Type B Chloramphenicol Acetyltransferases Are Responsible for Chloramphenicol Resistance in Riemerella anatipestifer , China

causes serositis and septicaemia in domestic ducks, geese, and turkeys. Traditionally, the antibiotics were used to treat this disease. Currently, our understanding of susceptibility to chloramphenicol and the underlying resistance mechanism is limited. In this study, the gene was identified in 69/192 (36%) isolated from different regions in China, including CH-2 that has been sequenced in previous study. Sequence analysis suggested that there are two copies of gene in this strain. Only both two copies of the mutant strain showed a significant decrease in resistance to chloramphenicol, exhibiting 4 μg/ml in the minimum inhibitory concentration for this antibiotic, but not for the single gene deletion strains. Functional analysis of the gene via expression in BL21 (DE3) cells and site-directed mutagenesis indicated that His79 is the main catalytic residue of CAT in . These results suggested that chloramphenicol resistance of CH-2 is mediated by the genes. Finally, homology analysis of types A and B CATs indicate that comprises type B3 CATs.