Surging Mupirocin Resistance in Staphylococci – Does It Warrant Routine Testing?

Sir, The nose and extranasal sites are mostly colonized by Staphylococcus aureus. It has been proposed that about 30% of colonizers eventually develop infection.[1] Decolonization with mupirocin is recommended for colonized individuals and is being treated with nasal topical application 4 times daily for 5 days.[2] Pharmacological concentration of the drug used for the treatment leads to persistence of low-level drug being concentrated in pharynx and may initiate emergence of resistance.[3] Mupirocin resistance appears to be increasing worldwide, which ranges from 1% to 45%. This study was done to analyze the rates of mupirocin resistance among Staphylococcus isolates. In this study, a total of 100 nonduplicate staphylococcal isolates from different clinical samples collected from inpatients (84 pus, 8 sputum, 7 urine, and 1 tissue) were identified by standard microbiological techniques. Antibiotic susceptibility testing was done by Kirby–Bauer’s disk diffusion method as per the CLSI guidelines. Quality control strain used was S. arueus ATCC 25923. Low-level (MuL) and high-level resistance (MuH) were detected, respectively, by 5 μg and 200 μg mupirocin disks. Sensitivity and specificity with 5 μg was 100% and 98.1%, and for 200 μg, it was 100% and 92.3%, respectively.[4] Currently, there are no CLSI guidelines for outlining interpretive criteria for mupirocin. Studies defined the interpretative criteria are the following:[5] A zone diameter of >14 mm for both 5 μg and 200 μg disks was considered to be susceptible for mupirocin A zone diameter of <14 mm in the 5 μg disk but >14 mm in the 200 μg disk was considered to be MuL strains A zone diameter of <14 mm for both 5 μg and 200 μg was considered to be MuH strains. Among these 100 isolates, MuL was detected in 14% and MuH in 5%. Mupirocin resistance was noted as 38% and 5% among methicillin-sensitive S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) isolates, respectively [Table 1 and Figures 1, 2]. All staphylococcal isolates were sensitive to netilmicin and linezolid. Erythromycin-induced clindamycin resistance was noted in 16% of staphylococcal isolates. Multidrug resistance was observed in 65% of isolates and 19% showed mupirocin resistance.Table 1: Distribution of mupirocin resistance among StaphylococciFigure 1: Antibiogram showing sensitive zone for Mup 200 µg and resistance for Mup 5 µgFigure 2: Antibiogram showing resistance for Mup 200 µg and 5 µgOommen et al. noted MuH among 7% of staphylococcal isolates, which correlates with our study.[6] MuH is mediated through plasmid, whereas MuL is mediated through chromosomes rather than plasmid. Mupirocin resistance may also aid in the spread of multidrug resistance through coselection with other resistance genes. Mupirocin susceptibility is not being tested routinely in clinical care practice because MuH has been reported to be relatively rare, ranging from 1% to 5% of MRSA isolates.[7] A study by Rudresh et al. reported 26% of the MSSA isolates resistant to mupirocin similar to the present study (38%).[8] Minimum inhibitory concentration determination by agar dilution method is the “gold standard” approach in the detection of mupirocin resistance and genotypic methods such as polymerase chain reaction remain the confirmatory test. In meager resource setting, disk diffusion method is a cost-effective, alternative method. Combined use of 5 μg and 200 μg mupirocin disks increased the accuracy of the resistance detection. It is necessary that use of mupirocin should be limited and employed only within the guidelines of defined infection control protocols. Ongoing monitoring of resistance is necessary especially where there is widespread use of decolonization regimen. Limitations Sample size is small Lack of a molecular testing Quality control for MuH was not included Correlation between clinical samples and nasal sample from the same patients was not studied Follow-up of these patients are not done. Conflicts of interest There are no conflicts of interest. Funding This work was supported by the Tamil Nadu State Research Committee, King Institute of Preventive Medicine and Research, Guindy, Chennai, Tamil Nadu. Author’s contribution All the authors have substantial contributions to each of the three components mentioned below: 1. Concept and design of study or acquisition of data or analysis and interpretation of data; 2. Drafting the article or revising it critically for important intellectual content; and 3. Final approval of the version to be published. The prominent roles of each also included the following. PSK: Concept and design of study, literature review, drafting/ editing and finalizing the manuscript; NR: Literature review, clinical analysis, data analysis and interpretation, drafting/editing and finalizing the manuscript. AVS: Literature review, clinical analysis, data analysis and interpretation, drafting/editing and finalizing the manuscript.