Phenotypic loss of protein A production was tested in six methicillin-resistant (Mc) (MRSA) isolates and their isogenic methicillin-sensitive (Mc) variants by a radiolabelled IgG-binding assay with washed cells and by Western blotting of supernates prepared from lysed washed cells. Genomic DNA was probed for homology with the protein A gene in RI digests and for homology to the methicillin resistance gene in III digests. The Mc variants had lost homology with . An isogenic pair of Mc and Mc strains, and derivatives of 8325-4 with site-specific mutations of the accessory gene regulator locus and were tested for adherence to human peritoneal mesothelial cells in monolayer culture. The isogenic pair were also tested for adherence to HEp-2 and Vero cell monolayers in assays with H thymidine-labelled bacteria. Mc isolates produced protein A which was absent from three strains that had become Mc. This correlated with deletion of the locus. homology, but reduced production of protein A, was retained in one Mc strain which also showed reduced adherence to HEp-2, Vero and mesothelial cells (p < 0·05) compared with the parent Mc strain. A mutation in strain 8325-4 did not significantly affect adherence to mesothelial cells but mutation in increased adherence significantly in both Spa and Spa strains.


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