Microbes at home

Microbes are living things too small to see without a microscope: bacteria, fungi, protozoa, as well as viruses, viroids and prions. They live everywhere: in soil, water and air, inside our bodies, and even in extreme places such as boiling hot springs (thermal vents), deep in the soil, under snow, and in very acidic water. Some cause disease, but many are useful, and we use them every day.

Curd. Warm milk with a spoonful of curd added turns into curd overnight. The small amount of curd is a starter: it carries millions of lactic acid bacteria (LAB), such as Lactobacillus. At a suitable warm temperature they multiply and produce acids, which coagulate and partly digest the milk proteins. LAB also increase vitamin B₁₂ in the curd, so curd is more nutritious than milk. In the stomach, too, LAB help keep harmful microbes in check.

Fermented doughs. The dough for idli and dosa is left overnight to ferment: bacteria in it produce carbon dioxide, which puffs it up. The dough for bread is fermented by baker's yeast, Saccharomyces cerevisiae, which also produces CO₂.

Other fermented foods. Toddy, a traditional drink of southern India, is made by fermenting sap from palms. Fish, soyabean and bamboo shoots are fermented to make foods in many parts of India.

Cheese. Different microbes give cheeses their own textures and flavours. The large holes in Swiss cheese are made by carbon dioxide from the bacterium Propionibacterium sharmanii. Roquefort cheese is ripened by a fungus growing in it, which gives its special flavour.