On Fusidic Acid Resistance in Staphylococcus Aureus

Abstract: Controlling bacterial infections with antibiotics is central to modern health care. However, increasing bacterial resistance to antibiotics threatens effective therapy. This thesis concerns the use of the antibiotic fusidic acid, and novel analogues of fusidic acid, to treat topical infections caused by the bacterial pathogen Staphylococcu aureus. It also addresses genetic mechanisms by which S. aureus develops resistance to fusidic acid.Pre-clinical microbiological tests were made on two structurally different groups of fusidic acid analogues developed by Leo Pharma. These drugs were tested against S. aureus and Streptococcus pyogenes strains, measuring MIC, in vitro concentration-dependent bacteriocidal or bacteriostatic effects, and in vivo efficacy in clearing topical infections. We developed a new superficial skin infection animal model (the ‘tape-stripping model’) designed for testing topical antibiotics, including the novel fusidic acid analogues, against S. aureus and S. pyogenes. Some new compounds giving promising results will be further tested and developed by Leo Pharma. Fusidic acid inhibits protein synthesis by binding to elongation factor EF-G on the ribosome. Previously described resistance mechanisms are mutations in the gene coding for EF-G (fusA), or, in some strains, the presence of a gene (fusB, fusC or fusD) coding for a protein that protects EF-G from fusidic acid.We discovered two novel classes of spontaneous FusR mutants in S. aureus with the small colony variant (SCV) phenotype which is associated with persistent infections. The FusR SCV’s are very frequent, slow growing, cross-resistant to aminoglycosides, and auxotrophic for hemin or menadione. Some of the FusR SCV mutations are in structural domain V of EF-G (classic fusA mutations map overwhelmingly in domain III). The remaining FusR SCV’s are unmapped but their additive effect on MIC together with the fusB plasmid suggests the possibility that their mechanism of resistance is also associated with the translation machinery.