Garvicin A, a novel class IId bacteriocin from Lactococcus garvieae that inhibits septum formation in L. garvieae strains

Lactococcus garvieae 21881, isolated in a human clinical case, produces a novel class IId bacteriocin, garvicin A (GarA), which is specifically active against other L. garvieae strains, including fish- and bovine-pathogenic isolates. Purification from active supernatants, sequence analyses, and plasmid-curing experiments identified pGL5, one of the five plasmids found in L. garvieae [M. Aguado-Urda et al., PLoS One 7(6):e40119, 2012], as the coding plasmid for the structural gene of GarA (lgnA), its putative immunity protein (lgnI), and the ABC transporter and its accessory protein (lgnC and lgnD). Interestingly, pGL5-cured strains were still resistant to GarA. Other putative bacteriocins encoded by the remaining plasmids were not detected during purification, pointing to GarA as the main inhibitor secreted by L. garvieae 21881. Mode-of-action studies revealed a potent bactericidal activity of GarA. Moreover, transmission microscopy showed that GarA seems to act by inhibiting septum formation in L. garvieae cells. This potent and species-specific inhibition by GarA holds promise for applications in the prevention or treatment of infections caused by pathogenic strains of L. garvieae in both veterinary and clinical settings.

Characterization of plasmids in a human clinical strain of Lactococcus garvieae

The present work describes the molecular characterization of five circular plasmids found in the human clinical strain Lactococcus garvieae 21881. The plasmids were designated pGL1-pGL5, with molecular sizes of 4,536 bp, 4,572 bp, 12,948 bp, 14,006 bp and 68,798 bp, respectively. Based on detailed sequence analysis, some of these plasmids appear to be mosaics composed of DNA obtained by modular exchange between different species of lactic acid bacteria. Based on sequence data and the derived presence of certain genes and proteins, the plasmid pGL2 appears to replicate via a rolling-circle mechanism, while the other four plasmids appear to belong to the group of lactococcal theta-type replicons. The plasmids pGL1, pGL2 and pGL5 encode putative proteins related with bacteriocin synthesis and bacteriocin secretion and immunity. The plasmid pGL5 harbors genes (txn, orf5 and orf25) encoding proteins that could be considered putative virulence factors. The gene txn encodes a protein with an enzymatic domain corresponding to the family actin-ADP-ribosyltransferases toxins, which are known to play a key role in pathogenesis of a variety of bacterial pathogens. The genes orf5 and orf25 encode two putative surface proteins containing the cell wall-sorting motif LPXTG, with mucin-binding and collagen-binding protein domains, respectively. These proteins could be involved in the adherence of L. garvieae to mucus from the intestine, facilitating further interaction with intestinal epithelial cells and to collagenous tissues such as the collagen-rich heart valves. To our knowledge, this is the first report on the characterization of plasmids in a human clinical strain of this pathogen.