Microorganisms and Disease

All around us, and even inside our own bodies, live tiny living things that are far too small to be seen with the naked eye. These are called microorganisms or microbes, and they can be seen only with the help of a microscope. Microorganisms are found almost everywhere — in air, water, soil, food, and on and inside the bodies of plants and animals. Despite their tiny size, they play an enormous role in nature, in our health, and in industry. The study of these tiny organisms is called microbiology.

Microorganisms are classified into five major groups. Bacteria are extremely small, single-celled organisms with no true nucleus; they come in shapes like rods, spheres, and spirals, and reproduce very rapidly by simply splitting in two. Viruses are even smaller than bacteria and are unusual because they can reproduce only inside the living cells of a host (a plant, animal, or even a bacterium); outside a host they are inactive, which is why scientists place them on the borderline between living and non-living.

The remaining three groups are larger and more complex. Fungi include single-celled forms such as yeast and many-celled forms such as moulds and mushrooms; the greenish or blackish growth on stale bread or rotting fruit is a fungus. Protozoa are single-celled, animal-like microbes such as Amoeba and Paramecium, and some, like the malaria-causing organism, are harmful. Algae are simple, plant-like microbes that contain chlorophyll and make their own food by photosynthesis; examples include the green threads (Spirogyra) seen floating in ponds.

Microorganisms can be both useful and harmful to us. Some help in making curd, bread, and medicines, and in keeping the soil fertile, while others cause diseases in humans, plants, and animals, and spoil our food. Because they are present everywhere and multiply so quickly, understanding the different types of microorganisms is the first step to using the helpful ones and protecting ourselves from the harmful ones — the central theme of this chapter.

Although we often blame microbes for disease, a great many of them are extremely useful and are essential for life on Earth. One of the most important services microbes provide is in keeping soil fertile through nitrogen fixation. Plants need nitrogen to grow, but they cannot use the nitrogen gas in the air directly. Certain bacteria, such as Rhizobium, live in the root nodules of leguminous plants (peas, beans, gram) and convert atmospheric nitrogen into compounds the plant can absorb. Some free-living soil bacteria and blue-green algae (cyanobacteria) also fix nitrogen, naturally enriching the soil.

Microbes are also nature's great recyclers through decomposition. Bacteria and fungi break down the dead bodies of plants and animals, and other organic wastes, into simple substances that mix back into the soil. This decomposition clears away dead matter and returns nutrients to the soil, keeping the cycle of nature going. Without decomposer microbes, dead leaves, bodies, and waste would pile up everywhere and the nutrients locked inside them would never be released for new life.

In our kitchens and food industries, microbes help make many foods and products. The bacterium Lactobacillus turns milk into curd, multiplying in warm milk and changing it into a thick, tasty, easily digestible food. Yeast, a fungus, is used to make bread rise: it ferments the sugar in the dough, releasing carbon dioxide gas that forms tiny bubbles and makes the bread soft and spongy. Microbes are also used in making cheese, idli and dosa batter, and many other fermented foods, as well as in producing alcohol.

Perhaps the most life-saving use of microbes is in making medicines, especially antibiotics. Antibiotics are substances produced by certain fungi and bacteria that can kill or stop the growth of disease-causing microbes; the famous antibiotic penicillin comes from a fungus. Microbes are also used to prepare vaccines, which protect us from many serious diseases. From enriching the soil, to recycling waste, to making food and medicine, useful microorganisms quietly support our daily lives in countless ways.

While many microbes are helpful, a number of them are harmful because they cause diseases and spoil our food. Microorganisms that cause disease are called pathogens. Harmful microbes affect not only humans but also plants and animals, and the diseases they cause can spread from one individual to another. Pathogens belong to all the major groups: some diseases are caused by bacteria, some by viruses, some by fungi, and some by protozoa.

