CHAPTER XXI 



USE OF PHAGES IN EPIDEMIOLOGICAL 

 STUDIES* 



Species of pathogenic organisms are usually constant in their 

 ecology and produce characteristic reactions in their hosts, and 

 it is often possible to recognize a bacterial species by these re- 

 actions. It must be borne in mind, however, that the definition 

 of species in bacteria is not based on such firm foundations as is 

 that of higher organisms. As far as the pathogenic bacteria are 

 concerned, much of the existing classification has resulted from 

 expediency — the search for methods of distinguishing with con- 

 fidence the pathogenic from the nonpathogenic flora with which 

 they may be fortuitously associated, and of obtaining them in 

 pure culture for diagnostic study. As the efficacy of these 

 methods depends on the more or less constant characters of the 

 bacteria concerned, they are understandably useful to the bac- 

 terial taxonomist. But the final validity of bacterial species, in 

 the sense in which the term is employed in the classification of 

 the higher organisms, still remains in doubt. 



Communicable bacterial diseases present epidemiologists with 

 the task of determining the avenues by which particular epi- 

 demic strains of bacteria have gained access to patients. More- 

 over, in a given outbreak of an infectious disease such as ty- 

 phoid fever, it is important to know whether the organisms iso- 

 lated from patients all have a common origin, that is, whether 

 the epidemic springs from one or multiple sources. Clearly, in 

 an infection caused by a single bacterial species, what is needed 

 is a method of classification at the intraspecific level. In other 

 words, the species should be divisible into "types." In an effi- 



* Chapter contributed by E. S. Anderson. 

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