QUITE apart from their uses in industry and their rather spectacular relations with human diseases, bacteria are important in everyday life. They cause foodstuffs to go bad, they sour our milk, ‘make ‘a cheese, and more notably still, they purify sewage. Indeed, we might almost say that without the bacterial processes by which sewage is purified, the civilisation of our big towns would be impossible. It is obvious that crude sewage will contain very nearly as many bacteria as it is possible for it to hold. Under proper conditions of storage the bacteria will kill themselves by using the available food materials and give as a result
water that is perfectly clear, and valuable substances that are used as fertilisers. The supply of pure milk has perhaps focused attention on bacterial activity in a way that nothing else could have done : we are told everywhere that milk has been ‘pasteurised,’ by which we understand that it has been subjected to a process of sterilisation which kills a large number of the bacteria normally contained in it. Further, we are told milk is supplied from ‘tuberculin ‘tested cows, by which we understand that the cows have been examined for the amount of tubercle bacteria they contain. We assume from the results obtained that our daily milk supply will not be infecting our bodies with large quantities of bacteria that will cause tuberculosis.