MICROBIAL FOOD POISONING 489 



tant than is recognized at present. The subject is still in a very 

 doubtful state. 



To establish by laboratory investigation the poisonous character of 

 foods requires toxicological training and experienced judgment, a dis- 

 cussion of which would lead beyond the scope of the present chapter. 

 For a general review of this field of work and references to further 

 information the articles cited at the end of this chapter should be 

 consulted. 



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 suflB.cient 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. 



