20 BACTERIA 



Certain anaerobic bacteria grow in the presence of oxygen if 

 other particular varieties of aerobic bacteria are present. 



Attenuated tetanus bacilli become virulent if cultivated with 

 Bacterium vulgae. Again, complicated chemical changes, as the 

 decomposition of nitrites with the evolution of nitrogen cannot 

 be accomplished by certain bacteria severally, but jointly, this 

 is quickly brought about. 



Pfeiffer has shown that certain chemical substances (foods, 

 albumins, etc.), attract bacteria (positive chemotaxis) , while 

 other substances, as turpentine, repel them (negative chemotaxis). 

 Oxygen repels anaerobes and is particularly attractive to aerobes. 



FREE AGENTS PREJUDICIAL TO THE LIFE OF 

 BACTERIA 



High temperatures are surely germicidal: 6oC. coagulates 

 mycoprotein of bacteria and other common albumins. The degree 

 of temperature at which bacteria are killed is called the thermal 

 death-point. Most vegetative forms die after a short exposure at 

 6oC., though some require a higher temperature, e.g., tubercle 

 bacillus. 



Spores resist boiling, often for hours. Spore-bearing bacilli 

 from the soil often survive a temperature of ii5C. moist heat 

 (steam), from thirty to sixty minutes. Bacteria resist dry heat 

 of i75C. from five to ten minutes. 



Cold inhibits bacteria; destroys some; but is not a safe germi- 

 cidal agent, as typhoid bacilli have been isolated from melted ice 

 in which they had been frozen for months. 



Ravenel exposed bacteria to the extreme cold of liquid air 

 ( 3i2F.) and found that typhoid bacilli survived an exposure 

 of sixty minutes; diphtheria, thirty minutes, and anthrax spores, 

 three hours; during this exposure, however, many were destroyed. 



Light is inimical to the life of bacteria, direct sunlight being the 

 most germicidal, as it destroys some, reduces the virulence of 



