The effect of an agr mutation on expression of type 5 capsular polysaccharide (CP) by Staphylococcus aureus Newman was investigated in different complex and synthetic media. CP expression by the agr mutant was strongly reduced in certain media but slightly in others, indicating that CP synthesis is positively controlled by agr. CP expression occurred in the post-exponential growth phase in both wild-type and mutant strains, suggesting that other regulatory systems could act in conjunction with agr.
Summary. Phenotypic loss of protein A production was tested in six methicillin-resistant (McR) Staphylococcus aureus (MRSA) isolates and their isogenic methicillin-sensitive (McS) 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 (spa) in EcoRI digests and for homology to the methicillin resistance gene (mec) in Hind111 digests. The McS variants had lost homology with mec. An isogenic pair of McR and McS strains, and derivatives of S. aureus 8325-4 with site-specific mutations of the accessory gene regulator locus (agr) and spa, 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 3H thymidine-labelled bacteria. McR isolates produced protein A which was absent from three strains that had become McS. This correlated with deletion of the spa locus. Spa homology, but reduced production of protein A, was retained in one McS strain which also showed reduced adherence to HEp-2, Vero and mesothelial cells (p < 0.05) compared with the parent McR strain. A spa mutation in strain 8325-4 did not significantly affect adherence to mesothelial cells but mutation in agr increased adherence significantly in both Spa+ and Spa-strains.
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