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.

Roles of Indole as an Interspecies and Interkingdom Signaling Molecule

“…The expression of indole-producing tryptophanase has been reported to increase biofilm formation and antibiotic resistance in the marine pathogen Edwardsiella tarda [40], whereas indole was found to decrease biofilm formation by, and the virulence of, the marine pathogen Vibrio anguillarum [41]. Several Gram-positive bacteria also produce indole, which inhibits antibiotic-resistant spore formation by Paenibacillus alvei [8] and Stigmatella aurantiaca [7].…”

Local and Universal Action: The Paradoxes of Indole Signalling in Bacteria

“…Resistance increases the minimum inhibitory concentration (MIC) of the antibiotic, while tolerance increases the minimum duration for killing (MDK) [23]. Studies have been focussed mainly on Gram-negative species, where indole is often found to increase antibiotic resistance and tolerance via the activation of efflux pumps and induction of oxidative stress responses [35,39,46,[52][53][54][55][56][57].…”

Indole decreases the virulence of pathogenic vibrios belonging to the Harveyi clade

“…As a variety of bacteria and some plants produce large quantities of indole, indole signaling is widespread in prokaryotic and eukaryotic communities (Lee et al, 2015). Indole plays an important role in bacterial pathogenesis, and diverse aspects of bacterial physiology controlled by indole have been revealed, including motility (Kim & Park, 2013), biofilm formation (Lee et al, 2007), plasmid stability (Chant & Summers, 2007), antibiotic tolerance (Vega et al, 2011) and lipopolysaccharide production (Han et al, 2011). We previously reported that motility and biofilm formation are the most notably virulence‐related phenotypes controlled by indole in V. campbellii quorum sensing model strain ATCC BAA‐1116 (Yang et al, 2017).…”