Analysis of AI-2/LuxS–Dependent Transcription in Campylobacter jejuni Strain 81-176

Abstract

Autoinducer-2 (AI-2) is a quorum-sensing signal molecule that controls a variety of cellular activities in response to cell density in both gram-negative and gram-positive bacteria. The production of AI-2 is dependent upon LuxS, the last enzyme in the AI-2 biosynthesis pathway. For this study, we constructed a luxS null mutation (ΔluxS) in Campylobacter jejuni strain 81-176, and showed that it abolished AI-2 production. The ΔluxS mutant had a longer doubling time in Mueller-Hinton (MH) broth and reduced swarming on MH soft agar at 37°C compared to the wild type (wt), whereas growth rate or swarming at 42°C was not affected. The ΔluxS mutant was also more sensitive to hydrogen peroxide (H₂O₂) and cumene hydroperoxide than the wt by disc inhibition assays at 42°C, though minimum inhibitory concentration comparisons were inconclusive. Differences in genome-wide gene expression between wt and ΔluxS mutant with and without H₂O₂ treatments were compared using DNA microarrays. The genes that showed differential expressions (wt/ΔluxS) include operons/pathways involved in AI-2 synthesis and S-adenosylmethionine (SAM) metabolism (metE, metF, and pfs), flagellar assembly/regulation, stress response (ahpC, tpx, and groES), ABC transporters/efflux systems, and two genes of unknown function located downstream of luxS (Cj1199 and Cj1200). The wt/ΔluxS expression ratios of ahpC (encoding alkyl hydroperoxide reductase) and tpx (encoding thiol peroxidase) were increased only with H₂O₂ treatment, consistent with our finding that the ΔluxS mutant exhibits higher sensitivity to oxidative stress than wt. Our microarray results agreed with the ΔluxS mutant phenotypes, and suggested that LuxS plays a role in central metabolism involving SAM metabolism, but it is uncertain whether AI-2 functions as a true quorum-sensing signal in C. jejuni.