106 EFFECT OF OTHER AGENTS ON BACTERIA 



5. The Higher the Temperature the Sooner the Bacteria Perish. 

 Death due to drying is probably in some cases due to a non-reversible 

 reaction which follows the well-known temperature law of Van't 

 Hoff and Arrhenius. In other cases it is undoubtedly due to the 

 increased osmotic pressure produced by the removal of the moisture. 



6. Old cultures, unless they be spore bearers, succumb sooner to 

 drying than do young cultures. 



7. The influence of drying upon bacteria varies greatly with the 

 species. Whereas the gonococcus, pneumococcus, spirochete of 

 syphilis, cholera spirilla, and Pfeiffer bacillus can withstand drying 

 only a few hours, the typhoid, diphtheria, and tubercle bacilli may 

 survive days; and tetanus, anthrax, and many soil organisms may 

 survive drying for months or even years. Ammonifying, nitrifying, 

 and nitrogen-fixing bacteria have been isolated in great numbers 

 from soils which have been kept in tight bottles air-dry for more than 

 fifty years. Even the non-spore-forming types of Azotobacter will 

 withstand desiccation over sulphuric acid for a considerable time. 



Osmotic Pressure. Bacteria vary greatly in their ability to with- 

 stand great osmotic changes. Some are quickly plasmolyzed in 

 solutions having low osmotic pressure, whereas others can grow in 

 strong sugar or salt solutions. This factor plays a great part in the 

 preserving of fruits by means of sugar, of pickles and cabbage by 

 means of salt, and many fruits by drying. Those fruits which have 

 the highest carbohydrate content, such as grapes and prunes, are 

 especially easy to preserve by drying. 



FIG. 16. Plasmolysis of various bacterial cells. (Buchanan's Household 



Bacteriology.) 



Probably the great osmotic pressure in the soil solution of alkali 

 soils plays a great part in retarding the bacterial activity of these 

 soils. In this case, however, there is also a physiological factor in 

 which the living protoplasm of the cell is so changed in its chemical 

 and physical properties that it cannot function normally. It is 

 found that equivalent osmotic concentrations of sodium and potas- 

 sium salts act very differently upon some bacteria. 



Pressure. Bridgman found that the application of very great 

 hydrostatic pressure resulted in the coagulation of white of egg. 



