Microorganisms, Biotechnology and Discoveries

Microorganisms are living things so small that we need a microscope to see them. They live everywhere: in soil, water, air, our food and even inside our body. Most of them are harmless and many are friends (they make curd, bread and medicines), but a few cause serious diseases. The two groups the examiner loves are bacteria and viruses, and the easiest way to remember them is this: bacteria are tiny living cells, while viruses are not even complete cells.

Bacteria are single-celled prokaryotes: they have no true nucleus and no membrane-bound organelles. Their DNA lies as a single circular chromosome in the cytoplasm (often with small extra rings called plasmids), they have 70S ribosomes, and the cell wall is made of peptidoglycan. They multiply by binary fission, and under good conditions one cell can divide every 20 minutes or so. Because they are so simple and so quick, they are the favourite tool of genetic engineers (you will see this in the biotechnology topic).

Gram staining (Hans Christian Gram, 1884) divides bacteria into two groups. Gram-positive bacteria have a thick peptidoglycan wall, hold the crystal violet stain and look purple. Gram-negative bacteria have a thin wall (plus an outer membrane), lose the violet stain and take up the safranin counter-stain, so they look pink/red. Thick wall = holds purple.

Viruses sit on the border between living and non-living. Outside a host they are just a particle, and they can even be crystallised (W. M. Stanley did this with tobacco mosaic virus in 1935). Inside a living cell they take over the cell's machinery and make copies of themselves. So a virus is an obligate intracellular parasite: it can multiply only inside a living host cell. A virus has only one kind of nucleic acid (either DNA or RNA, never both) wrapped in a protein coat called the capsid. It has no cytoplasm, no ribosomes and no metabolism of its own. A virus that infects bacteria is a bacteriophage.

After bacteria and viruses, three more groups of microorganisms matter for exams: fungi, protozoa and algae. All three are eukaryotes (they have a true nucleus). The easiest way to separate them is by the way they get food. Fungi cannot make food and absorb it from dead or living matter. Protozoa are tiny animal-like single cells that eat or absorb food. Algae contain chlorophyll and make their own food by photosynthesis.

Fungi (study: mycology) have no chlorophyll, so they are heterotrophs. Most are saprotrophs: they pour digestive enzymes on dead matter, break it down outside the body and then absorb the soluble food. A fungus body is made of thread-like hyphae, a mass of which is called a mycelium. The cell wall is made of chitin (the same material as insect skeletons). Fungi reproduce by spores; yeast, which is a single-celled fungus, also reproduces by budding. Rhizopus (bread mould) shows the typical plan: upright hyphae carry a round sporangium full of spores.

Two partnerships you must know. Lichen = alga (or cyanobacterium) + fungus living together; the alga makes food, the fungus gives shelter and water. Lichens are indicators of air pollution (they disappear where sulphur dioxide is high). Mycorrhiza = fungus + roots of a higher plant; the fungus helps the plant absorb phosphate and water and gets sugar in return.

Protozoa are single-celled, animal-like organisms (protozoology is their study). Know the movement organ because it is an examiner's favourite: Amoeba moves with false feet (pseudopodia), Paramecium with hair-like cilia, Euglena and Trypanosoma with a whip-like flagellum, and Plasmodium (sporozoa) has no organ of movement in the adult stage. Paramecium has a fixed slipper shape, a groove for taking in food, and a contractile vacuole that pumps out excess water. Euglena has chloroplasts and also eats food in the dark, so it is called a link between plants and animals.

Algae (study: phycology) are simple, plant-like organisms with chlorophyll but without true roots, stems or leaves; their body is a thallus. They live mostly in water. They are grouped by their pigments: green algae (Spirogyra, Chlamydomonas, Ulva, Chlorella), brown algae (Laminaria, Fucus, kelp) and red algae (Gelidium, Gracilaria, Porphyra). Blue-green 'algae' are really cyanobacteria (bacteria), which is a favourite trap.

Many of the foods on your plate exist because of friendly microbes: curd, idli, bread, cheese, vinegar, wine and beer. The key process behind them is fermentation: microbes break down sugars without oxygen (anaerobically) and release energy for themselves, and the by-products (alcohol, acid or gas) give the food its taste and texture. Louis Pasteur showed in the 1850s-60s that fermentation is carried out by living microbes, and Eduard Buchner (1897) showed that it is done by enzymes (zymase) inside yeast.

Curd formation: a drop of curd (starter) adds a few Lactobacillus to warm milk. They feed on lactose and make lactic acid; the acid makes casein (milk protein) clot, so milk thickens and turns slightly sour. This is why curd sets better in summer than in winter: the bacteria grow best in warmth.

