582 MICROBIOLOGY OP' FOODS 



Several different classes of food poisonings may be recognized 

 according to the source of the poisonous substance. 



The material of plants or animals may be naturally poisonous to 

 man as a result of the physiological activity of their own living sub- 

 stance. Poison of this kind may be constantly present throughout 

 the tissues, or it may be confined to certain parts, or it may occur only 

 at particular times or seasons. Some instances of poisoning with fish 

 and with mushrooms belong to this class, and possibly also some of the 

 instances of poisoning with potatoes of high solanin content. 



Plants and animals may feed upon substances not poisonous to 

 themselves, and these substances may remain a constituent part of 

 their bodies to poison man when consumed by him. Some poisonings 

 with freshly killed game are considered to be of this nature. 



Any food may contain foreign poison added to it by design or by 

 accident, such for example as the salts of the various poisonous metals. 

 The amount of tin or lead passing into solution in canned or tinned 

 foods may conceivably be sufficient to cause poisoning, but there is no 

 reliable evidence that it has ever occurred. 



Animals may be infected with pathogenic bacteria or with other 

 parasites capable of infecting man, and the use of food products from 

 such animals may cause disease. Tuberculosis, trichinosis and 

 tapeworm may be acquired in this way. . 



Any food may serve as the passive carrier of infectious agents, such 

 as B. typhosus, and some foods may even favor the multiplication of 

 pathogenic bacteria gaining access to them. 



A food may undergo chemical changes due to microorganisms in- 

 capable of infecting man, resulting in the production of poisonous sub- 

 stances in the food. Undoubtedly the great majority of instances of 

 food poisonings belong in this class. The bacteria causing these changes 

 have been designated as pathogenic saprophytes. 



The last three classes comprise the microbial food poisonings, and 

 these are the kinds of food poisoning with which we are at present more 

 particularly concerned. 



INFECTIONS OF FOOD-PRODUCING ANIMALS TRANSMISSIBLE TO MAN 



Animals dead of infectious diseases or slaughtered in the last stages 

 of disease are not ordinarily used for food, nor is the milk of such 



