158 PATHOGENIC ORGANISMS 



The organisms that pass the morphological and biochemical 

 tests must be tested for virulence to guinea pigs. Two pigs 

 are used, one for a subcutaneous or intra-peritonial injection of 

 the twenty-four hour broth culture alone (1 c.cm.) and the other 

 for a mixture of the culture with 1 c.cm.. of a diphtheritic anti- 

 toxin of high titre. The unprotected pig usually dies within 

 thirty-six hours, and almost invariably within forty-eight hours, 

 if the culture is one of typical B. diphtherise. The protected 

 animal should show no definite symptoms and remain alive. 



Diphtheroid Bacilli. On many occasions bacilli have been 

 found in milk having the characteristic granular staining prop- 

 erties of some forms of B. diphtherise but sharply differentiated 

 from this organism by the absence of virulence. Bergey 15 

 investigated a number of these organisms which were apparently 

 identical with B. diphtheria, and divided them into three 

 groups according to their biochemical properties. Two groups 

 showed fermentative activity markedly different to the diph- 

 theritic group and that of the third was identical but non- 

 pathogenic. Savage 16 investigated a number of the diph- 

 theroid organisms found in milk sediments. These were 

 apparently identical and closely resembled B. diphtherise in 

 staining properties and microscopical appearance except for an 

 absence of blue granules in preparations stained with Neisser's 

 stain. The bacilli were Gram positive, non-motile, and devel- 

 oped on nutrient agar as small, discrete, translucent colonies. 

 On serum they were slightly coloured and such organisms did 

 not give the typical microscopical appearance found with the 

 growths on agar. Litmus milk was unaffected and, except for 

 a trace of acid in lactose, neither gas nor acid was produced 

 in the usual test media. They were non-pathogenic to mice. 

 Klein 17 found a bacillus in milk which he called B. diphther- 

 oides. This organism differed morphologically from B. diph- 

 therise, Hoffmann's bacillus, and the xerosis group. No 

 growth was observed on gelatine at 21 C. or on agar at temper- 

 atures less than 25 C. On agar at 37 C. the growth was slow 

 and no colonies appeared until the third day when they devel- 



