GENERAL FOOD RELATIONSHIPS— MET ABOLISH 67 



Those bacteria whose food consists of dead material, either 

 organic or inorganic, are spoken of as saprophytes, while 

 those whose natural habitat, without reference to their 

 food, is in or on other living organisms are called parasites. 

 These two terms are frequently used as though they tvere 

 opposites. This cannot be true, since they do not refer to 

 the same thing. Saprophyte, as just stated, has reference 

 to food, while parasite refers to j^l^ce of abode. Usually the 

 place of abode determines the character of the food, but 

 there is no necessary connection. Many parasites are 

 saproph}' tes, hence the terms are not mutually exclusive. In 

 fact, strictly speaking, all parasitic bacteria are saprophytes, 

 since the food must enter the cell in solution and living matter 

 is not soluble. Hence the actual food is dead material. 



The host is the organism in or on which the parasite lives. 

 Parasites may be of several kinds. Those which neither do 

 injury nor are of benefit to the host are called non^pathogenic 

 parasites or commensals; many of the bacteria in the intestines 

 of man and other animals are of this class. Those which 

 do injury to the host are called pathogenic or disease-pro- 

 ducing, as the organisms causing the transmissible diseases 

 of animals and plants.^ Finally, we have those parasites 

 which are of benefit to and receive benefit from the host. 

 These are called symbionts or symbiotic parasites and the 

 mutual relationship symbiosis. Certain of the intestinal 

 bacteria in man, and especially in herbivorous animals, are 

 undoubted symbionts, as are also the ''root- tubercle bacteria" 

 already mentioned. 



The term strict parasite refers to those parasites which 

 imder natural conditions do not reproduce apart from the 

 host. Facultative parasites may reproduce under natural 

 conditions either in or on the host or apart from it. There 

 is no known parasitic bacterium which has not been grown 

 artificially apart from its host. 



Bacteria bring about a great many changes on their food 



^ The term pathogenic is also applied to certain non-parasitic saprophytic 

 bacteria whose products cause disease conditions, as one of the organisms 

 causing a type of food poisoning in man (Clostridium hotulinum), which also 

 probably causes "forage poisoning" in domestic animals. 



