Protein A of Staphylococcus aureus contains a polymorphic Xr-region characterized by a
tandem repeat of eight amino acid units. In this study, the diversity of genes encoding the
repeat regions and their relatedness among S. aureus strains was analyzed. Ten different
protein-A types characterized by repeat numbers 4–13 were identified in a total of 293 clinical
isolates. The protein-A type with 10 repeat units (10 repeats) in the Xr-region was most
frequently detected in methicillin-resistant S. aureus, whereas the majority of methicillin-
susceptible strains were distributed almost evenly into protein-A types with 7–11 repeats.
Strains that belonged to a single coagulase type were classified into multiple protein-A types,
e.g. strains with the common coagulase types II and VII were differentiated into 7 and 8
protein-A types, respectively.
Nucleotide sequence analysis of the Xr-region of 42 representative strains revealed the
presence of 37 different genotypes (spa types), which were constituted by a combination of
several of 24 different repeat unit genotypes. Based on the similarity in arrangement of repeat
unit genotypes, 34 strains with different repeat numbers were classified into 5 genetic clusters
(C1–C5). The clusters C1, C2 and C3 consisted exclusively of strains with identical coagulase
types II, III, and IV, respectively. These findings suggested that the protein-A gene of S.
aureus has evolved from a common ancestral clone in individual clusters independently.