METABOLIC CHARACTERIZATION OF THE GENUS BRUCELLA III

The one hundred year journey of the genus Brucella (Meyer and Shaw 1920)

The genus Brucella, described by Meyer and Shaw in 1920, comprises bacterial pathogens of veterinary and public health relevance. For 36 years, the genus came to include three species that caused brucellosis in livestock and humans. In the second half of the 20th century, bacteriologists discovered five new species and several 'atypical' strains in domestic animals and wildlife. In 1990, the Brucella species were recognized as part of the Class Alphaproteobacteria, clustering with pathogens and endosymbionts of animals and plants such as Bartonella, Agrobacterium and Ochrobactrum; all bacteria that live in close association with eukaryotic cells. Comparisons with Alphaproteobacteria contributed to identify virulence factors and to establish evolutionary relationships. Brucella members have two circular chromosomes, are devoid of plasmids, and display close genetic relatedness. A proposal, asserting that all brucellae belong to a single species with several subspecies debated for over 70 years, was ultimately rejected in 2006 by the subcommittee of taxonomy, based on scientific, practical, and biosafety considerations. Following this, the nomenclature of having multiples Brucella species prevailed and defined according to their molecular characteristics, host preference, and virulence. The 100-year history of the genus corresponds to the chronicle of scientific efforts and the struggle for understanding brucellosis.

Differential phenotyping of Brucella species using a newly developed semi-automated metabolic system

BACKGROUND

A commercial biotyping system (Taxa Profile™, Merlin Diagnostika) testing the metabolization of various substrates by bacteria was used to determine if a set of phenotypic features will allow the identification of members of the genus Brucella and their differentiation into species and biovars.

RESULTS

A total of 191 different amines, amides, amino acids, other organic acids and heterocyclic and aromatic substrates (Taxa Profile™ A), 191 different mono-, di-, tri- and polysaccharides and sugar derivates (Taxa Profile™ C) and 95 amino peptidase- and protease-reactions, 76 glycosidase-, phosphatase- and other esterase-reactions, and 17 classic reactions (Taxa Profile™ E) were tested with the 23 reference strains representing the currently known species and biovars of Brucella and a collection of 60 field isolates. Based on specific and stable reactions a 96-well "Brucella identification and typing" plate (Micronaut™) was designed and re-tested in 113 Brucella isolates and a couple of closely related bacteria.Brucella species and biovars revealed characteristic metabolic profiles and each strain showed an individual pattern. Due to their typical metabolic profiles a differentiation of Brucella isolates to the species level could be achieved. The separation of B. canis from B. suis bv 3, however, failed. At the biovar level, B. abortus bv 4, 5, 7 and B. suis bv 1-5 could be discriminated with a specificity of 100%. B. melitensis isolates clustered in a very homogenous group and could not be resolved according to their assigned biovars.

CONCLUSIONS

The comprehensive testing of metabolic activity allows cluster analysis within the genus Brucella. The biotyping system developed for the identification of Brucella and differentiation of its species and biovars may replace or at least complement time-consuming tube testing especially in case of atypical strains. An easy to handle identification software facilitates the applicability of the Micronaut™ system for microbiology laboratories.

The differentiation of Brucella species by substrate specific tetrazolium reduction

In the development of a tetrazolium reduction assay to replace substrate stimulated oxygen uptake for the identification of Brucella species, nine tetrazolium salts were evaluated. Only the more readily reduced compounds (MTT and INT) detected increased metabolic activity with the more fastidious, slow growing strains and with B. suis strains on L-arginine and DL-ornithine. The assay was optimised with MTT. MTT reduction profiles offered with the medium on which the cells were grown. Cells grown on TSA gave profiles more similar to the published respirometric results than those grown on SDA. The optimal substrate concentration was 0.84 g l-1 and prolonged (> 3 h) exposure to substrate was necessary before adding MTT. MTT concentration was not critical and the OD was proportional to the MTT concentration between 0.03 and > 0.5 g l-1. MTT reduction was linear for 60 min after its addition. The reaction was, therefore, stopped after 60 min by adding formaldehyde solution. The optimised assay was evaluated with 71 strains of Brucella, representing all the species and biovars of the genus. Each strain was assigned to its previously identified species and sub-groups were defined.

