Overexpression of mltA in Edwardsiella tarda reduces resistance to antibiotics and enhances lethality in zebra fish

Aims: The aim of this study was to investigate the role of membrane-bound lytic murein transglycosylase A (MltA) in a bacterial fish pathogen Edwardsiella tarda. Methods and Results: An mltA in-frame deletion mutant (ΔmltA) and an mltA overexpression strain (mltA+) of Edw. tarda were constructed through double-crossover allelic exchange and by transformation of a low-copy plasmid carrying the intact mltA into the ΔmltA mutant, respectively. Either inactivation or overexpression of MltA in Edw. tarda resulted in elevated sensitivity to β-lactam antibiotics and lower viability in oligotrophic or high osmotic environment than wild-type strain. Autolysis induced by EDTA was reduced in ΔmltA strain, while mltA+ strain was virtually flimsy, indicating that MltA is responsible for the lysis effect. Moreover, mltA+ strain exhibited significant increases in lipopolysaccharide (LPS) biosynthesis and virulence to zebra fish compared with wild-type strain. Conclusions: The results indicated that MltA plays essential roles in β-lactam antibiotics and environmental stresses resistance, autolysis, LPS biosynthesis and pathogenicity of Edw. tarda. This is the first report that MltA has a virulence-related function in Edw. tarda. Significance and Impact of the Study: This study provided useful information for further studies on pathogenesis of Edw. tarda.