NUTRITION OF BACTERIA 39 



translated, means heat-loving. They probably first appeared on our 

 planet at a time when its surface was considerably warmer than it is 

 now. They rarely multiply at temperatures below 40 to 50 C., and 

 are found in abundance only in the tropics, in hot springs, and in soil 

 which has been exposed to the direct rays of the sun for some time. 

 They are quite common in the intestinal contents of animals. 



Thermophile bacteria probably multiply considerably in sponta- 

 neously fermenting manure, and assist in bringing about by their 

 metabolism the marked elevation of temperature. Some thermophile 

 bacteria, in the absence of free oxygen, may be able to multiply at 

 temperatures as low as 34 C. Such bacteria may possibly multiply 

 in the intestines of man and domestic animals. 



Thermotolerant Microorganisms. There are some bacteria which 

 multiply at quite high temperatures, but which have their optimum 

 at 35 to 37 C. These are called thermotolerant (heat- tolerating). Bac- 

 teria in their vegetative form, with the exception of the thermophile 

 and thermotolerant, are, as a rule, not very resistant to heat. They 

 are generally killed at 55 to 60 C. if exposed in the moist state to 

 this temperature for about ten minutes. The exact temperature 

 and time has to be ascertained experimentally for each species, and 

 again for the spores of such species as sporulate. This temperature 

 when applied for a few minutes (generally five or ten) is called the 

 thermal death point of the bacterium or of its spore. While heat easily 

 damages the vegetative form of the bacteria, cold, as a rule, has very 

 little effect upon bacteria and their spores. They may be frozen at a 

 very low temperature without any effect, and will be found alive after 

 thawing. However, repeated freezing and thawing in rapid succes- 

 sion kills some pathogenic bacteria. 



Nutrition of Bacteria. All bacteria depend for their existence and 

 multiplication upon certain food materials and moisture. Many of 

 them may be dried out completely (^dthooit-Jbeujg_Mlle(^, but they 

 (cannot multiply) under such conditions, and only do so after they 

 have again had access to moisture. Other bacteria when dried die 

 very soon, as, for instance, the glanders and the plague bacilli. Spores 

 of certain pathogenic bacteria, like those of anthrax, tetanus, black- 

 leg, etc., can exist for years in a dried condition, and when conditions 

 become favorable, germinate and display all of their general typical 

 and special pathogenic properties. Some bacteria can exist for a 

 long time in their vegetative form in a desiccated state, as, for instance, 

 the tubercle bacillus. Desiccated bacteria and their spores must 

 be considered as being in a condition where there is no metabolism 

 and where life is latent, somewhat like life in higher plants in winter 

 or in hibernating animals. 



When bacteria grow in artificial^ cultures they exhaust the soil 

 within a certain time, which, together with the accumulation of their 

 metabolic products, produce conditions destructive to most of them. 

 They die sooner if they are raised in the incubator than if raised at 