Isolation, identification, and characterization in Israel of Brucella melitensis biovar 1 atypical strains susceptible to dyes and penicillin, indicating the evolution of a new variant

During characterization by conventional biotyping tests of numerous Brucella melitensis isolates obtained in Israel in the last 2 years, we identified several strains of the biovar 1 serotype that are atypically susceptible to dyes and penicillin. Their coisolation from two brothers in a family that rears sheep and from the milk of one of their sheep and the prevalence of such strains in two distinct geographical zones in Israel provide epidemiological support for the notion that a new variant has been identified.

An outbreak of Brucella abortus biovar 2 in Canadian cattle

An outbreak of brucellosis caused by Brucella abortus biovar 2 was identified in cattle in Alberta in December 1986. This was the only clinical infection discovered since the national cattle herd was declared brucellosisfree in 1985. It was the first report of B. abortus biovar 2 in Canadian cattle. The outbreak, involving three herds containing purebred Hereford cattle, was spread by the private treaty sale of untested cattle, and was identified following investigation of an abortion. The source of infection for the outbreak was not established, but several possibilities were identified including infected herds present in the area during the mid-1970's, latent infection originating in a Saskatchewan herd during the early 1960's, American cattle imported during the early 1970's, and brucellosis-infected bison in Wood Buffalo National Park. The containment and elimination of this nidus of infection appears to have been successful, and the national cattle herd at the time of writing is free of the disease.

Atypical isolates of Brucella abortus from Canada and the United States characterized as dye sensitive with M antigen dominant

A total of 41 Brucella isolates, examined by standard biotyping procedures, were found to be similar to Brucella abortus biovar 2 in dye sensitivity but had a dominant M antigen. Oxidative metabolic tests performed on 39 of the isolates confirmed them as B. abortus. Additional biochemical and bacteriophage susceptibility studies were performed on 35 of the isolates. The isolates had identical reactions in the various tests, except for one isolate which was resistant to lysis by all phage strains used. Two isolates were injected into guinea pigs and shown to be virulent. The isolates described in this study appear similar to atypical Brucella isolates previously reported in the United Kingdom and the United States and may form the basis of a new biovar, B. abortus biovar 10.

Identification of the causative agents and the epidemiology of porcine brucellosis

Brucella organisms that had been isolated from swine or from human beings exposed to infected swine, and that had been previously identified as members of the species Br. melitensis, were examined for their oxidative metabolism, for their growth patterns on media containing basic fuchsin and thionin, and for their hydrogen sulfide production. These organisms displayed the oxidative metabolic pattern that characterizes and identifies the members of the species Br. suis. They also are identical to Br. suis, type 1, in their tolerance to increased concentrations of thionin in the growth media. They are similar to Br. suis, type 2, in their inability to produce hydrogen sulfide, and share with other members of the species Br. suis the characteristic preference for a porcine host. These findings are evidence that these organisms are not Br. melitensis but are a biotype of Br. suis. Their correct identification is important to understanding the epidemiology of brucellosis.

Metabolic characterization of Brucella strains that show conflicting identity by biochemical and serological methods

Each of 87 strains of brucellae examined for its utilization of amino acid and carbohydrate substrates displayed a metabolic pattern that characterized it as to its species identity, irrespective of its serological and biochemical characters. Strains that displayed the metabolic pattern of Br. abortus were lysed by Brucella bacteriophage type abortus strain 3. Strains that displayed the metabolic pattern of Br. melitensis were not lysed by this phage.On this basis of identification, it is shown that there occur strains of Br. abortus that do not produce hydrogen sulfide, do not require carbon dioxide and are thionin-resistant, thereby duplicating the features generally regarded as characteristic of Br. melitensis. They can be identified by their metabolism and phage susceptibility. The distribution of Br. abortus and Br. melitensis antigens, as measured by agglutination with monospecific antisera, is also not always related to other species-identifying characteristics. Therefore, neither the serological method nor the biochemical method can be considered a reliable guide to the identification of Brucella species.