412 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



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 accord- 

 ing to the source of the poisonous substance. 



The substance of plants or animals may be naturally poisonous to 

 man as a result of the physiological activity of their own living substance. 

 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 mush- 

 rooms 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 patho- 

 genic 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 par- 

 ticularly concerned. 



