86 SAPROPHYTISM, PARASITISM, AND PATHOGENISM 



less pronounced than that of the saprophytic bacteria. 1 Indeed, 

 intense chemical activity would be incompatible with their continued 

 parasitic existence, for the damage to their host would be insupport- 

 able. The parasitic and pathogenic bacteria do not, for example, 

 produce widespread liquefaction of the tissues, even when large 

 numbers of them are actually growing in the body of the host. The 

 growth of invasive organisms in the animal body is characterized by 

 subtle changes in the composition of the tissues of the host and the 

 development of these reciprocal reactions between host and parasite, 

 which collectively are included in the newly developed science of 

 Immunology. 



It would appear, therefore, that in their evolution toward parasit- 

 ism, those bacteria which could thrive without producing deep-seated 

 and rapid degradation of proteins, that is to say, whose metabolism 

 approached more closely the intracellular metabolism of their host, 

 would be the more adaptable to a parasitic existence, and this is in 

 accord with what is known of the chemistry of these organisms. 

 Their metabolism approaches rather closely that of their host. 



H. THE CYCLE OF PARASITISM. 



The cycle of parasitism for bacteria whose life cycle is such that 

 but a limited excursion outside their host is possible for them and 

 this appears to be the case for the majority of organisms parasitic on 

 man consists of three separate and well-defined stages, as Theobald 

 Smith 2 has so clearly pointed out. They must first reach an appro- 

 priate host; secondly multiply at least temporarily thereon, and 

 thirdly escape to other suitable hosts. Each phase of this parasitic 

 existence must be exactly fulfilled, otherwise the cycle is broken and 

 that particular strain dies out. It is not surprising, therefore, that 

 the bacteria habitually parasitic for man are found variously upon 

 the surface of the body in the upper respiratory tract, the gastro- 

 intestinal tract, or upon the mucous surfaces which are in direct 

 communication with the exterior. Escape from the body of the host 

 to other hosts is readily accomplished from these positions. 3 



Under special conditions, parasitic bacteria may actually invade 

 the body of the host and become, therefore, temporarily pathogenic. 



1 Theobald Smith, Am. Med., October 22, 1904, viii; Kendall, Boston Med. and 

 Surg. Jour., 1913, clxix, 749. 



2 Theobald Smith, loc. cit. 



3 Theobald Smith, loc. cit. 



