Mutation of tryptophanase gene tnaA in Edwardsiella tarda reduces lipopolysaccharide production, antibiotic resistance and virulence

Abstract

A fish pathogen Edwardsiella tarda LTB-4 produced various indole alkaloids, including indole, 2-(1H-indol-3-yl)ethanol, 4-di(1H-indol-3-yl)methylphenol, tri(1H-indol-3-yl)methane and 2-[2,2-bis(1H-indol-3-yl)]ethylphenylamine. Indole was the most abundant among these indole alkaloids. E. tarda LTB-4 produced indoles during its whole growth phase and maintained a high level (around 35.5 µM) during the stationary phase. The relevant tryptophanase (TnaA) gene tnaA was cloned from LTB-4 and conditionally expressed in Escherichia coli; the recombinant TnaA catalysed L-tryptophan to indole. A tnaA in-frame deletion mutant ΔtnaA was constructed through double cross-over allelic exchange by means of the suicide vector pRE118; deletion of tnaA caused some phenotypic changes including decreased swarming and twitching motility, lipopolysaccharide production and multiple antibiotic resistances. Also, subtherapeutic doses of chloromycetin, carbenicillin and tetracyline could cause the decrease of bacterial growth, but greatly induce the production of indole by E. tarda. Most importantly, attenuated virulence of the ΔtnaA mutant to zebra fish by increasing the LD50 for about 55-fold indicated that TnaA involved in the virulence of E. tarda.