Читать книгу The Fundamentals of Bacteriology - Charles Bradfield Morrey - Страница 21

The maximum moisture is absolutely pure water, and no organism can thrive in this alone owing to the factor of too low osmotic pressure and to the further factor of absence of food material. There are many bacteria which thrive in water containing only traces of mineral salts and a large class whose natural habitat is surface water. These “water bacteria” are of great benefit in the purification of streams. They are as a class harmless to men and animals. Some of the disease-producing bacteria like Bacterium typhosum (of typhoid fever) and Vibrio choleræ (of Asiatic cholera) were undoubtedly originally water bacteria, and it is rather striking that in these diseases conditions are induced in the intestine (diarrheas) which simulate the original watery environment. The minimum moisture condition is absolute dryness, and no organism can even exist, not to say develop, in such a condition since water is an essential constituent of living matter. Some bacteria and especially most spores may live when dried in the air or by artificial means for months and even years, while some are destroyed in a few hours or days when dried (typhoid, cholera, etc.). The optimum amount of moisture has not been determined with any great accuracy and certainly a rather wide range in percentage of water is permissible with many, though a liquid medium is usually most favorable for artificial growth. The “water bacteria” have been mentioned. In the soil a water content of 5 to 15 per cent. seems to be most suitable for many of the organisms which aid in plant growth. In animals and man the organisms infecting the intestinal tract prefer a high percentage of moisture as a rule, especially those causing disease here. Those found on the surface of the body (pus cocci) need a less amount of water, while those invading the tissues (tuberculosis, black-leg, etc.) seem to be intermediate in this respect. In artificial culture media a water content of less than 30 per cent. inhibits the growth of most bacteria.

As a general rule those bacteria which require the largest percentage of water are most susceptible to its loss and are most readily killed by drying. The typhoid and cholera organisms die in a few hours when dried, while pus cocci and tubercle bacilli live much longer.