Hello, and welcome to your biology lesson for today. In this session, we will explore the fascinating world of bacteria and fungi, and discover how these tiny organisms play enormous roles in our lives, our health, our food, and our industries. We will journey through their structures, their remarkable abilities, and their economic importance to human society.
Let us begin with bacteria, the most primitive and yet incredibly successful organisms on Earth.
Bacteria are single-celled organisms that belong to the group called prokaryotes. This means they lack a true nucleus, their genetic material floats freely in the cytoplasm without any nuclear membrane enclosing it. Despite their simplicity, bacteria are everywhere, in the air we breathe, the water we drink, the soil beneath our feet, and even inside our own bodies. In fact, there are more bacteria living on your skin right now than the total number of humans on Earth.
These microscopic organisms are incredibly small, averaging about two micrometres in length, that is, two-thousandths of a millimetre. They come in four main shapes that you should remember. Spherical bacteria are called cocci, rod-shaped ones are bacilli, spiral-shaped bacteria are spirilla, and comma-shaped ones are called vibrio. Some bacteria live alone, while others form pairs called diplococci, chains called streptococci, or clusters known as staphylococci.
The structure of a bacterial cell is elegantly simple. Picture a tiny cell with a stiff outer wall made of peptidoglycan, not cellulose like plant cells. Inside this wall lies a thin cell membrane surrounding the cytoplasm. There is no true nucleus, just a single circular chromosome of DNA attached to the cell membrane. The cytoplasm contains vacuoles and granules, and sometimes a protective slimy layer called a capsule covers the cell wall. Some bacteria possess whip-like flagella that pierce through the cell wall and enable them to swim actively in liquid environments.
Most bacteria cannot make their own food since they lack chlorophyll. They are heterotrophic, obtaining nourishment either as saprotrophs from decaying dead matter, or as parasites from living hosts. They secrete powerful enzymes that dissolve food externally, then absorb the nutrients in solution. Some bacteria need oxygen to respire, these are aerobic, while others are anaerobic and are actually killed by exposure to air.
Bacteria reproduce primarily through a remarkable process called binary fission. The circular DNA duplicates, the cell expands, and then it pinches in the middle to form two identical daughter cells. Under ideal conditions, some bacteria divide every thirty minutes. At this rate, a single bacterium could theoretically produce over two hundred and eighty trillion descendants in just twenty-four hours.
When conditions become unfavourable, bacteria form spores. The cell contents shrink into a spherical mass surrounded by a thick, hard protective wall. These spores are incredibly resilient, they can survive extreme dryness, boiling water, freezing ice, and even poisonous chemicals. When favourable conditions return, the spore germinates and a new bacterium emerges. Importantly, spore formation is not reproduction, it is simply a survival strategy to escape harsh conditions.
Now let us explore how bacteria benefit humanity, beginning with their crucial role in medicine.
Bacteria are instrumental in producing antibiotics, serums, and vaccines that save countless lives.
An antibiotic is defined as a chemical substance produced by a living microorganism that can stop the growth of or kill disease-causing bacteria and fungi. The story of antibiotics began in nineteen twenty-nine when Alexander Fleming noticed that a mould called Penicillium chrysogenum accidentally contaminated his bacterial culture and destroyed the bacteria. This discovery led to penicillin, the first antibiotic. Note carefully that penicillin comes from a fungus, not a bacterium. However, the antibiotic streptomycin, discovered by Selman Waksman, does come from the bacterium Streptomyces griseus. A good antibiotic should kill a wide variety of disease-causing organisms, produce minimal side effects, and not harm the normal bacteria of the host.
Serums are another vital medical product. Serum is blood plasma from which fibrinogen has been removed, and it may contain antibodies called antitoxins. When pathogenic bacteria release poisonous proteins called toxins, the body produces antitoxins to neutralise them. To prepare serum, a small dose of bacterial toxin is injected into a healthy animal like a horse or cow. The animal's immune system produces antitoxins, and after several injections, blood is drawn and the serum is separated. This serum can then protect humans against specific diseases. Snake-bite treatments use similar anti-venom serums.
Modern genetic engineering has revolutionised medicine through bacteria. Human genes can be introduced into bacteria like Escherichia coli, commonly called E. coli. These modified bacteria multiply rapidly and produce human proteins. Insulin was the first such product, now essential for millions of diabetics worldwide. Other genetically engineered products include blood clotting factors for treating haemophilia.
