818 BRUCELLA 



The colonial appearance of Brucella depends on the smoothness or roughness of the 

 strain. The difference between smooth and rough colonies is not great, and is best brought 

 out by examination under a binocular plate microscope using obhquely transmitted Ught. 

 Colonies of antigenicaUy smooth strains of Br. abortus on potato agar are small, bluish 

 and translucent with regular margins and a smooth gUstening surface. The mdividual 

 cells are uniformly short rods arranged singly. Colonies of antigenicaUy rough strains 

 are of much the same size as the smooth colonies, but are less convex, more opaque, and 

 have a dull granular appearance. The individual cells are usually somewhat larger than 

 those of the smooth type, and occasional long slender rods may be observed. Inter- 

 mediate colonial and morphological forms have also been described (Mingle and Mantei 

 1941). The colonial differences are intensified on glycerol glucose agar. On this medium 

 S and R colonies, if examined by obUque transmitted light against a dark background, 

 produce the appearance of an irregular mosaic of light and dark, curved and angular areas 

 (Henry 1933). 



Growth Requirements. — Growth is generally improved by the addition of natural 

 animal protein to the medium. The most satisfactory media, particularly for the 

 growth of Br. abortus and Br. melitensis, are liver extract agar — first described by 

 Holth (1911), subsequently by Stafseth (1920), and frequently referred to as Huddle- 

 son's medium — 2 per cent, glycerol agar (Zeller and Stockmayer 1933), Fleming's 

 (1919) chocolate agar (Henry et al. 1932), 5 per cent, serum agar and Bacto-tryptose 

 agar. Zobell and Meyer (1932) have described a synthetic medium in which the 

 metabolism of Brucella strains may be studied. Br. melitensis and Br. suis are 

 said not to grow in an amino-acid glucose inorganic salt medium unless nicotinamide, 

 thiamin and pantothenic acid are added ; Br. abortus requires biotin as well (Kerby 

 1939, Koser et al. 1941, Koser and Wright 1942). 



The range of temperature consistent with growth is 20°-40° C. ; the optimum 

 being about 37° C. At 20° C. growth is very slow. The effect of H-ion concentra- 

 tion is rather difficult to dissociate from that of COg. Many strains of Br. abortus 

 require for their optimum development a concentration of 5-10 per cent. CO 2 in 

 the atmosphere. This has the effect of turning an alkaline medium acid. For the 

 growth of these organisms an initial H-ion concentration of pH 6-6 is desirable. 

 The other members of the group usually grow as well on an alkaline as on a slightly 

 acid medium ; but since, as will be pointed out directly, even melitensis strains are 

 often benefited by a small amount of extra CO2, it is advisable for practical purposes 

 to adjust media to pH 6-6-6-8. 



CO2 Requirements. — One of the most interesting features of the Brucella group 

 is their peculiar respiratory behaviour. Ever since its original isolation by Bang 

 (1897), Br. abortus has presented certain difficulties in cultivation. No growth 

 occurs on a solid medium under aerobic conditions. If, however, the tube is 

 suitably sealed (Preisz 1903) (see Fig. 173), or if it is attached by rubber tubing 

 to another tube inoculated with an organism such as B. subtilis (Nowak 1908), 

 growth occurs after a delay of a few days. These observations were generally 

 interpreted as showing that Br. abortus was microaerophilic, and could not grow 

 till the partial pressure of oxygen over the culture had been lowered to a suitable 

 extent. A similar interpretation was also placed on the fact that, when inoculated 

 into a serum agar gelatin shake medium, it grew in the form of a band situated 

 about \ cm. below the surface (Fig. 174). 



Credit is due to Huddleson (1921) for showing that this organism requires for its 

 development a partial pressure of CO 2 higher than that normally present in the 



