Staphylococcus aureus is a major cause of human bacterial infection. Invasive infections caused by methicillin-resistant S. aureus (MRSA) are associated with a high mortality rate. Although virulence determinants have been identified in different MRSA strains, the mechanism behind MRSA virulence still remains to be fully elucidated. Rather than confining analyses to culture-based analysis, finding relevant and novel ways of assessing virulence in MRSA strains in in vivo studies of virulence are essential as they allow host-pathogen interaction responses. Larvae of the wax moth Galleria mellonella provide a convenient and affordable model system for studying microbial pathogenesis. The larvae are not only readily obtainable from breeders of insects as food for exotic pets, but they can also be conveniently housed in the laboratory, are easily infected via accurate injection and can survive incubation at 37 ° C. I have utilized the wax moth larvae and a non-destructive scoring system (the Caterpillar Health Index [CHI]) to investigate the in vivo virulence of laboratory and clinical isolates of S. aureus. These results have subsequently been compared with virulence gene profiles for the clinical and reference isolates in this study.