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 134.5 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 Rontgen 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.

Further, the virulence of an organism is modified by the condition of the patient into whose tissues it is introduced.  So long as a person is in good health, the tissues are able to resist the attacks of moderate numbers of most bacteria.  Any lowering of the vitality of the individual, however, either locally or generally, at once renders him more susceptible to infection.  Thus bruised or torn tissue is much more liable to infection with pus-producing organisms than tissues clean-cut with a knife; also, after certain diseases, the liability to infection by the organisms of diphtheria, pneumonia, or erysipelas is much increased.  Even such slight depression of vitality as results from bodily fatigue, or exposure to cold and damp, may be sufficient to turn the scale in the battle between the tissues and the bacteria.  Age is an important factor in regard to the action of certain bacteria.  Young subjects are attacked by diphtheria, tuberculosis, acute osteomyelitis, and some other diseases with greater frequency and severity than those of more advanced years.

In different races, localities, environment, and seasons, the pathogenic powers of certain organisms, such as those of erysipelas, diphtheria, and acute osteomyelitis, vary considerably.