Mutations in gltB and gltC reduce oxidative stress tolerance and biofilm formation in Listeria monocytogenes 4b G

Abstract

The foodborne pathogen Listeria monocytogenes has the capability to persist on surfaces in food-processing environments, and the organism is resistant to environmental stresses. In this study, a Tn917 insertion mutant of L. monocytogenes 4b G showing reduced biofilm formation and sensitivity to oxidative stress was identified and characterized. The transposon insertion site within the gltB gene was identified by inverse PCR. The gltC gene is located upstream and is reported to be transcribed divergently from gltB. Mutants with deletions in gltB and gltC were constructed and both showed reduced biofilm formation and increased sensitivity to H2O2 compared to the wild-type. In the wild-type strain, gltB and gltC expressions were induced approximately 8-fold and 14-fold by quantitative RT-PCR, respectively, with exposure to H2O2, providing further evidence that their gene products may be involved in the response to oxidative stress. In addition, after the induction by H2O2 and compared with the wild-type, the gltB expression in ΔgltC and the gltC expression in ΔgltB were down-regulated about 4-fold (p<0.05) and 3-fold (p<0.05) respectively. These data demonstrate a possible mutual regulation between gltB and gltC expressions under oxidative stress conditions, partly explaining the similar oxidative stress responses of ΔgltB and ΔgltC. Furthermore, ΔgltB and ΔgltC exhibited decreased adherence to a glass surface compared to the wild-type, while the cell motility of wild-type and mutant strains was similar. It is hypothesized that some cell surface characteristics unrelated with cell motility may be introduced into the mutants by the inactivation of gltB or gltC, which might lead to the reduction in biofilm formation. We conclude that both gltB and gltC are involved in the biofilm formation as well as the oxidative stress tolerance in L. monocytogenes 4b G, by pathways that remain yet unclear.