120 CONDITIONS OF GROWTH. 



parasitic species have a more restricted range, which approaches the 

 normal temperature of the animals in which they habitually de- 

 velop. At 40 C. (104 F.) growth, as a rule, ceases, but there are 

 some notable exceptions to this rule. 



Miquel some years ago found a bacillus in the water of the Seine 

 which grew at a temperature of 69 to 70 C. ; Van Tieghem reports 

 having observed species in thermal waters capable of growth at a 

 still higher temperature (74 C.) ; and Globig has more recently ob- 

 tained from garden earth several species which multiplied at 65 C. 

 Some of the species found by the last-named observer were even 

 found to require a temperature of about 60 for their development ; 

 and yet this temperature is quickly fatal to a large number of the 

 best known species. 



Low temperatures, while arresting the growth of bacteria, do not 

 destroy their vitality. This has been demonstrated by numerous ex- 

 periments, in which they have been exposed for hours in a refrigerat- 

 ing mixture at 18 C. Frisch has even subjected them to a tempe- 

 rature of 87 C. by the evaporation of liquid carbon dioxide, and 

 found that they still grew when placed in favorable conditions. 



Parasitism. The strict parasites grow only in the bodies of liv- 

 ing animals, or in artificial media kept at a suitable temperature. 

 As examples we ma}' mention the bacillus of tuberculosis, the bacil- 

 lus of leprosy, the micrococcus of gonorrhoea, the spirillum of re- 

 lapsing fever. There is also a large class of facultative para- 

 sites which, when introduced into the body of a susceptible animal, 

 multiply in it, and may continue to live as parasites so long as they 

 are transferred from one animal to another, but which are also able 

 to live as saprophytes independently of a living host. To this class 

 belong the pus cocci, the bacillus of typhoid fever, the spirillum of 

 cholera, and many others. 



It seems extremely probable that the strict parasites were at one 

 time capable of living a saprophytic existence, and that their restric- 

 tion to a parasitic mode of life has been effected in course of time in 

 accordance with the laws of natural selection. This view is sup- 

 ported by the fact that the tubercle bacillus, which has been regarded 

 as a strict parasite, which can only be cultivated artificially under 

 very special conditions, has been shown to be capable of modification in 

 this regard to such an extent that when cultivated for a time in a favor- 

 able medium bouillon with five per cent of glycerin it will even grow 

 in ordinary bouillon made from the flesh of a calf or a fowl (Roux). 



Reaction of Medium. Some bacteria grow readily in a medium 

 having an acid reaction, while the slightest trace of acidity prevents 

 the development of others. As a rule, the pathogenic species require 

 a neutral or slightly alkaline culture medium. 



