32 BIOLOGY AND TECHNIQUE 



carried on more rapidly than the substitution of waste products by ab- 

 sorption, and a gradual weakening of vital energy,' or even a gradual 

 death of the bacterium, may take place. When certain bacteria form 

 spores, they become very much more resistant against both high and 

 low temperatures, probably because a true resting stage has been 

 reached, during which metabolism has been reduced to a minimum, 

 there being practically no nutritive material taken in and corresponding- 

 ly little destruction taking place within the body of the microorganism. 



The optimum temperature for various bacteria depends upon the 

 habitual environment, in which the particular species is accustomed to 

 exist. Thus, for the large majority of bacteria pathogenic for human 

 beings, the optimum temperature is at or about 37.5 C. There are 

 a large number of bacteria common in water, however, which grow 

 hardly at all at the body temperature, but thrive most luxuriantly at 

 temperatures of about 20 C. F. Forster, l moreover, described certain 

 phosphorescent bacteria, isolated from sea-water, which grow readily at 

 C., or a little above. On the other hand, Miquel 2 has described non- 

 motile bacilli, which he isolated from the water of the Seine, which grew 

 rapidly at temperatures ranging about 70 C., and the so-called "muce- 

 dinees thermophiles, " described by Tsiklinski, 3 develop most readily at 

 temperatures very little above this. It is thus plain that the tempera- 

 tures favored by various bacteria depend to a large extent upon an 

 adaption of these bacteria through many generations to specific en- 

 vironmental conditions. A good illustration of this is furnished by the 

 bacillus of avian tuberculosis, a microorganism differing essentially 

 from the bacillus of human tuberculosis in that its optimum growth 

 temperature lies at 4l-42 C., a temperature which exceeds the op- 

 timum temperature for the human type by as much as the normal tem- 

 perature of birds exceeds that of man. The same principle is illustrated 

 by the facts that the bacteria which have a very low optimum tem- 

 perature are usually those isolated from water, and the so-called ther- 

 mophile or high-temperature bacteria are obtained from hot springs and 

 from the upper layers of the soil, where, according to Globig, 4 occasion- 

 ally temperatures ranging from about 55 C. occur. 



As stated before, one and the same species may develop within a 



1 F. Forster, Cent. f. Bakt., ii, 1887. 



2 Miquel, Bull, de la Stat. Munic. de Paris, 1879. 



3 Tsiklinski, Ann. Past,, 1889. 



4 Globig, Zeit. f . Hyg., iii. 