Vaccines represent another triumph of bacterial biotechnology. A vaccine is a preparation consisting of weakened germs or dead germ substances. Vaccination is the introduction of these weakened or dead substances into the body to develop resistance to a particular disease. When vaccinated, a person experiences a mild form of the disease, stimulating the body to produce antitoxins that provide future immunity. The TAB vaccine for typhoid uses killed bacteria, while the BCG vaccine for tuberculosis uses living weakened bacteria. Toxoids, which are inactivated toxins, are used to create immunity against diphtheria and tetanus.
Bacteria are equally indispensable in agriculture, particularly through their remarkable role in the nitrogen cycle.
Plants need nitrogen to build proteins, but they cannot use the free nitrogen gas that makes up seventy-nine percent of our atmosphere. They can only absorb nitrogen in the form of nitrates from the soil. This is where bacteria become essential partners in agriculture.
Nitrogen-fixing bacteria called Rhizobium live in small nodules on the roots of leguminous plants like beans and peas. These bacteria capture free nitrogen from the soil and air and convert it into soluble nitrates that plants can use. When legume crops are ploughed back into the soil, they enrich it with nitrogen for future crops. Free-living soil bacteria like Azotobacter and Clostridium also convert atmospheric nitrogen into ammonia, which becomes amino acids and nitrates.
Nitrifying bacteria perform another crucial service. First, bacteria convert nitrogenous wastes and dead organic matter into ammonia. Then bacteria like Nitrosomonas convert ammonia compounds into nitrites. Finally, bacteria like Nitrobacter transform nitrites into nitrates that plants can absorb. This entire process is called nitrification.
Denitrifying bacteria complete the cycle by breaking down soil nitrates and releasing nitrogen gas back into the atmosphere. Pseudomonas is one such bacterium. Without this step, soil nitrogen would eventually become locked in compounds and unavailable to plants.
Beyond the nitrogen cycle, bacteria drive decay and putrefaction, nature's recycling system. Decay is the complete breakdown of organic matter without foul smell, while putrefaction is incomplete breakdown that produces unpleasant odours. Sewage treatment plants use bacteria to decompose human waste, producing biogas for cooking and nutrient-rich manure for agriculture. Biogas plants work similarly, with bacteria breaking down cow dung to release methane gas while producing excellent fertiliser. In our intestines, beneficial bacteria synthesise vitamins, especially B-complex vitamins and vitamin K.
In industry, bacteria contribute to processes like tea curing, where specific bacteria develop different tea flavours, and leather tanning, where bacteria help break down soft perishable parts of animal hides.
However, bacteria are not always beneficial. They spoil food through fermentation and decay, especially in warm weather, sometimes causing dangerous food poisoning. Botulism is a particularly serious form caused by bacteria in improperly preserved canned foods. Distended cans that gush gas when opened should always be discarded.
We preserve food through various methods. Sterilisation by boiling kills most bacteria, though spores require higher temperatures and pressure. Salting prevents biodegradation by curing foods like fish and pickles. Dehydration removes water that microbes need to grow. Pasteurisation heats milk to about sixty degrees Celsius for thirty minutes, killing most harmful bacteria while preserving flavour, though it does not completely sterilise the milk. Refrigeration slows bacterial growth, with deep freezing at minus twenty to minus thirty degrees Celsius providing long-term preservation. High sugar or salt concentrations cause plasmolysis, killing any invading microbes. Chemical preservatives like sodium benzoate also protect tinned foods.
Bacteria cause significant diseases in plants, animals, and humans. Plants suffer from black rot of mustard and cauliflower, and bacterial blight of cowpea. Cattle are afflicted by anthrax, which causes body swelling and reduced milk yield, and bovine tuberculosis, which damages the lungs. Human bacterial diseases include whooping cough, cholera, tuberculosis, diphtheria, typhoid, pneumonia, and tetanus. Tragically, bacteria can also be weaponised as bioweapons, with germ bombs potentially releasing disease-causing bacteria like anthrax to cause widespread epidemics.
Now let us turn our attention to fungi, organisms that are more evolved than bacteria but equally fascinating and economically important.
Fungi are eukaryotes with true nuclei, and unlike bacteria, most are multicellular. They lack chlorophyll and cannot make their own food, so they are heterotrophic. The most familiar fungi are moulds that grow on bread, fruits, and even leather.
Bread mould, Rhizopus, consists of a network of thread-like structures called hyphae. The entire mass of hyphae is called mycelium. These penetrating threads secrete enzymes that digest food externally, a process called extracellular digestion, then absorb the nutrients. Reproduction occurs both asexually through spores produced in sporangia, and sexually through the union of gametes from different mycelial strains.
