OBJECTIVE: CA-MRSA strains often carry PVL genes. However, the pathophysiological role of PVL in CA-MRSA infections remains undefined and controversial. We previously reported the coexistence, in a large Canadian health-care region, of two phenotypically, genotypically and clinically similar USA400 (a major clonal group of CA-MRSA) sibling strains with and without PVL genes, suggesting that PVL genes are not necessarily the key determinants associated with the increasing dissemination of CA-MRSA. Here, we report our experimental findings, using various in vitro cell culture and animal models, of the growth properties of these sibling strains.

METHODS: Three phenotypically and genotypically characterized clinical isolates of each USA400 sibling strain were selected for testing. The human laryngeal carcinoma cell (HEp-2) and the human lung epithelial cell (A549) lines were used to study bacterial in-vitro invasion/survival and cytotoxicity, respectively. Our previously developed two invertebrate models (Caenorhabditis elegans and Drosophila melanogaster) and the murine intradermal infection model (BALB/c mouse), were employed as host models for studying bacterial in-vivo virulence. Experiments were performed in triplicate and repeated at least 3 times. Nematode and fly survival data were analyzed by Kaplan-Meier and log rank tests. Full thickness biopsies of the core cutaneous lesions from infected mice were performed and subjected to detailed histopathological examination.

RESULTS: Both PVL+ and PVL- USA400 strains showed a high nematocidal activity by virtue of killing a majority (>90%) of the nematodes over 9 days, with no significant difference in killing rate between these 2 groups (p>0.05). The USA400 sibling strains also exhibited a high fly-killing activity in our Drosophila model, with more than 85% of flies dying within 60 hours of infection, and with no significant difference in killing rate between PVL+ versus PVL- strains (p>0.05). In the murine model, both strains caused the formation of pustular cutaneous lesions with localized abscess formation without ulceration, in contrast to a colonization strain which only caused localized skin oedema and inflammation. No difference was observed in human laryngeal carcinoma cell invasion/survival ability and in human lung epithelial cell cytotoxicity between the PVL+ and PVL- strains (all p>0.05).

CONCLUSIONS: Our study demonstrates that PVL genes are not the major virulence factor associated with the invasion/cytotoxicity and pathogenesis of CA-MRSA strains in various in vitro cell culture and animal models, respectively, suggesting that other virulence factors may play a greater role in this regard.