| Summary: | <p>Abstract</p> <p>Background</p> <p><it>Brucella </it>spp. are highly similar, having identical 16S RNA. However, they have important phenotypic differences such as differential susceptibility to antibiotics binding the ribosome. Neither the differential susceptibility nor its basis has been rigorously studied. Differences found among other conserved ribosomal loci could further define the relationships among the classical <it>Brucella </it>spp.</p> <p>Results</p> <p>Minimum inhibitory concentration (MIC) values of <it>Brucella </it>reference strains and three marine isolates to antibiotics binding the ribosome ranged from 0.032 to >256 μg/ml for the macrolides erythromycin, clarithromycin, and azithromycin and 2 to >256 μg/ml for the lincosamide, clindamycin. Though sequence polymorphisms were identified among ribosome associated loci 23S <it>rrn</it>, <it>rpl</it>V, <it>tuf</it>-1 and <it>tuf</it>-2 but not <it>rplD</it>, they did not correlate with antibiotic resistance phenotypes. When spontaneous erythromycin resistant (ery<sup>R</sup>) mutants were examined, mutation of the peptidyl transferase center (A2058G Ec) correlated with increased resistance to both erythromycin and clindamycin. <it>Brucella </it>efflux was examined as an alternative antibiotic resistance mechanism by use of the inhibitor L-phenylalanine-L-arginine β-naphthylamide (PAβN). Erythromycin MIC values of reference and all ery<sup>R </sup>strains, except the <it>B. suis </it>ery<sup>R </sup>mutants, were lowered variably by PAβN. A phylogenetic tree based on concatenated ribosomal associated loci supported separate evolutionary paths for <it>B. abortus, B. melitensis</it>, and <it>B. suis/B. canis</it>, clustering marine <it>Brucella </it>and <it>B. neotomae </it>with <it>B. melitensis</it>. Though <it>Brucella ovis </it>was clustered with <it>B. abortus</it>, the bootstrap value was low.</p> <p>Conclusion</p> <p>Polymorphisms among ribosomal loci from the reference <it>Brucella </it>do not correlate with their highly differential susceptibility to erythromycin. Efflux plays an important role in <it>Brucella </it>sensitivity to erythromycin. Polymorphisms identified among ribosome associated loci construct a robust phylogenetic tree supporting classical <it>Brucella </it>spp. designations.</p>
|
|---|
