The expressions of hla, hlg and sak were higher in the stationary phase than in the mid-log phase for all strains (Figure 4A), which is consistent with previous studies [21–23]. The expressions of sspA and hysA were higher in the mid-log phase for some strains, suggesting that

the find more expression of these genes varied among strains. We subsequently compared the virulence gene expression of S. aureus strains against that of M92 in vitro (Figure 4B). All strains were found to have lower hla expression than M92 in vitro, but varied in the expression of other genes, with no specific pattern noted. When in vivo virulence gene expression was examined, it was noted that hla expression was significantly higher in all high virulence strains (USA300, USA400 check details and CMRSA2; p values: 0.0013, 0.038 and 0.0015, respectively) but not in the low virulence strain CMRSA6 as compared with M92 (Figure 4C). High in vivo MK-8776 cost expression of sak and sspA were also observed in the high virulence strains but not all of them exhibited significant difference (sak, p values: 0.006, 0.007 and 0.0698 for USA300, USA400 and CMRSA2, respectively;

sspA, all p > 0.05) (Figure 4C). The other genes displayed different gene expression patterns in different strains without correlation with fly killing activity. CMRSA6, a low virulence strain, showed lower in vivo gene expression compared with M92 for all genes tested. Figure 4 Comparison of 5 virulence gene expression profiles between different MRSA strains. (A) Fold-change in the transcriptional level for each

gene in MRSA at stationary phase relative to the level in bacteria at mid-log phase in vitro (BHI broth); (B) Fold-change in the transcriptional level for each gene of MRSA strains relative to the level of M92 at mid-log phase in vitro (BHI broth); (C) Fold-change in the transcriptional level of each gene in MRSA strains relative to the level of M92 at 18 hour in the flies post infection (in vivo). The asterisk indicates a statistically significantly difference (p < 0.05) Avelestat (AZD9668) of the in vivo virulence gene expression in the MRSA strains as compared with M92 (Student’s t-test). Hemolysin α (hla): USA300 vs M92, p=0.0013; USA400 vs M92, p=0.038; and CMRSA2 vs M92, p=0.0015. Staphylokinase (sak): USA300 vs M92, p=0.006; USA400 vs M92, p=0.007; CMRSA2 vs M92, p=0.0698. Discussion Needham and co-workers [14] have shown that a limited number of S. aureus lab strains caused fly death following injection of bacteria into the dorsal thorax of the flies, suggesting it is a useful model for high-throughput analysis of S. aureus virulence determinant. In this study, we compared the virulence of MRSA strains with different genetic backgrounds using the fly model and demonstrated that they had different fly killing activities, where USA300, USA400, and CMRSA2 strains had greater killing activities compared to CMRSA6 and M92.