#Conditions of Bacterial Life.#--Bacteria require for their growth and development a suitable food-supply in the form of proteins, carbohydrates, and salts of calcium and potassium which they break up into simpler elements. An alkaline medium favours bacterial growth; and moisture is a necessary condition; spores, however, can survive the want of water for much longer periods than fully developed bacteria. The necessity for oxygen varies in different species. Those that require oxygen are known as _aërobic bacilli_ or _aërobes_; those that cannot live in the presence of oxygen are spoken of as _anaërobes_. The great majority of bacteria, however, while they prefer to have oxygen, are able to live without it, and are called _facultative anaërobes_. The most suitable temperature for bacterial life is from 95° to 102° F., roughly that of the human body. Extreme or prolonged cold paralyses but does not kill micro-organisms. Few, however, survive being raised to a temperature of 134½° F. Boiling for ten to twenty minutes will kill all bacteria, and the great majority of spores. Steam applied in an autoclave under a pressure of two atmospheres destroys even the most resistant spores in a few minutes. Direct sunlight, electric light, or even diffuse daylight, is inimical to the growth of bacteria, as are also Röntgen rays and radium emanations. #Pathogenic Properties of Bacteria.#--We are now only concerned with pathogenic bacteria--that is, bacteria capable of producing disease in the human subject. This capacity depends upon two sets of factors--(1) certain features peculiar to the invading bacteria, and (2) others peculiar to the host. Many bacteria have only the power of living upon dead matter, and are known as _saphrophytes_. Such as do nourish in living tissue are, by distinction, known as _parasites_. The power a given parasitic micro-organism has of multiplying in the body and giving rise to disease is spoken of as its _virulence_, and this varies not only with different species, but in the same species at different times and under varying circumstances. The actual number of organisms introduced is also an important factor in determining their pathogenic power. Healthy tissues can resist the invasion of a certain number of bacteria of a given species, but when that number is exceeded, the organisms get the upper hand and disease results. When the organisms gain access directly to the blood-stream, as a rule they produce their effects more certainly and with greater intensity than when they are introduced into the tissues.