172 Microbes and You 



Surface tension affects microbial growth in terms of clumping, 

 morphology, staining properties, cultural characteristics, and even 

 virulence. The lower the surface tension, the easier it is for a sub- 

 stance to come into intimate contact with bacteria. We make use 

 of this knowledge in the preparation of disinfectants when we 

 suspend the active principle in alcohol (a tincture) instead of in 

 water (an aqueous solution). Alcohol may be said to be wetter 

 than water, in this regard. At 18° C. pure water has a surface 

 tension of 73.0 dynes, glycerol has 65.2 dynes, and ethvl alcohol 

 has only 21.7 dynes of surface tension. Any substance that lowers 

 surface tension tends to gather at the surface, including the surface 

 of bacterial cells. If the surface tension is lowered, some bacteria 

 are killed. In fact, pneumococci are dissolved, and this principle 

 is employed as a test for separating pneumococci from other cocci. 



Since bacteria are practically all surface, and surface reactions 

 play such an important part in their metabolism, any change in the 

 nature of the bacterial surface will influence growth of the organ- 

 ism. A man weighing 70 kilograms has a total surface area of 

 about 1.5 meters. The ratio of surface area to weight is about 1:50 

 in this instance. Bacteria have a surface area to weight ratio of 

 9000:1. It is not surprising, therefore, that small cells can ac- 

 complish such a great deal of work. Escherichia coll has been 

 calculated to have a surface area of 0.000,004,5 square millimeters, 

 and a mass of 0.000,000,000,5 milligrams. 



ROCKING AND SHAKING 



Gentle rocking and slow aeration will stimulate bacterial growth 

 in liquid cultures, possibly due to the fresh food supply made 

 available to organisms as waste products are removed from their 

 immediate vicinity. The rocking may also cause more rapid 

 separation of organisms after binary fission has taken place. 

 Vigorous agitation, however, is definitely detrimental to bacteria. 

 A constant trembling motion exerted by the running of heavy 

 machinery has been reported to kill some bacteria after four 

 days exposure to the vibrations. The destructive action caused 

 by violent agitation is apparently due to denaturation of cell 



