Mechanisms of Biofilm formation by Listeria monocytogenes

Listeria monocytogenes is a food-borne bacterial pathogen which has the ability to attach to and form biofilm on food-processing surfaces. It is thought that L. monocytogenes biofilm formation in food processing environments is a major source of contamination because, once established, the biofilm appears to have greater resistance to disinfectants and other cleaning agents. In the first stage of this thesis, L. monocytogenes wild type 10403s and Tn917transposon mutant strains were surveyed for their ability to attach to polystyrene surfaces at different temperatures (37°C, 30°C and 18°C). The results indicated that two mutants, B265 and I366, showed a significant reduction in their attachment at 18°C and 30°C but not at 37°C. These data have revealed that there is a temperature-dependent involvement of some genes in surface attachment. Subsequently Arbitrary PCR was used to analyse sequences flanking the transposon insertions in both attachment-deficient transposon mutants. Two open reading frames for a putative NADH oxidase in B265 mutant and for a putative penicillin-binding protein in I366 mutant were identified. In the second stage of investigation, deletion mutants were subsequently made successful in these ORFs. Deletion mutation in each of these ORFs also resulted in a significant decrease in attachment of L. monocytogenes to polystyrene at 18°C and 30°C with no effect at 37°C. In the third stage, L. monocytogenes strains were screened for their ability to attach to polystyrene at different NaCl concentrations and pH values. Both the wild type and deletion mutants showed a decrease in attachment at high NaCl concentrations and a loss of attachment at high pHs. But neither of the mutants differed from wild type which showed a similar pattern of response. The molecular basis of biofilm mechanisms of L. monocytogenes is considered and this has been ascribed to the presence or absence of specific genes. Recently, there has been a growing interest in understanding the molecular basis of these processes. This study has demonstrated that NADH oxidase and penicillin-binding protein may play a role in L. monocytogenes attachment.