Effects of Physical Forces on Bacteria 161 



given culture may outlast the others. It becomes a sort of contest 

 like trying to hold your breath, and individuals vary in their 

 abilities. The heat resistance of bacteria depends upon (1) tem- 

 perature, (2) length of exposure, (3) degree of moisture present, 



(4) pH of the menstrum in which the bacteria are being heated, 



(5) the type of medium in which the organisms are suspended, 

 and (6) the innate nature of the organisms. It is easier to heat-kill 

 bacteria in water than it is to accomplish the same end in milk or 

 in cream, which tend to protect the organisms from the lethal 

 effects of the heat. 



MOISTURE 



A cartoon character was once pictured as drinking copious 

 amounts of water, and when pressed for an explanation he admitted 

 that he had just eaten some dirt and he was swallowing the water 

 to drown the germs. Unfortunately, bacteria are not killed that 

 easily, in fact they thrive in the presence of a high percentage of 

 water. Bacteria are more aquatic than terrestrial, but in a water- 

 logged soil where oxygen is not sufficiently soluble, some aerobic 

 bacteria have to give way in favor of anaerobic organisms which 

 thrive when free oxygen is lacking. Some oxygen can diffuse into 

 liquids exposed to the air, and motile bacteria can congregate at 

 the level where oxygen concentrations are optimum. 



While most bacteria thrive in high concentrations of moisture, 

 it is surprising how little water is necessary to keep organisms 

 alive. Like a goldfish removed from his bowl and exposed to the 

 drying effects and the oxygen differences in the air, bacteria dried 

 in the air will eventually die. If the exposure is not prolonged, 

 both the goldfish and the microbe might recover when returned 

 to their normal habitat. 



Delicate pathogens are more susceptible to desiccation than are 

 most saprophytes. This is an important point for clinical bac- 

 teriologists to bear in mind. Swabs delivered to the laboratory 

 for bacteriological culturing should not be allowed to lie around 

 before being placed in or on appropriate media. Negative results 

 may occur where positive findings are indicated. The lethal effect 

 of desiccation depends upon ( 1 ) the medium in which the bacteria 