Yeasts are unicellular fungi, typically spherical and found in sugary solutions. The yeast cell has a distinct cell wall, nucleus, and vacuoles. Yeasts respire anaerobically, without oxygen, breaking down glucose into ethanol and carbon dioxide. This process, fermentation, is the foundation of several major industries.
The economic importance of fungi is truly remarkable.
In breweries, yeast ferments sugars to produce alcoholic beverages. Wine comes from fermented grapes, beer from barley maltose, using Saccharomyces cerevisiae. The chemical equation is simple, glucose yields ethanol and carbon dioxide. Wine contains about twelve percent alcohol, which eventually kills the yeast. Higher alcohol spirits require distillation. While mild consumption may stimulate, excessive drinking causes serious harm, particularly liver cirrhosis.
In bakeries, yeast transforms bread-making. When added to dough, yeast ferments sugar and produces carbon dioxide gas. The dough rises to three times its original volume, a process called leavening. During baking, these gas bubbles expand, creating the light, spongy texture we love in bread.
Cheese-making relies heavily on fungi and bacteria. Lactic acid bacteria curdle milk, separating it from whey. The curd is processed into cottage cheese, salted to remove moisture and prevent spoilage, then ripened with specific microorganisms that impart distinctive flavours. Various Penicillium species create flavoured cheeses. Cheese is nutritionally valuable, rich in fat, protein, calcium, phosphorus, and vitamins A and B.
Mushroom cultivation represents another major fungal industry. Not all mushrooms are edible, many are poisonous, but cultivated varieties provide excellent nutrition. The white button mushroom, Agaricus bisporus, is most common globally. In India, three varieties dominate, white button, paddy straw mushroom, and oyster mushroom.
Mushroom cultivation proceeds through five steps. First, composting mixes agricultural waste like wheat straw with chicken manure and fertilisers, maintained at fifty degrees Celsius for about a week. Second, spawning introduces mushroom mycelium, the mushroom seed, into the compost. Third, casing spreads a thin soil layer that supports the mushroom, maintains humidity, prevents drying, and regulates temperature. Fourth, cropping and harvesting involves waiting for mycelium growth, then pin heads, then button stage mushrooms of marketable size. Fifth, preservation extends shelf life through vacuum cooling, gamma radiation, or freeze-drying.
Mushrooms are nutritionally excellent, containing about eighty-five to ninety-five percent water, three percent protein, four percent carbohydrate, minimal fat, and abundant minerals and vitamins. They are particularly rich in niacin, pantothenic acid, and biotin, vitamins well-preserved during cooking and processing. Remember, never eat wild mushrooms, they may be fatally poisonous. Always purchase from reliable commercial sources.
Other fungi contribute to medicine, notably Penicillium chrysogenum as the source of penicillin. However, many fungi are harmful, spoiling food and causing diseases like ringworm. Penicillium and Aspergillus commonly rot citrus fruits. That green mould on an orange is Penicillium, but do not mistake it for medicinal penicillin.
Let us briefly recap the key insights from today's lesson.
First, bacteria are prokaryotic single-celled organisms without true nuclei, incredibly diverse in shape and habitat, reproducing rapidly by binary fission and forming resilient spores for survival.
Second, bacteria are medically vital, producing antibiotics like streptomycin, enabling serum and vaccine production, and serving as factories for genetically engineered medicines like insulin through Escherichia coli, commonly called E. coli.
Third, bacteria drive the nitrogen cycle through nitrogen fixation, nitrification, and denitrification, making atmospheric nitrogen available to plants and sustaining agriculture.
Fourth, bacteria serve industry in tea curing, leather tanning, and biogas production, while requiring careful control to prevent food spoilage and disease.
Fifth, fungi are eukaryotic, mostly multicellular, heterotrophic organisms, with yeasts enabling fermentation for alcohol and bread production, and various species contributing to cheese-making and antibiotic production.
Sixth, mushroom cultivation provides nutritious food through controlled indoor farming, while proper food preservation methods protect us from harmful bacterial and fungal contamination.
As we conclude, remember that these microscopic organisms, invisible to our eyes, shape our world in profound ways. From the antibiotics that cure our infections to the bread on our tables, from the nitrogen that feeds our crops to the mushrooms that nourish us, bacteria and fungi are indispensable partners in human civilization. Understanding them empowers us to harness their benefits while protecting ourselves from their harms. Keep curious, keep questioning, and keep exploring the magnificent hidden world of biology. Until next time, stay well and keep learning.