DETERMINATION OF THE PATHOGENICITY OF AEROBES viijs-ll 



If, as some still believe, the normal life cycle of a bacterial cell 

 consists mainly of an increase in size with age, with minor morphologi- 

 cal and physiological changes, the relationship of pathogenicity to 

 the life cycle has little significance. If, however, the life cycle is 

 represented by complicated ontogenetic changes (dissociations), 

 each phase or stage representing distinct characteristics and varying 

 in stability and in response to the environment, the relationship of 

 pathogenicity assumes considerable importance. This problem 

 concerns Leaflet VII only in so far as the variations affect the study of 

 pathogenicity. 



As regards pathogenicity, dissociation may occur as readily in this 

 property as do morphologic and physiological changes in the cell or 

 colony and may be associated with one or more of these latter changes. 

 The relationship of pathogenicity to R, S, M and G colony types and 

 to the morphology, size and age of the cell may have to be determined 

 for each culture. With some organisms, e.g.. Salmonella typhosa and 

 Corynebacteriiim diphtheriae, the smooth colony type is the most 

 pathogenic, whereas the mucoid phase of Diplococcus pneumoniae and 

 Klebsiella pneumoniae and the rough phase of other organisms, such as 

 Bacillus anthracis, are the most pathogenic. The relation of the G 

 phase to pathogenicity has not been clearly established but in staphy- 

 lococci, e.g., it appears to be non-pathogenic. 



Acquired variations. Acquired or new variation represents 

 changes in the average cell in response to environmental changes. 

 When they are favorable to survival of the organism they are adapta- 

 tions. 



Organisms not ordinarily pathogenic may acquire some degree of 

 pathogenicity in animal passage but they are not strictly pathogenic. 

 Consequently, the history of an organism in vivo should always be 

 reported. Organisms grown in immune serum may increase in 

 pathogenicity and resist agglutinating and other antibodies. 



Some organisms lose pathogenicity quickly, particularly when 

 grown on artificial culture media. They are usually most pathogenic 

 in the late logarithmic phase. To reduce this tendency to lose patho- 

 genicity the culture medium and incubation temperature should favor 

 optimum growth and should be similar to conditions existing in body 

 fluids and tissues (See Felton, 1932). Tissue culture or fresh blood, 

 either unheated or inactivated at 57°C for 1 hour to destroy transient 

 organisms, used alone or added to the culture medium are valuable 

 in maintaining pathogenicity. Transferring from one animal to 

 another should be done quickly. 



Antigenic variations. In addition to those changes in antigenic 

 specificity associated with different phases, there is some evidence 

 that bacteria may adsorb antigen from the environment with result- 

 ing change in antigenicity (See, e.g., Burky, 1934 and Rosenow, 

 1945). False serologic reactions have resulted from foreign antigen, 

 such as agar. Two different organisms may have a common anti- 

 gen from being grown on the same medium. Thus, a common anti- 

 gen may not necessarily indicate a natural relationship (See, e.g., 

 Dubos, 1945). Extraneous or unnatural antigens or their antibodies 



