Licence: In copyright
Credit: Manual of surgery / by Alexis Thomson and Alexander Miles. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![ever, while they prefer oxygen, are able to live without it, and are c&W&d facultative anaerobes. A temjjerature ranging from 95° to 102° F. is the most suitable for bacterial life. Extreme or prolonged cold paralyses but does not kill micro-organisms. Few, however, survive being raised to a temperature of 134J° F. Boiling for ten to twenty minutes will kill not only bacteria, but even their most resistant spores. Direct sunlight, electric light, or even diffuse daylight is inimical to the growth of bacteria. Pathogenic Properties of Bacteria.—We are now only concerned with pathogenic bacteria—that is, bacteria capable of 2?roducing 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 sa]oropliytes. Such as do flourish 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. It is also the case that if organisms gain access directly to the blood-stream 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 robust 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.](https://iiif.wellcomecollection.org/image/b21933194_0001_0040.jp2/full/800%2C/0/default.jpg)
