High-level mupirocin resistance in Staphylococcus aureus: evidence for two distinct isoleucyl-tRNA synthetases

Abstract

Mupirocin resistance in Staphylococcus aureus results from changes in the target enzyme, isoleucyl-tRNA synthetase (IRS). Twelve strains of S. aureus comprising four susceptible (MICs c 4 iig/ml), four intermediate level-resistant (MICs between 8 and 256 ,ug/ml), and four highly resistant (MICs 512 ,ug/ml) isolates were examined for their IRS content and the presence of a gene known to encode high-level mupirocin resistance. Ion-exchange chromatography of cell extracts showed a single IRS active peak in mupirocin-susceptible strains, with 50% inhibitory concentrations (IC50s) of 0.7 to 3.0 ng of mupirocin per ml. In strains showing intermediate mupirocin resistance, similar single IRS activity peaks were observed, but these were less sensitive to inhibition, and the mupirocin IC50s for them were 19 to 43 ng/ml. Strains that were highly resistant to mupirocin displayed two distinct peaks; one was similar to that found with susceptible strains (IC50, 0.9 to 2.5 ng/ml), but an additional peak with an IC50 of 7,000 to 10,000 ng/ml was also observed. A strain cured of the plasmid encoding high-level mupirocin resistance lacked the resistant IRS peak. Restriction digests, produced by endonuclease NcoI, of total bacterial DNA isolated from the highly resistant strains hybridized with a mupirocin resistance gene probe, whereas DNA isolated from the intermediate level-resistant and susceptible strains did not. These results demonstrate that two different IRS enzymes were present in highly mupirocinresistant S. aureus strains. In strains expressing intermediate levels of resistance, only a chromosomally encoded IRS which was inhibited less by mupirocin than IRS from fully susceptible strains was detected.Mupirocin (pseudomonic acid A) is a narrow-spectrum topical antibiotic active predominantly against gram-positive pathogens, particularly staphylococci, including methicillinresistant Staphylococcus aureus, and streptococci (20). Previous studies (11,13) have shown that mupirocin inhibits bacterial isoleucyl-tRNA synthetase (IRS), which charges the appropriate tRNA species with isoleucine. This inhibition decreases or abolishes bacterial protein synthesis by preventing the incorporation of isoleucine into nascent peptides. The epoxide-bearing end of the mupirocin molecule resembles the carbon skeleton of isoleucine and may compete for the active site of the IRS enzyme (11). Further studies, however, are necessary to elaborate this mechanism of action. Previous data from our laboratoxy (8) showed that inhibition of IRS by mupirocin was time dependent and irreversible, suggesting that the mupirocin-IRS enzyme complex was highly stable.Since the introduction of mupirocin into clinical practice in 1985, there have been few reports of S. aureus strains exhibiting reduced susceptibility to the agent (17). These resistant strains can be divided arbitrarily into two distinct groups, those which exhibit intermediate levels of resistance (MICs, 8 to 256 ,ug/ml) and those which exhibit high levels of resistance (MICs, .512 p,g/ml).S...

Phenotypic and Genotypic Mupirocin Resistance among Staphylococci Causing Prosthetic Joint Infection

“…Topical mupirocin is an effective S. aureus nasal decolonization agent (14,21); however, mupirocin resistance may be associated with decolonization failure (4,7,25). Low-level resistance to mupirocin is associated with mutations in endogenous bacterial isoleucyl-tRNA synthetase; high-level mupirocin resistance is due to acquisition of mupA which encodes an exogenous isoleucyl-tRNA synthetase not inhibited by mupirocin (6,8). Coagulase-negative staphylococci (CNS) may act as a reservoir for mupA, which may be transferred to S. aureus; transfer of mupA from CNS to S. aureus has been demonstrated in vitro (17,23).…”

Molecular Detection of Antimicrobial Resistance

“…Resistance to mupirocin is phenotypically divided into two groups, low-level (MICs, 4 to 256 g/ml) and high-level (MICs, Ն512 g/ml). Low-and high-level resistance has been detected in both S. aureus and CNS (96,254,295). Low-level resistance to mupirocin in most cases is probably due to mutations in the host IRS.…”

“…Low-level resistance to mupirocin in most cases is probably due to mutations in the host IRS. Isolates resistant to high levels of mupirocin contain an additional biochemically distinct IRS enzyme that is less sensitive to inhibition by mupirocin (96). The mupA gene, which codes for the mupirocin-resistant IRS, is only 30% similar to the host IRS at the amino acid level.…”

“…The mupA gene, which codes for the mupirocin-resistant IRS, is only 30% similar to the host IRS at the amino acid level. mupA is known to be carried on transferable plasmids that vary in size in both S. aureus and CNS isolates that are epidemiologically unrelated (81,96,254). Since the concentrations achieved locally are likely to be in the region of 20,000 g/ml, it is unlikely that low-level resistance will have a major impact on staphylococcal clearance.…”

Mupirocin – Are We in Danger of Losing It?

“…distinct IleS-2 genes: the constitutive chromosomally based gene that may, itself, encode for variable levels of mupirocin resistance, and the acquired high-level resistance plasmid-based gene (10,11). Farmer et al (12) showed that the antibiotic concentration that halved isoleucyltRNA synthetase activity correlated well with the MIC of 3.3×10 -2 mg/L for mupirocin-susceptible strains, 1.3×10 -1 mg/L for low-level resistant strains, and 7.5 mg/L in highlevel resistant strains (12).…”