Objective      To investigate the underlying mechanism of variable virulence in Staphylococcus aureus strain XQ.      Methods      After subculturing in vitro

the virulence change in clinical isolated stain XQ of Staphylococcus aureus (S.aureus) was tested in BALB/c mouse models

and S.aureus strain ATCC25923 was used as control. Protein expression profiles in bacterial cells and supernatants were determined by SDS-PAGE analysis. The genes of accessory gene regulator (agr) were amplified by PCR and then sequenced

and the expression levels of virulence factor genes controlled by agr in XQ and XQM were also determined by quantitative real-time reverse transcription PCR (qRT-PCR).      Results      S. aureus XQ strain was more virulent than ATCC25923

however

after culturing in vitro

the virulence of the 180th generation of XQ strain (XQ

180

) was significantly decreased

and SDS-PAGE analysis demonstrated that the protein expression patterns in the culture supernatants were different between XQ and the virulence attenuated strain (XQ

M

). The sequencing results indicated an extra 188 bp repeated sequence was inserted into the 443th of nucleotide agrC gene

resulting in a truncated mutant of AgrC protein. What’s more

the gene expression levels controlled by agr were markedly reduced in XQ

M

strain.      Conclusion      XQ is a high-virulence strain of S. aureus

and prone to virulence variation during cultured in vitro. The virulence variation of S. aureus XQ is associated with the agrC mutant caused by a repeated sequence insertion.