EXTERNAL INFLUENCES 7 



Moore and Stenhouse Williams killed tubercle bacilli by 

 three weeks' exposure to an atmosphere enriched with 

 oxygen, and found the plague bacillus and stapliylococcus 

 growths were adversely affected (oxyphobia). 



As a rule, bacterial growth ceases at temperatures below 

 12 C. and above 42 C. The range of temperature within 

 which bacteria will grow is practically constant for each 

 species, but there is a more narrow margin (the ' optimum ' 

 temperature) in which each does best. The optimum 

 temperature differs more or less according to the species. 

 Normal inhabitants of the human body and organisms 

 pathogenic for man thrive best at blood-heat (37 C.), 

 the colon bacillus, which grows luxuriantly at 42 C., being 

 an exception. Bacteria obtained from the lower animals 

 generally have the normal temperature of the host as an 

 optimum. The body- temperature of a fowl is 42 C., 

 and avian tubercle bacilli thrive at 43 C., a temperature 

 at which human tubercle bacilli refuse to grow. 



Some bacteria isolated from dung and from heated hay 

 grow best at temperatures between 60 and 70 C. (ther- 

 mophilic organisms), while others can grow at C. 

 Growth at unnatural temperature may result in loss of some 

 characters. 



When obtaining their nourishment from some living 

 body or ' host,' organisms are known as ' parasites.' The 

 adjective ' obligate ' is prefixed if they can live only on 

 this ' host.' If the bacteria grow on dead organic matter, 

 they are called ' saprophytes.' These are also divided into 

 ' obligate ' and ' facultative ' saprophytes. The term 

 * obligate parasite ' requires to be used with some reserva- 

 tion. It merely indicates that hitherto attempts to cul- 

 tivate an organism on culture media have failed, and a 

 successful attempt thereat automatically transfers it to the 

 list of facultative parasites. 



Resistance to External Influences Cold. The germi- 

 cidal effect of low temperatures is small. Growth usually 

 ceases below 10 C., but even after exposure to- 252 C. 

 (the temperature of liquid hydrogen) for ten hours, or to 

 170 C. for several weeks, bacteria are not killed, and 

 when placed in favourable conditions show that no serious 

 impairment has taken place. When water freezes naturally 

 there is a high death-rate among the bacteria therein, 

 and repeated freezing and thawing has been found more 



