The Classification of the Brucella Group: A Systematic Study

Abstract

1. A systematic study has been made of 165 Brucella strains from different parts of the world, and a less intensive study, comprising only the H₂S formation, dye sensitivity, and agglutinin-absorption tests, of a special group of 156 strains from the north-east, east, and south-east of France.2. For purposes of differentiation little weight can be attached to the use of morphological appearances, abundance of growth in culture, pigment formation, the appearance of crystals in the medium, the production of alkali in peptone water, or the utilisation of glucose.3. The presence of 5–10 per cent, of carbon dioxide in the atmosphere is essential for the growth of most freshly isolated bovine abortus strains. This concentration frequently favours the growth of melitensis, though not of porcine abortus, strains. A concentration of 40 per cent. CO₂ almost invariably inhibits to some extent the growth of melitensis and porcine abortus strains, and frequently inhibits the growth of bovine abortus strains. It may be concluded that, while the inhibition of growth of a given strain by 40 per cent. CO₂ cannot be considered of differential value, the fact that growth in 40 per cent. CO₂ is as good as, or better than, that in air is definitely in favour of its being of bovine abortus type, while the occurrence of growth in 10–40 per cent. CO₂ but not in air almost certainly indicates that the strain belongs to this type.4. Most bovine abortus, para-abortus, and American porcine abortus strains produce H₂S freely for 3 or 4 days, while melitensis, paramelitensis, and Danish porcine abortus strains fail to produce any, or more than a small quantity on the first day only. Though the failure of a given strain to produce H₂S cannot be regarded as indicating that it is of melitensis, paramelitensis, or Danish porcine abortus type, the definite production of H₂S can be considered as very strong evidence that it is of bovine abortus or American porcine abortus type.5. The dye sensitivity method, introduced by Huddleson, is of very real value in differentiation, and is the only certain method available for distinguishing between the bovine and porcine abortus types. By its means it is possible to divide Brucella strains into three main groups—bovine abortus and para-abortus, porcine abortus, and melitensis and paramelitensis. Not all strains, however, within a given group behave alike, and a number of sub-groups can be established on the basis of special sensitivity to one or more dyes. The method not infrequently yields results that demand considerable experience in their interpretation, and it cannot always be relied upon for the correct typing of individual strains.6. The thermo-agglutination test is one of the simplest methods of detecting antigenic roughness. Any strain that fails to react negatively to this test is unsuitable for the production of smooth antiserum or for use in routine diagnostic agglutination work.7. The agglutinin-absorption method, performed by a strictly quantitative technique, enables a differentiation to be made between bovine and porcine abortus strains on the one hand, and melitensis strains on the other, provided that smooth strains are employed both for the preparation of antisera and for absorption. The use of direct agglutination by monospecific abortus and melitensis sera affords a rapid and accurate means of typing individual strains, and may prove of value in the examination of strains which, while partly rough and unsuitable for absorption experiments, still retain sufficient smooth antigen to be agglutinated by one or other serum. It is also of great service in the detection of mixed strains. Both methods yield identical results.8. In the examination of the main group of strains the serological method proved more valuable than any other method in the correct allocation of individual strains, and the results agreed closely with those afforded by the dye method. In the examination, however, of the special group of strains from the north-east, east, and south-east of France, there was frequently a marked disagreement between the results of the H₂S and dye tests on the one hand, and the serological method on the other, the chief divergence being that a number of strains reacting by the former methods as melitensis behaved serologically like abortus. It appears as if, in this particular area of France, strains occur having the metabolic properties of melitensis and the antigenic constitution of abortus.9. A study of the main group of strains by the various methods enumerated enabled them to be classified into the following groups: (a) bovine abortus with five sub-groups, (b) porcine abortus with two sub-groups, (c) melitensis with two sub-groups, (d) para-abortus with two sub-groups, and (e) paramelitensis.10. A study of the special group of strains from the eastern districts of France revealed the presence of ten sub-groups. Since it is rather doubtful to what main group many of the strains belong, they have been classified according to the host from which they were isolated.