DETERMINATION OF THE PATHOGENICITY OF AEROBES vii^s-lS 



even though it may be universally present. One may be a harmless 

 invader, constantly associated with the disease but not contributing 

 to it. Even when both organisms are essential to produce the disease, 

 one may be a saprophyte in dead tissues and may contribute to the 

 infectious process only by providing conditions essential to the growth 

 of the pathogen. Unless the organisms are also associated in other 

 diseases, serologic tests may be of differential value. Bacterial 

 antagonism also plays a role in pathogenicity in some instances. 



It should be determined whether the bacterial product causes 

 disease by its direct action on the tissues or by sensitizing them to it. 

 If the latter, then other organisms that produce a similar antigen 

 or a similar non-specific antigen may also account for the pathology. 

 A pathogenic organism can be differentiated because it grows in 

 the animal and produces sensitization (Hanger, 1928). 



Cultural co)isideratio7is. The cultivation of pathogenic bacteria 

 may not always be favorable for producing the pathogenic factors. 

 Corynebacterium diphtheriae and streptococci, e.g., grow luxuriantly 

 under certain conditions without producing toxin. Certain bacteria 

 require oxygen for toxin production. The toxin also may be pro- 

 duced and then disappear in a culture or may be destroyed by unfa- 

 vorable manipulation. Finally, an early toxin and a late one may 

 have different properties. 



Most pathogenic aerobes are facultative anaerobes or facultative 

 microaerophiles. Parasitic species may prefer tissues or cavities with 

 low oxygen tension. 



The Use of Biochemical Methods in Lieu of Animal 



Inoculation Tests to Study Certain Pathogenic 



Properties 



Because they give results parallel with certain pathogenic effects, 

 tests have been proposed, based on biochemical properties, that ap- 

 pear to be satisfactory as substitutes for animal inoculation experi- 

 ments, e.g., when a large number of cultures are to be tested as in 

 clinical work, when animal inoculation experiments are inconclusive, 

 as in non-hemolytic streptococci, or when animal inoculation experi- 

 ments involve considerable danger (see, e.g., Dozois and Rauss, 1935; 

 and De Angelis, 1937). For example, power to clot plasma is now 

 recognized as an excellent in vitro method for differentiating patho- 

 genic from non-pathogenic staphylococci. For a summary of recent 

 biochemical methods for staphylococci, see Chapman (1946). 



Resistance of streptococci to the bactericidal power of fresh, 

 diluted, defibrinated guinea pig blood and to different chemicals 

 is an excellent indicator of pathogenicity (probably toxicity). A 

 complete up-to-date discussion of this work will be found in Chapman 

 (1947). 



Although the writer is enthusiastic about carefully applied bio- 

 chemical tests of such organisms as staphylococci and streptococci 

 as substitutes for animal inoculation tests, he is aware of their 

 shortcomings and is not in favor of universal acceptance at the present 

 time. So many technical considerations enter into the reliability 



