72 BACTERIOLOGY. 



Although, as a g-eneral rule, bacteria do not grow below 

 10°, yet a number of notable exceptions are known. Sev- 

 eral phosphorescing bacteria grow and emit light even at 

 0°, the temperature of melting ice. Nearly a score of 

 organisms have been studied which can multiply at, or near, 

 the freezing-point. As is well known, meat kept in cold 

 storage may appear perfectly normal, but when taken out 

 of storage it will decompose much more rapidly than 

 ordinary fresh meat. This difference is due to the fact 

 that the bacteria in the preserved meat slowly multiply, in 

 spite of the prevailing low temperature. 



The temperature of 40° is usually considered as the 

 the maximum temperature for the growth of bacteria. The 

 majority of bacteria cannot grow above this limit. More- 

 over, a temperature of 45 to 50° will rapidly weaken, and 

 eventually kill all the ordinary forms of bacteria. There 

 are, however, certain bacteria which thrive exceptionally 

 well at such abnormal temperatures. Indeed, some forty 

 or more bacterial species have been isolated which grow at 

 58 to 60°. A few species have been observed to multiply 

 at 68° and even at 74°. 



The bacteria which can thus grow at unusually high 

 temperatures are non-pathogenic, and, are designated as 

 thermophilous. When it is remembered that the ordinary 

 vegetating bacteria are promptly killed, in a few minutes, 

 by a temperature of 60 — ^70° the existence of this group of 

 organisms will appear all the more remarkable. The 

 albumin which is present in the white of an egg, or in blood 

 serum will coagulate at a temperature of 65—70°. The 

 destruction of bacteria by this temperature is due, there- 

 fore, to the coagulation of the albuminous constituents of 

 the protoplasm. Evidently, the thermophilous bacteria 

 possess a markedly different chemical composition. 



This interesting group of organisms is chiefly repre- 

 sented by bacilli. Only a few micrococci have been found 

 to grow under these conditions. The thermophilous bac- 



