52 GENERAL CONDITIONS FOR GROWTH 



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 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 arti- 

 ficial 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. 



TEMPERATURE. 



The temperature conditions for bacterial existence and 

 growth have been determined more accurately than any of 

 the other general conditions. The maximum for existence 

 must be placed at or near 100°, since it is known that all 

 bacteria including spores may be killed by boiling in time. 

 Nevertheless, certain forms have been reported as thriving 

 in hot springs where the water temperature was 93°. This 

 is the highest known temperature for development. The 

 minimum for existence lies at or near the absolute zero 

 (—273°), since certain organisms have been subjected to the 



