126 MANUAL OF BACTERIOLOGY. 



The time of exposure is, as a rule, ten minutes. The tubes 

 should be removed quickly to cold water. Their contents 

 should afterward be inoculated into bouillon to determine 

 whether or not the organisms have been killed. In the 

 practical use of heat for sterilization or disinfection, the 

 exact thermal death-point is greatly exceeded. The time of 

 exposure is also longer than is absolutely necessary as deter- 

 mined by the results of the experiments. 



It is hardly safe to depend upon text-book statements in re- 

 gard to the thermal death-point of bacteria in practical disin- 

 fection. 



Moisture is indispensable to the growth of bacteria, and dry- 

 ing causes the death of certain kinds, as, for instance, the 

 spirillum of cholera, while others remain alive, but do not grow. 



Heim* found that the resistance of organisms to drying is 

 very much greater when the organism in question is contained 

 in the pathological material from animals which have suc- 

 cumbed to the disease, on the one hand, than when it is derived 

 from cultures, on the other. The pneumococcus which is very 

 sensitive to drying, and in fact is difficult to keep going on 

 culture media, remains alive for i6 months and preserves its 

 virulence for more than a year when it is contained in blood 

 from an animal which has died of the infection dried on silk 

 threads and kept in a desiccator containing calcium chloride. 

 Similar results were obtained with other organisms. 



Food. — There are a few species of bacteria that contain 

 chlorophyll, but it is wanting in most forms. On account of 

 the absence of chlorophyll, bacteria require, as part of their 

 food, organic compounds, such as sugar, as a source of carbon. 

 They are unable, with very few exceptions, such as the nitrify- 

 ing bacteria, to derive their carbon from the carbon dioxide 

 of the atmosphere, or from inorganic carbon compounds. Al- 

 though some species are able to obtain nitrogen from inorganic 



*Zeitschr. f. Hygiene. Bd. L., No. i, April 4, 1905, p. 123. 



