Hydrogen peroxide-mediated killing of Caenorhabditis elegans by Enterococcus italicus and Lactococcus garvieae isolated from food

In this study, we used the nematode Caenorhabditis elegans as a model to assess the pathogenic potential of two species isolated from food, Enterococcus italicus and Lactococcus garvieae, for which few indications on pathogenicity are available. We identified the conditions under which E. italicus and L. garvieae are able to kill the nematode and suggest that the production of hydrogen peroxide (H2O2) by these two bacteria was involved in the death of C. elegans in our model system. The efficacy of E. italicus and L. garvieae to kill C. elegans differed, most likely related to each species' distinct ability to accumulate H2O2 (4.9 mM and 0.9-1.1 mM, respectively). Genome analysis of both species revealed that the genome of E. italicus contains a gene encoding a NADH oxidase which shows high amino acidic similarity with H2O2 -forming NOX-1 enzymes, while that of L. garvieae contains a gene codifying for a water-forming NADH-oxidase (NOX-2). Reverse transcriptase-PCR experiments carried out in presence of flavin adenine dinucleotide (50 mM) confirmed the presence of the two different genes and likely explains the different toxicity of E. italicus and L. garvieae against C. elegans in our study. The results obtained show for the first time the production of H2O2 in E. italicus and L. garvieae and indicate its toxic effect in the nematode C. elegans.