An in vitro investigation of decolonisation treatment failure and low-level resistance to mupirocin in Staphylococcus aureus

Andrew Anyakwo, Lesley Price, Susan Lang

Research output: Contribution to conferencePoster

Abstract

Decolonisation of patients nasally colonised with MRSA using mupirocin is not always successful. Eradication rates of up to 80% have been reported, but relapse at 4-weeks is more frequent with strains displaying low-level mupirocin resistance (LLR) compared to mupirocin sensitive (MS) isolates. The aim of this study was to compare the efficacy of mupirocin in the treatment of surface attached sensitive S. aureus to those with LLR. The mupirocin minimum inhibitory concentration (MIC) of 17 clinical S. aureus (7 MRSA and 10 MSSA) was tested by microbroth-dilution. Subsequently, bacterial cells were attached to mucin-coated microtitre-plate wells for 4h in triplicate, planktonic cells removed and surface adherent cells challenged with 200mg/l mupirocin for 24h. After challenge, cell viability was determined using the metabolic-dye resazurin and the percentage cell death compared to untreated adherent cells in replicate.
Three MRSA exhibited LLR (MIC 32mg/l), whilst all other isolates were mupirocin susceptible (MIC 0.12-0.25mg/l). After exposure to 200mg/l mupirocin (at least 6-fold above the MIC) for 24h a statistically greater proportion of mucin-adherent LLR-MRSA cells remained viable (mean 25%; range 24-26%) compared to 12% MS-MRSA (9-16%) and 14% MSSA (3-41%) (p<0.05). Mupirocin tolerance was not related to MRSA-genotype; EMRSA-15 (mean 14%; range 9-16%) and EMRSA-16 (21%; 8-26%). All strains exhibited a greater tolerance to mupirocin when surface-attached compared to counterpart free-floating populations, however, a greater percentage of cells exhibiting LLR survived in comparison to susceptible cells. The tolerance exhibited by surface-attached LLR isolates might underlie the high rate of treatment relapse observed in patients
Original languageEnglish
Publication statusUnpublished - 2013

Keywords

  • MRSA
  • nasal decolonisation
  • mupirocin
  • tolerance
  • in vitro

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