Submitted to: Comparative Immunology Microbiology and Infectious Diseases
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/7/2008
Publication Date: 3/1/2010
Citation: Jensen, A.E., Halling, S.M. 2010. Effect of Polymyxin B and Environmental Conditions on Isolation of Brucella Species and the Vaccine Strain RB51. Comparative Immunology Microbiology and Infectious Diseases. 33(2):121-131. Interpretive Summary: Isolation of Brucella from field specimens is the laboratory "gold-standard" for diagnosing brucellosis. The recently introduced vaccine, RB51, did not grow on routine selective medium containing the antibiotic polymyxin B without controlling the culture environment. For accurate diagnosis of brucellosis when RB51 and hard-to-grow strains of Brucella are present, both growth conditions and selective medium must be carefully selected.
Technical Abstract: Brucella species are known to be resistant to polymyxin B (PB) but their relative sensitivity to PB and the polymyxin derivative, colistin (COL) has not been rigorously or systematically studied. Comparative sensitivity of the Brucella reference strains, vaccine strain RB51, and three Brucella isolates from marine mammals to these two cationic peptides were determined by Etest and reported as minimum inhibitory concentrations (MICs). Vast differences were found in sensitivity of Brucella to PB. Brucella were less sensitive to COL, but the same pattern of relative sensitivity was demonstrated. Brucella suis biovar 4 was extraordinarily resistant to PB (MIC greater than or equal to 1024 ug/ml) compared to the all the other reference strains (6 to 384 ug/ml). Both B. melitensis and B. suis were much less sensitive to PB than the other species. The most sensitive strain tested was the rough vaccine strain RB51 (0.25 ug/ml). As anticipated, rough Brucella strains were more sensitive to PB than the smooth. Brucella strains were generally more resistant to PB when grown in the presence of CO(2) than ambient air. Though the pH decreases the effectiveness of PB, some strains became more sensitive as the medium became more acid. Our observations extend basic knowledge of the differential resistance of Brucella to PB. Differences in relative sensitivity of Brucella species to PB and COL and culture environment for CO(2) independent strains need to be considered when culturing specimens for Brucella on selective medium containing PO or COL.