In humans, harmful microbes cause many familiar diseases. Bacteria cause illnesses such as tuberculosis (TB), cholera, and typhoid; viruses cause the common cold, influenza (flu), measles, and polio; the protozoan that causes malaria is spread by mosquitoes; and certain fungi cause skin diseases like ringworm. These pathogens enter the body through air, water, food, or contact, multiply inside us, and damage the body, producing the symptoms of illness.

Harmful microbes also attack plants and animals, which is a serious problem for farmers. Plant diseases such as citrus canker (caused by bacteria) and rust of wheat (caused by a fungus) reduce crop yields and can ruin harvests. In animals, microbes cause diseases such as foot-and-mouth disease in cattle and anthrax. These diseases lower food production and cause heavy losses, so controlling harmful microbes in farming is very important.

Apart from causing disease, harmful microbes also cause food spoilage. When bacteria and fungi grow on food, they break it down, change its taste, smell, and appearance, and make it unfit to eat. Spoiled food often develops a bad odour, mould, or a slimy surface, and eating it can lead to food poisoning because some microbes release harmful toxins. This is why food must be stored properly and preserved. Understanding harmful microbes helps us take steps — through cleanliness, safe food handling, and treatment — to protect our health and our food.

Some diseases can spread from an infected person to a healthy person; these are called communicable diseases (or infectious diseases). They are caused by pathogens — bacteria, viruses, protozoa, or fungi — that pass from one individual to another in various ways. Examples include the common cold, tuberculosis, cholera, typhoid, and malaria. Diseases that cannot spread from person to person, such as those caused by poor diet, are called non-communicable diseases; this topic focuses on the communicable ones.

Communicable diseases spread through several modes of transmission. Some spread through the air: when an infected person coughs or sneezes, tiny droplets carrying microbes are released, and a healthy person who breathes them in can catch the disease. Tuberculosis (TB) and the common cold spread this way. Others spread through water: when drinking water is contaminated with sewage or microbes, diseases like cholera and typhoid spread to those who drink it.

Diseases can also spread by contact and through vectors. Contact transmission happens when a healthy person touches an infected person, or shares their towels, utensils, or other belongings; many skin diseases spread this way. A vector is an animal — often an insect — that carries pathogens from one person to another without itself falling ill. The female Anopheles mosquito is the vector for malaria, and houseflies can carry microbes from waste to food, spreading diseases like cholera and typhoid.

Knowing how communicable diseases spread helps us prevent them. We can cover the mouth and nose while coughing or sneezing, drink only clean or boiled water, wash hands before eating, avoid sharing personal items with the sick, and control disease-carrying vectors by keeping surroundings clean and not letting water collect (where mosquitoes breed). Vaccination protects against many such diseases. By breaking the chain of transmission, we can stop these diseases from spreading through a community.

Our body is constantly exposed to harmful microbes, yet most of the time we stay healthy. This is because the body has a powerful defence network called the immune system, which recognises invaders and destroys them. The immune system has two lines of defence. The first is innate (inborn) immunity, which we are born with; it acts the same way against all invaders and includes barriers like the skin, the mucus lining our nose and throat, stomach acid, and white blood cells that swallow up microbes. It is non-specific, meaning it attacks any germ in the same general way.

The second line is adaptive (acquired) immunity, which the body develops over time as it meets different microbes. Unlike innate immunity, it is specific — it produces a tailored defence against each particular microbe and, importantly, it remembers that microbe. Any substance that the body recognises as foreign and that triggers an immune response is called an antigen; microbes and their parts carry antigens on their surface. When the body detects an antigen, the immune system swings into action against it.

In response to an antigen, special white blood cells produce defensive proteins called antibodies. Each antibody is shaped to lock onto a particular antigen, like a key fitting a lock. By binding to the antigen, antibodies neutralise the microbe or mark it for destruction. After the infection is over, the body keeps memory cells that remember that antigen, so if the same microbe attacks again, the body produces antibodies much faster and we may not even fall ill. This is why we usually get diseases like chickenpox only once.

