MOISTURE 63 



MOISTURE. 



The viaximum 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 Bacilliis typhosus (of 

 typhoid fever) and Microspira comma (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 abso- 

 lute dryness, and no organism can even exist, not to say 

 develop, in such a condition since water is an essential con- 

 stituent 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 permissable 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 arti- 

 ficial culture media a water content of less than 30 pqr 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 



