Classification of Brucella spp. isolated from marine mammals by DNA polymorphism at the omp2 locus

A number of recent reports have described the isolation and characterizatio n of Brucella strains from a wide variety of marine mammals such as seals, porpoises, dolphins and a minke whale. These strains were identified as bru cellae by conventional typing tests. However, their overall characteristics were not assimilable to those of any of the six currently recognized Bruce lla species and it was suggested that they comprise a new nomen species to be called 'Brucella maris'. In the present study we analysed DNA polymorphi sm at the omp2 locus of 33 marine mammal Brucella strains isolated from sea ls, dolphins, porpoises and an otter. The omp2 locus contains two gene copi es (named omp2a and omp2b) coding for porin proteins and has been found par ticularly useful for molecular typing and identification of Brucella at the species, biovar, or strain level. PCR-restriction fragment length polymorp hism (RFLP) and DNA sequencing showed that strains isolated from dolphins a nd porpoises carry two omp2b gene copies instead of one omp2a and one omp2b gene copy or two similar omp2a gene copies reported in the currently recog nized species. This observation was also recently made for a minke whale Br ucella isolate. The otter and all seal isolates except one were shown to ca rry one omp2a and one omp2b gene copy as encountered in isolates from terre strial mammals. By PCR-RFLP of the omp2b gene, a specific marker was detect ed grouping the marine mammal Brucella isolates. Although marine mammal Bru cella isolates may represent a separate group from terrestrial mammal isola tes based on omp2b sequence constructed phylogenetic trees, the divergence found between their omp2b and also between their omp2a nucleotide sequences indicates that they form a more heterogeneous group than isolates from ter restrial mammals. Therefore, grouping the marine mammal Brucella isolates i nto one species 'Brucella maris' seems inappropriate unless the currently r ecognized Brucella species are grouped. With respect to the current classif ication of brucellae according to the preferential host, brucellae isolated from such diverse marine mammal species as seals and dolphins could actual ly comprise more than one species, and at least two new species, B. pinnipe diae and B. cetaceae, could be compatible with the classical criteria of ho st preferentialism and DNA polymorphism at their omp2 locus. (C) 2001 Editi ons scientifiques et medicales Elsevier SAS.