VII48-4 MANUAL OF METHODS FOR PURE CULTURE STUDY 



General 



Koch's postulates. Koch's postulates constitute the accepted 

 standard for demonstrating the relation of a microorganism to disease. 

 They are: (1) the organism must always be present where the disease 

 occurs; (2) the organism must be obtained in pure culture from 

 pathological tissue; (3) this pure culture must cause the disease w^hen 

 injected into a favorable region or tissue of a normal susceptible ani- 

 mal; and (4) the organism must be recovered from the latter. 



Rivers (1937) pointed out that strict adherence to Koch's postu- 

 lates may hinder the study of pathogenicity, particularly with regard 

 to viruses and to the synergistic effect of two organisms. He stated 

 that error may result even when Koch's postulates apparently have 

 been fulfilled and that fulfillment is not always essential or desirable. 



Koch's postulates are inapplicable to certain microorganisms for 

 the reasons mentioned in the introduction and because it may be 

 difficult to establish the presence of the pathogen in the diseased tis- 

 sue even though it may be present in large numbers. (See, e.g.. Chap- 

 man, 1945). In these instances, strong circumstantial evidence 

 may be presented as "proof" of pathogenicity. However, extreme 

 caution must be observed in drawing any such conclusions for the 

 following reasons: about 15% of animals purchased through usual 

 channels die from causes unconnected with the injections (Chapman, 

 unpublished studies); infections are common in laboratory animals 

 (see, e.g., Farris et al, 1945); the pathologic effects may be caused 

 by some ingredient of the culture medium (Rigdon, 1938) and patho- 

 genicity for one animal may not be comparable with pathogenicity 

 for another. 



Bacterial pathogenicity. Every organism that lives the normal 

 length of life for the species passes through a life cycle. There is con- 

 siderable difference of opinion among bacteriologists concerning 

 physiological and pathogenic relationships to different stages of 

 growth but regardless of what one considers a "life cycle" and whether 

 "microbic dissociation" and filterable forms are included in it, there 

 is evidence that in some pathogenic bacteria at least, differences in 

 the growth phase are associated with differences in pathogenicity. 

 (See, e.g., Dubos, 1945). 



The pathogenicity of a microorganism can be maintained by growth 

 in a suitable medium, animal passage, maintenance at low oxygen 

 tension, low temperature, frequent subculturing, and drying in 

 animal organs. Increase in pathogenic properties by animal passage 

 is limited by exposure and selection (Zinsser and Wilson, 1932). 



Different pathogenic properties. "Virulence" is used loosely to 

 signify pathogenicity but, since different types of pathological effects 

 are caused by different agents it is desirable to use more precise terms 

 which designate, for example, the specific toxic power, ability to 

 multiply rapidly in the body, etc. 



Bacterial toxins. It should be determined whether the pathogenic 

 principle is associated with intra- or extra-cellular products or is 

 intimately connected with the bacterial cell wall. Injection of 

 filtrates differentiates the first two from the last. Many factors, such 



