Exfoliatin-Producing Strains Define a Fourth agr Specificity Group in Staphylococcus aureus

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Abstract
The staphylococcal virulon is activated by the density-sensingagr system, which is autoinduced by a short peptide (autoinducing peptide [AIP]) processed from a propeptide encoded byagrD. A central segment of the agr locus, consisting of the C-terminal two-thirds of AgrB (the putative processing enzyme), AgrD, and the N-terminal half of AgrC (the receptor), shows striking interstrain variation. This finding has led to the division of Staphylococcus aureus isolates into three different agr specificity groups and to the division of non-aureus staphylococci into a number of others. The AIPs cross-inhibit the agr responses between groups. We have previously shown that most menstrual toxic shock strains belong toagr specificity group III but that no strong clinical identity has been associated with strains of the other two groups. In the present report, we demonstrate a fourth agr specificity group among S. aureus strains and show that most exfoliatin-producing strains belong to this group. A striking common feature of group IV strains is activation of the agrresponse early in exponential phase, at least 2 h earlier than in strains of the other groups. This finding raises the question of the biological significance of the agr autoinduction threshold.