CHAPTER IV. 

 ACTION OF EXTERNAL INFLUENCES ON BACTERIA. 



1. INFLUENCE OF TEMPERATURE. 



THE examination of the influence of temperature must always be 

 preceded by an observation of the temperature at which the organism 

 ordinarily thrives, for bacteria are in this like human beings, some are 

 adapted to live in what, to other bacteria, would be a cold climate ; 

 on the other hand, other organisms are adapted to a tropical life, 

 represented in this case by hot springs, etc., in which bacterial life 

 is not wanting. Hence, for every species there will be a minimum, 

 optimum and maximum temperature at which growth takes place. It 

 is usual to have three standards of temperature, viz. room-temperature, 

 varying from 18 C. to 22 C. ; lower-temperature incubator, 30 C. to 

 33 C. ; arid higher-temperature incubator, 35 C. to 38 C. Pathogenic 

 bacteria thrive best at the last named temperature, which is approxi- 

 mately that of the human body. On the other hand, soil-bacteria, 

 water-bacteria and air-bacteria require a lower temperature. They 

 grow well at room-temperature, but somewhat better and quicker 

 at the medium range, viz. 30 C. to 33 C. Of course a large number 

 of pathogenic bacteria are present in the soil, water, and air, but 

 they are either in the resting condition or else not growing at or 

 near their optimum range of temperature. Whilst the average thermal 

 death-point of bacteria is about 55 C. there are some, termed thermo- 

 pliilic, which can stand much higher temperatures. With regard to 

 low temperature, the first recorded experiments are those of Forster 

 in 1887. He found that in commercial milk there were 1000 bacteria 

 per 1 c.c., in drain water 2000 per c.c., in garden soil 140,000 per grain, 

 and in street mud an innumerable quantity per grain, which were not 

 only alive, but reproductive at C. Further, Miguel found that a 



