, spirilla are spiral, and vibrio are comma-shaped. Viruses are much smaller than bacteria and are called acellular, because they are not made of cells. A virus is simply a nucleic acid, either DNA or RNA, covered by a protein coat called the capsid. A virus cannot reproduce on its own; it must enter a living host cell and use that cell's machinery to multiply, which is why viruses are called obligate intracellular parasites. Fungi are eukaryotes and include yeasts, moulds and mushrooms, and they grow best in warm, moist conditions. Protozoa are single-celled eukaryotes such as Amoeba and Paramecium.
Diseases Caused by Microorganisms
Many microorganisms cause disease in humans, animals and plants. Among bacterial diseases, tuberculosis, cholera, typhoid, tetanus, leprosy and diphtheria are important. Among viral diseases, measles, mumps, polio, rabies, influenza, dengue, chikungunya, chickenpox, hepatitis and AIDS are common. Malaria and amoebic dysentery are caused by protozoa, ringworm and athlete's foot are caused by fungi, and diseases such as sleeping sickness and kala-azar are caused by protozoa as well. Some diseases are spread by vectors: malaria by the female Anopheles mosquito, dengue and chikungunya by the Aedes mosquito, and plague by the rat flea. Diseases spread through contaminated water and food include cholera, typhoid, jaundice and diarrhoea. Diseases spread through air include tuberculosis, common cold, influenza and measles. Diseases may also spread by direct contact, by blood, and by animals. Plant diseases such as rust of wheat and citrus canker also reduce crop yield.
Useful Microorganisms
Microorganisms are extremely useful to human beings. In the making of curd, the bacterium Lactobacillus converts milk into curd and also improves its nutritional value. Yeast is used in the baking industry to make dough rise and in the brewing industry to make alcohol by fermentation. In the absence of oxygen, yeast converts sugar into alcohol and carbon dioxide; this process is called fermentation and is used to make wine, beer and vinegar. Many antibiotics are obtained from microorganisms; penicillin comes from the fungus Penicillium, and streptomycin comes from a bacterium. Microbes are also used to make vitamins and enzymes. In agriculture, bacteria such as Rhizobium fix atmospheric nitrogen in the root nodules of pulses and enrich the soil, and other bacteria and fungi decompose dead matter and convert it into humus. In sewage treatment and biogas plants, microbes break down waste and produce biogas. Microbes are therefore both friends and foes of humankind.
Immunity and Vaccination
Immunity is the ability of the body to fight against a disease. When a pathogen enters the body, the body produces substances called antibodies that act against it, and the body then remembers that pathogen. Immunity may be innate, which a person has from birth, or acquired, which develops after an infection or after vaccination. In vaccination, a weakened or killed pathogen, or a part of it, is introduced into the body so that the body produces antibodies and develops memory without suffering the disease. Edward Jenner developed the first vaccine against smallpox. Vaccines are now available for many diseases such as polio, measles, tuberculosis, tetanus, diphtheria and hepatitis, and they have saved millions of lives. A person who carries a pathogen without showing symptoms is called a carrier and can spread the disease to others.
Prevention of Disease
Most communicable diseases can be prevented by simple habits. Personal hygiene includes washing hands with soap, keeping the body and clothes clean, and covering the mouth while coughing. Safe drinking water and properly cooked food prevent water-borne and food-borne diseases. Vector control, by removing stagnant water, using mosquito nets and spraying insecticides, checks diseases such as malaria and dengue. Vaccination protects against many serious diseases. Isolation of infected persons and the disinfection and sterilisation of instruments prevent the spread of infection. A balanced diet, clean surroundings and adequate rest keep the body's natural immunity strong. In hospitals and laboratories, sterilisation is carried out by heat, chemicals or radiation to destroy all microorganisms. Public health measures such as proper drainage, garbage disposal and health education are equally important.
Examination Trends
In competitive examinations, microbiology questions usually test the kind of microorganism that causes a particular disease, the vector of malaria, dengue and plague, the microbe used in curd, bread, alcohol and antibiotics, the scientist associated with a discovery, the difference between a bacterium and a virus, and the meaning of fermentation, vaccination and immunity. Objective questions frequently ask the shape of bacteria, the pathogen of tuberculosis, the cause of AIDS, the microbe that fixes nitrogen, the source of penicillin, and the names of vaccines and their diseases. A smaller number of questions test sterilisation, pasteurisation and the names of diseases that affect plants.
Common Mistakes to Avoid
- Do not call a virus a cell; a virus has no cell structure and is acellular.
- Remember that antibiotics work against bacteria but not against viruses.
- Do not confuse the vector of malaria (Anopheles) with that of dengue (Aedes).
- Do not interchange the roles of bacteria and yeast in curd and alcohol production.
- Distinguish between innate and acquired immunity before answering.
How to Prepare This Topic
Make a table of diseases in three columns: disease, causative microbe and mode of spread or vector. Learn the useful microbes separately, with the product each one gives. Remember the important scientists, Anton van Leeuwenhoek, Robert Koch, Edward Jenner and Alexander Fleming, with their contributions. Write the differences between bacteria and viruses in a small chart, because this is asked very often. Revise the methods of prevention and the meaning of sterilisation and pasteurisation. Finally, attempt previous years' question papers so that the exact wording used in examinations becomes familiar.
Frequently Asked Questions
What is the difference between a bacterium and a virus?
A bacterium is a single-celled prokaryotic organism that has a cell wall, cytoplasm and genetic material, and it can reproduce on its own. A virus is much smaller, has no cell structure, and can multiply only inside a living host cell by using that cell's machinery.
Which microorganisms are used to make curd and alcohol?
The bacterium Lactobacillus is used to make curd from milk. Yeast, a fungus, is used to make alcohol by fermentation and is also used in baking to make bread rise.
Which mosquito spreads malaria and which spreads dengue?
Malaria is spread by the bite of the female Anopheles mosquito, which carries the parasite Plasmodium. Dengue is spread by the Aedes mosquito, which is active mainly during the day.
What is fermentation?
Fermentation is the breakdown of sugar into alcohol or acid by microorganisms in the absence of oxygen. Yeast converts sugar into alcohol and carbon dioxide, and this process is used in the making of wine, beer and bread.
Who discovered penicillin and from where is it obtained?
Penicillin was discovered by Alexander Fleming. It is obtained from the fungus Penicillium notatum, and it was the first antibiotic to be widely used to treat bacterial infections.