This memory is the basis of vaccination. A vaccine contains dead or weakened microbes, or their harmless parts, carrying the antigen but unable to cause the disease. When a vaccine is given, the immune system makes antibodies and memory cells against that antigen, just as it would for a real infection — but without the illness. Later, if the real microbe enters the body, the immune system is already prepared and destroys it quickly. Vaccination has helped control and even wipe out deadly diseases, making the immune system one of the body's most remarkable protectors.

Antibiotics are medicines used to fight diseases caused by bacteria. They are special substances, originally produced by some microbes themselves, that can kill bacteria or stop them from growing and multiplying inside the body. The discovery of antibiotics is one of the greatest achievements in medicine. In 1928, the scientist Alexander Fleming accidentally noticed that a mould (the fungus Penicillium) growing on a dish had killed the bacteria around it. From this mould came the first antibiotic, penicillin, which went on to save millions of lives.

Antibiotics work by attacking parts or processes that bacteria need to survive — for example, by damaging the bacterial cell wall or interfering with how bacteria make the substances they need. Some antibiotics kill bacteria outright, while others simply stop them from multiplying so that the body's immune system can clear the remaining ones. Because they target features special to bacteria, antibiotics can fight a bacterial infection without harming the patient's own cells.

A very important point is that antibiotics work only against bacteria, not against viruses. Diseases like the common cold, flu, and measles are caused by viruses, so taking antibiotics for them does no good — viruses do not have the bacterial features that antibiotics target. Taking antibiotics unnecessarily for viral illnesses wastes the medicine and can do harm. Antibiotics should be taken only when a doctor prescribes them for a bacterial infection, and the full course should always be completed, even if the patient starts feeling better.

A growing danger today is antibiotic resistance, also called antimicrobial resistance (AMR). When antibiotics are overused or misused — taken when not needed, or stopped halfway through a course — some bacteria survive and become resistant, meaning the antibiotic can no longer kill them. These resistant bacteria multiply and spread, making infections much harder to treat. To slow AMR, we must use antibiotics only when truly needed, exactly as prescribed, and always finish the full course. Using antibiotics responsibly keeps these life-saving medicines working for everyone.

Food is spoiled mainly by microbes — bacteria and fungi — that grow on it, break it down, and make it unfit and unsafe to eat. Food preservation is the set of methods we use to stop or slow the growth of these microbes so that food stays fresh and edible for a longer time. Microbes need warmth, moisture, and air to grow, so most preservation methods work by removing one of these or by killing the microbes already present. Preserving food prevents waste, keeps food safe, and lets us store and transport it across seasons and places.

Some methods preserve food using heat and cold. Pasteurisation is used for milk: the milk is heated to a certain temperature for a short time and then cooled quickly; this kills most harmful microbes without spoiling the taste, and the milk keeps longer. Refrigeration keeps food in a cold fridge, where the low temperature greatly slows down the growth of microbes, so items like vegetables, milk, and cooked food stay fresh for several days. Freezing slows microbes even more, preserving food for much longer.

Other methods work by changing the conditions microbes need. Pickling preserves vegetables, fruits, and meats by storing them in salt, oil, vinegar, or spices; the high salt or acid content stops microbes from growing, which is how pickles, jams, and similar foods last for months. Canning seals food inside airtight cans or jars after heating, so that no microbes can get in or survive, allowing food to be stored for a long time. Dehydration (drying) removes the water from food — as in dried grains, pulses, and dried fruits — and since microbes cannot grow without moisture, dried food keeps well.

A more modern method is irradiation, in which food is exposed to controlled radiation that kills microbes and pests without making the food harmful or radioactive; it is used for spices, onions, and some other foods to extend their shelf life. Common salt and sugar are also used as natural preservatives — salting fish and meat, and adding sugar to make jams and squashes, both stop microbial growth. By choosing the right preservation method, we can keep food safe, reduce wastage, and enjoy seasonal foods all year round.