816 BRUCELLA 



of CO2 on its growth, its cytotrophism (see Buddingh and Womack 1941), its 

 failure to develop anaerobically, its production of HjS, its very weak fermentative 

 ability, its antigenic affinity to Br. melitensis and Br. abortus, and its high patho- 

 genicity for man in the laboratory qualify it perhaps even better for inclusion 

 in the Brucella group. Since, however, it has not been studied with the same 

 thoroughness as the other members, we shall exclude it from the following descrip- 

 tion of the group characteristics, and describe it separately at the end of the 

 chapter. 



Morphology and Staining. — The bacilli are short and slender ; the axis is 

 straight ; the ends are rounded ; the sides may be parallel or convex outwards. 

 In length they vary from about 0-6 to 1-5 //, and in breadth from 0-5 to 0-7 ja.. 

 The short forms may appear as oval cocci, or, if they are about to divide, as diplo- 

 cocci. As a rule they are arranged singly, in pairs end-to-end, or in small 

 groups ; sometimes short chains of 4-6 members may be seen, especially in liquid 

 media. Owing to the frequent coccoid appearance, their bacillary nature may be in 

 doubt, but it may be noted that in size they are smaller than any of the Gram- 

 negative cocci. Moreover, when arranged in pairs, their long diameter is in the 

 same axis as that in which they are lying, in distinction to the Gram-negative 

 diplococci, whose long axis is generally at right angles to that in which they are 

 lying. 



Br. melitensis is generally considered to be more coccal in form than Br. abortus, 

 and for this there is some justification. The difierence in size and shape, how- 

 ever, is so slight as to render it impossible to distinguish with certainty between 

 individual strains. Duncan (1928) has pointed out that these organisms, when 

 grown on agar or glucose agar, show no marked morphological differences ; but if 

 they are cultivated on a relatively rich medium, such as Fildes' peptic digest blood 

 agar, the Br. abortus strains frequently develop long bacillary forms, reaching 

 2-0 or 3-0// in length, whereas Br. melitensis strains usually retain their coccal shape 

 and rarely exceed \-0 jjl in length. 



The organisms stain fairly well with the ordinary dyes. Bipolar staining is 

 not uncommon, and occasionally irregularity in the depth of coloiir is seen. In 

 old cultures irregular forms may be noted. They are Gram-negative, non-acid-fast, 

 non-motile and non-sporing. The presence of capsules in freshly isolated smooth 

 strains has been described (Huddleson 1940, Mickle 1940). 



Cultural Reactions. — Apart from their different CO2 requirements, the members 

 of this group resemble each other closely in their cultural characteristics. None of 

 them is difl&cult to grow ; none grows profusely. On agar the colonies are small, 

 translucent and undifferentiated. In broth there is a moderate turbidity with a 

 slight powdery or viscous sediment, which disintegrates completely on shaking ; 

 after about 2 weeks in the incubator or at room temperature the deposit becomes 

 extremely viscous, and can be disintegrated only with difficulty. According to 

 Thomsen (1933), if the organisms are grown in flasks of broth instead of in tubes 

 the suis and C02-sensitive abortus types give rise in 1 to 3 weeks to a mealy or 

 scaly surface pellicle and a heavy deposit that is difficult to disintegrate by shaking. 

 Aerobic abortus strains form no pellicle, but produce a uniform turbidity and a slight 

 deposit that is easily disintegrated. Strains of Br. melitensis give rise to a fairly 

 dense turbidity, a moderately heavy deposit, and a granular, usually incomplete, 

 surface growth. Growth in gelatin is poor, and is unaccompanied by liquefaction. 

 Perhaps the most striking peculiarity is the yellowish colour that develops on potato 



