Microorganisms: Friend and Foe Notes

Microorganisms are classified into four major groups — bacteria, fungi, protozoa, and some algae. Viruses are also microscopic but reproduce only inside a host cell, so they are on the border of living and non-living.

They may be single-celled (bacteria, many protozoa like Amoeba and Paramecium) or many-celled (many fungi and algae). Some cause diseases: bacteria (typhoid, tuberculosis), protozoa (malaria, amoebic dysentery), viruses (common cold, influenza, COVID-19), and fungi (ringworm).

Microorganisms live almost everywhere — in soil, water, air, inside our bodies, and even in hot springs and polar ice.

In the kitchen: the bacterium Lactobacillus turns milk into curd; yeast, which respires anaerobically and releases carbon dioxide, makes bread and idli/dosa batter rise and is used to produce alcohol from sugars (fermentation).

In medicine: antibiotics such as penicillin are made from fungi and bacteria and kill or stop disease-causing microbes (they do not work on viral diseases). Vaccines contain dead or weakened microbes that train the body's immune system to fight the real disease later — as with polio, tetanus, measles, and hepatitis vaccines.

In the environment and agriculture: bacteria and fungi decompose dead plants and animals and animal waste, returning nutrients to the soil. Certain bacteria (Rhizobium in the root nodules of pulses) and blue-green algae fix atmospheric nitrogen into compounds plants can use, increasing soil fertility.

Disease-causing microbes are called pathogens. They spread through air (cold, TB), water (cholera, typhoid), food, contact, and through carriers such as the housefly and the female Anopheles mosquito (malaria) and Aedes mosquito (dengue).

Microorganisms also spoil food. Some, like certain bacteria and fungi, produce toxins in food that cause food poisoning. Food is therefore preserved to stop microbial growth: adding salt (pickles, salted fish, meat), adding sugar (jams, jellies), adding chemical preservatives (sodium benzoate, sodium metabisulphite), heating and cooling (boiling, pasteurisation of milk, refrigeration, deep freezing), and removing moisture (drying, dehydration). Sealed cans and airtight packing keep microbes out.

Pasteurisation, invented by Louis Pasteur, involves heating milk to about 70 °C for a short time and then quickly cooling it, which kills most microbes without spoiling the taste.