Summary: The gene, encoding a major outer-membrane protein in , was PCR-amplified from strains representing all species and known biovars by using primers selected according to the 16M published sequence. Amplification of was achieved from DNA of all species with the exception of , the only species where expression of was not detected by reactivity with an mAb specific for an epitope located in Omp-31. Southern blot hybridization of plasmid probes, bearing inserts (4.4-17 kb) containing 16M and adjacent DNA of different sizes, with III-digested total DNA showed that a large fragment, comprising the entire gene and flanking DNA, was actually absent in strains. The size of this DNA fragment has been determined to be about 10 kb. Southern blot hybridization with the different plasmid probes identified species-specific markers for and . At the biovar level, a specific marker for bv. 1 was also identified. Additionally, PCR-RFLP studies of revealed specific markers for and bv. 2. Using a combination of PCR-RFLP patterns and Southern blot hybridization profiles species were differentiated with the sole exception of which was not differentiated from bv. 1, 3, 4 and 5. Results presented in this paper demonstrate the potential of for differentiating the brucellae and show that lacks a large DNA fragment of about 10 kb containing and flanking DNA. In such a large deletion, other genes in addition to are probably involved. Sequencing of this DNA fragment will help to identify the missing genes in which could possibly be involved in the differences of pathogenicity and host preference seen in species.


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