The same microbes that help us can also spoil food, so people learned to stop their growth. Pasteurisation (named after Louis Pasteur) heats milk to about 63 °C for 30 minutes, or about 72 °C for 15 seconds, and then cools it quickly. It kills disease-causing microbes without changing the taste much, but it does not kill every microbe. Preservation works by taking away what microbes need: low temperature (refrigeration slows growth), dehydration (no water, no growth), salt or sugar (draws water out of microbes by osmosis), acid or oil (pickles), airtight heating (canning) and chemical preservatives such as sodium benzoate.

Microbes also affect our health through food. Food poisoning can be caused by Salmonella and Staphylococcus, and botulism by Clostridium botulinum, which can grow in badly canned food. Even in the mouth, bacteria feed on sugar left on teeth, form plaque (bacteria plus food particles) and make acid that dissolves the enamel. If decay is left untreated, microorganisms can invade the inner pulp of the tooth.

Before 1928, a small cut or a case of pneumonia could kill a healthy person, because there was no medicine that could kill bacteria inside the body. Then Alexander Fleming noticed that a mould had killed the bacteria around it on a culture plate, and the age of antibiotics began. An antibiotic is a chemical made by one microorganism that kills or stops the growth of other microorganisms, mainly bacteria. The word was coined by Selman Waksman in 1942.

An antibiotic works on the structures that bacteria have but human cells do not: for example, penicillin stops bacteria from building their peptidoglycan cell wall, so the bacterium bursts. Viruses have no cell wall and no metabolism of their own, so antibiotics cannot touch them. That one idea answers many questions: antibiotics for flu or cold are useless, and wrong or half-finished courses create antibiotic-resistant bacteria (AMR).

Vaccines teach the body to fight a germ before the real infection arrives. A vaccine contains a weakened or killed germ, or just a harmless part of it (a toxoid, a protein, or its genetic instructions). The immune system makes antibodies and memory cells, so a later attack is stopped quickly. This is active immunity. A ready-made antibody injection (such as antivenom for snakebite, made in horses) gives quick but short passive immunity. The word vaccine comes from Latin vacca (cow), because Jenner used cowpox.

Biotechnology means using living organisms, or parts of them such as enzymes, to make useful products. Making curd and wine is old (traditional) biotechnology. The modern kind began when scientists learned to cut a gene out of one organism and put it into another. That is genetic engineering, and an organism carrying a gene from another species is transgenic or a genetically modified organism (GMO). The word 'biotechnology' was coined by the Hungarian engineer Karl Ereky in 1919.

Why bacteria? Bacteria have small circular DNA rings called plasmids that can carry foreign genes, they multiply every 20 minutes or so, and they read the genetic code in the same way as other organisms. So a bacterium given the human insulin gene starts making human insulin. The first genetically engineered medicine, human insulin (Humulin), was made in E. coli and approved in 1982; the first recombinant DNA organism was made by Stanley Cohen and Herbert Boyer in 1973.

GM crops are checked in India by the Genetic Engineering Appraisal Committee (GEAC) under the Ministry of Environment. The main purpose of GM crops is to improve yield, pest or herbicide resistance, shelf life and nutrition. The debates are about safety, biodiversity, seed ownership and farmers' costs.

Cloning means making a genetically identical copy. Dolly the sheep, born in July 1996 at the Roslin Institute, Scotland (Ian Wilmut and Keith Campbell), was the first mammal cloned from an adult body cell. She died in 2003. In India, the National Dairy Research Institute, Karnal produced cloned buffalo calves (Samrupa and Garima, 2009), and Noori, a cloned pashmina goat, was born at SKUAST, Srinagar (2012). In plants, tissue culture grows a whole plant from a tiny piece because plant cells are totipotent; it is used for banana, orchids and disease-free potato.

Biofertilisers are living microbes that are added to soil or seed to raise the supply of nutrients, mainly nitrogen and phosphorus. They are cheap, they do not pollute the water, and they restore soil health, which is why they are promoted as an eco-friendly alternative to chemical fertilisers. The nitrogen-fixing ones convert atmospheric nitrogen (N2), which plants cannot use, into ammonia using the enzyme nitrogenase.

Biology exam questions are full of 'who found what and when', 'the study of what is called what' and 'the scientific name of this organism'. This topic gives you all three in tables. Do not try to cram them in one go; read each table once with the small stories attached (for example, Hooke saw dead cork cells, Leeuwenhoek saw living ones), and the pairs will start sticking by themselves.

Binomial nomenclature, introduced by Carl Linnaeus, gives each organism a two-part name: the genus (first letter capital) then the species (all small), written in italics or underlined separately, as in Homo sapiens. The lion, tiger and leopard share the genus Panthera because they are close relatives; they differ in species. The rules are governed by ICN (plants), ICZN (animals) and ICNP (bacteria).