822 BRUCELLA 



more than 2 per cent. The acid produced is more than neutralized by the alkali 

 formed as the result of protein breakdown, so that it is not detected by the usual 

 indicators. McAlpine and Slanetz (1928a) have recommended the glucose utiliza- 

 tion test as a means of differentiating between the abortus, melitensis, and suis types, 

 but most workers have found it unreliable, and it has now been generally discarded. 

 There is evidence that arabinose and xylose are fermented by members of the 

 Brucella group (Mallardo 1930, Coleman et al. 1930, McNutt and Purwin 1931, 

 Silberstein 1932), but the reaction is of no differential significance. 



The methyl red and Voges-Proskauer tests are negative. No indole is formed. 

 According to Zobell and Meyer (1932), all types reduce nitrates to nitrites. Nitrites 

 are also rapidly reduced, so that the Griess-Ilosvay test on nitrate broth cultures may 

 be negative. American suis strains are more active than the other types in reducing 

 nitrites. Ammonia is produced to a variable extent from peptone, urea, and 

 asparagin. Catalase is formed, being strongest with Br. stiis and weakest with 

 Br. abortus. According to Huddleson and Stahl (1943), the degree of catalase 

 activity is closely associated with virulence. The reducing action of these organ- 

 isms is comparatively weak (Habs 1930), and in broth cultures methylene blue 

 is often not decolorized. Tuttle and Huddleson (1934) found that liver extract 

 broth cultures showed a negative drift to a limiting potential after 8 days of 

 + 0-15 to + 0-09 volt. Br. suis appeared to be slightly more active than the 

 abortus or melitensis types, but the difference was insufficient to be of value in 

 species identification (see also Bau and Wang 1935-36). It may be noted that 

 some strains of Br. abortus reduce basic fuchsin. Huddleson (1931) thought that 

 this was a property of non-pathogenic strains, but our observations do not bear this 

 out. 



H2S Production. — One of the most important differential criteria, to which 

 attention was first drawn by Huddleson and Abell (1927), and Huddleson (1929), 

 is the production of HgS. This test should be carried out on liver agar using lead 

 acetate papers (see p. 369). Freshly isolated strains of Br. abortus and Br. suis 

 (American variety) give off HgS for at least the first 4 days, while strains of melitensis 

 produce either none at all, or only during the first 24 hours of incubation. The 

 Danish variety of Br. suis forms no HgS. In the laboratory, abortus and American 

 suis strains sometimes lose their ability to produce HjS ; the test should therefore 

 be made as soon after isolation as possible. The interpretation of this test can be 

 summed up by saying that, while failure of a given strain to produce HgS beyond 

 the first day does not exclude its being of abortus or American suis type, the con- 

 tinued production of HjS after the first day affords a strong presumption that it is 

 not of melitensis or Danish suis type. This test has now been widely used, and to 

 those who have realized its limitations it has given satisfaction (Favilli 1930, Kristen- 

 sen 1931, Taylor, Lisbonne, and Roman 1932, Zobell and Meyer 1932, Zeller and 

 Stockmayer 1933, Wilson 1933, Olin and Lindstrom 1934, Pagnini 1934, di Mino 

 1935). 



Antigenic Structure. — It would be idle to recapitulate here the confusion that 

 reigned for so long over the antigenic relationship of members of this group. Pre- 

 vious to 1918, when Evans demonstrated an antigenic affinity between Br. melitensis 

 and Br. abortus, most workers had concerned themselves with comparison of meli- 

 tensis and so-called paramelitensis strains, while for many years subsequently 

 progress was hindered by a failure to realize the difference in antigenic structure 

 between strains in the smooth and rough phases. 



