Staphylococcus aureus strain designation by agr and cap polymorphism typing and delineation of agr diversification by sequence analysis

The allelic variations of the regulatory operon agr (groups I-IV) and the cap polymorphism (capsular types 5 and 8) were used as a typing scheme for rapid strain designation in Staphylococcus aureus. In combining 10 agr subgroups resolved by restriction fragment length polymorphism (RFLP) analysis with the two cap polymorphisms 12 types could be defined. To assess whether this type designation is informative for the population structure of the species S. aureus, agr and cap types were determined in clonal lineages defined by pulsed-field gel electrophoresis (PFGE) of a collection of 219 isolates. agr groups and cap types were both linked to certain clone complexes. However, little correlation was found between the two polymorphic loci. By PFGE cluster analysis 11 prevalent and 52 sporadic clones were defined. Most of the prevalent clones (9/11) could be discriminated by agr/cap typing. Thus, this technique allows a first subdivision of isolates and an inter-center comparable designation of S. aureus clones preceding a more detailed clonal analysis by PFGE or multi-locus sequence typing (MLST). To get insight into agr diversification, sequence analysis of the variable and conserved part of agr from selected S. aureus clones was performed. Strains of agr-I displayed the highest sequence divergence on the nucleotide and amino acid level, suggesting an early diversification of this group. When analyzing the relationship between the four agr interference groups we could show: (i) one intermediate between agr-I and agr-IV alleles; (ii) agr-IV sequences seem to bridge the agr-I and -III groups and (iii) two cases of horizontal transfer of the variable gene cassette from an agr-I strain to an agr-II strain. Thus, stepwise evolutionary progression and rare events of recombination were evident in the diversification of the agr locus.