96 INFLUENCE OF TEMPERATURE ON BACTERIA 



stance, and hence decrease proportionally the speed of the catalyzed 

 reaction. 



The protoplasm composing the bacterial cell consists of carbo- 

 hydrates, lipins, proteins, and ash having a definite structural 

 arrangement within the living cell. The cellular protoplasm is, 

 therefore, a colloid existing during life in the soluble condition, but 

 when heated there occurs an irreversible reaction with the forma- 

 tion of a gel. This heat-coagulation of the protein is explained by 

 Robertson as essentially a phenomenon of dehydration, the first 

 stage of which consists of internal neutralization through the loss 

 of the elements of water from end-groups ( NH 2 and COOH) , 

 thus: 



H 2 N RCOH N R COOH = HN R COH N R CO +H 2 O 



1 1 



In the second stage, or true coagulation, there is a polymerization 

 of the amino-acids with the formation of the irreversible gel. 



2H 2 NR COH NR COOH = H 2 N R COH N R COH N 

 R COH NR COOH + H 2 O 



Relation to Heat. From the above theoretical consideration we 

 should expect to find, and do actually find, an upper temperature 

 limit at which all organisms cease to function. This upper limit 

 varies considerably with the species of bacteria and the condition 

 under which it is being held. B. phosphorescens will not grow above 

 37 C., B. tuberculosis above 42 C., B. thermophilis above 72 C., 

 and Setchell has found bacteria living in the water of hot springs 

 at a temperature of 89 C. 



This great variation in temperature requirement of bacteria has 

 led to their division into four classes : 



1. Thermophilic, or heat-loving bacteria, are those that develop 

 at relatively high temperatures, usually above 45 to 50 C. These 

 organisms occur in the water of hot springs, in decaying piles of 

 compost or manure, in fermenting ensilage, in the intestinal contents 

 of man and animals. To this class belong the non-motile bacilli 

 isolated by Miquel from the Seine, which grew rapidly at tem- 

 peratures around 70 C., as does also the so-called " Mudedinus 

 thermophiles," described by Tsiklinsky, which develop readily at 

 temperatures slightly above this. Most of the thermophiles are 

 spore-bearing bacilli of little or no practical importance. 



2. Psychrophilic bacteria are those which grow best at relatively 

 low temperatures, usually below 10 C. They are most common in 

 cold waters such as those of springs, wells, the depths of lakes or 

 oceans and the soils of arctic regions. Forster has described certain 

 phosphorescent bacteria, which he isolated from sea water which 

 grow readily at 10 C. Many bacteria of the soil must belong to this 

 class, as Conn and Brown have repeatedly shown that soil bacteria 



