250 NUTRITION AND METABOLISM 



and the physiological effects are also quite different, though they both 

 cause poisoning. 



Toxins are not essential products of the metabolism of pathogens. 

 Strains of pathogenic bacteria can be bred which do not produce toxins 

 as chromogens can be bred without pigment, or lactic bacteria which 

 do not produce acid. The strains which lose their pathogenicity grow 

 better on artificial media but are less able to produce disease in the 

 animal. They may regain the power of producing toxin if passed 

 through the body of the animal. The real object of toxin production 

 by microorganisms is not known; the microorganisms derive no ap- 

 parent benefit. 



PHYSICAL PRODUCTS or METABOLISM 



PRODUCTION OF HEAT. It has long been known that fermentation 

 produces heat. The rise of temperature is usually not very great. In 

 lactic fermentation it amounts to about i, in alcoholic fermentation to 

 2 or 3, but in certain processes the heat liberated is considerable, as 

 in the fermentation of manure, of ensilage, of vinegar, and in others. 



The cause of heat formation is quite evident from the discussion on 

 page 199. Decomposition of organic matter means a liberation of 

 energy which is used for the continuation of life processes; the utiliza- 

 tion is, as a rule, incomplete, and a part of the energy appears in the 

 form of heat. The amount of heat produced can be measured directly 

 with the thermometer if great care is taken that no heat is lost by 

 radiation or by evaporation of water. 



Much heat is produced in the vinegar fermentation. In the quick- 

 vinegar process (page 644) the temperature rises sometimes as high as 

 10 to 15 above the temperature of the room and the vinegar manu- 

 facturer uses the heat produced by the bacteria to keep the generators 

 at the optimum temperature. If the process is not controlled carefully, 

 the vinegar bacteria are likely to produce sufficient heat to kill 

 themselves. 



The heat produced in the fermentation of manure, especially horse 

 manure, is used in the hot-beds to cultivate and force young plants. 

 In the manure pile, great heat production is not desirable because high 

 temperatures will volatilize the ammonia; the tight packing of manure 

 which keeps out the oxygen will prevent too strong bacterial action. 

 The highest temperature in silos which has been recorded is about 70, 



