Economic Importance of Bacteria

Although bacteria are too small to see, many are quietly useful. In the dairy, Lactobacillus bacteria convert the sugar in milk (lactose) into lactic acid; the acid makes the milk proteins set, turning warm milk into curd and giving it a sour taste. The same souring helps make cheese. In industry, bacteria are used to make vinegar, to cure tobacco and coffee, and to soften animal hides in leather tanning. Without these tiny helpers, much everyday food and many products would not exist.

Plants need nitrogen to make proteins, but they cannot use the nitrogen gas in the air directly. Rhizobium bacteria, living in nodules on the roots of leguminous plants such as peas and beans, fix this free nitrogen into nitrates the plant can absorb. Other soil bacteria carry on the work: Nitrosomonas changes ammonia (from rotting matter) into nitrites, and Nitrobacter changes nitrites into nitrates. This whole chain, the nitrogen cycle, keeps soil fertile naturally. One group works against us: denitrifying bacteria like Pseudomonas break nitrates back down to nitrogen gas, draining the soil.

Some bacteria and fungi make antibiotics - chemicals that kill or stop disease-causing germs. Streptomyces bacteria give us Streptomycin, used against tuberculosis. Bacteria are also nature's cleaners: bacteria of decay break down dead plants, animals and waste, returning nutrients to the soil. In sewage treatment plants, bacteria digest the waste so the water can be cleaned. By breaking down what would otherwise pile up, they act as the planet's recyclers and scavengers.

A small number of bacteria cause real trouble. Disease-causing (pathogenic) bacteria give humans tuberculosis, cholera, typhoid and tetanus, and infect animals and crops too - Xanthomonas causes black rot of cauliflower and mustard. Other bacteria spoil our food: they sour milk, rot vegetables and turn food poisonous if it is left warm. This is why we refrigerate, boil, pasteurise and tin food - all these methods slow or stop bacterial growth and keep food safe.