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Indian Journal of Medical Microbiology, Vol. 28, No. 4, October-December, 2010, pp. 372-375 Brief Communication Mupirocin resistance in clinical isolates of staphylococci in a tertiary care centre in south India SK Oommen1, B Appalaraju1, K Jinsha2 1 Department of Microbiology, PSG Institute of Medical Sciences & Research Centre, Peelameedu, Coimbatore - 641 004, India Date of Submission: 02-Jul-2010 Code Number: mb10109 PMID: 20966573 Abstract The present study was carried out to determine the rates of high-level and low-level mupirocin resistance in Staphylococcus spp. (MuH and MuL) in southern India. A prospective study was carried out on Staphylococcus spp. isolated for a period of three months in the microbiology laboratory of an 800-bedded tertiary care hospital. One hundred sixty-seven non-duplicate Staphylococcus spp. isolated from different specimens were tested for mupirocin susceptibility using 5 and 200 μg discs and by agar dilution. Rates of MuH were found to be two percent in methicillin-resistant Staphylococcus aureus (MRSA) and 28% in methicillin-resistant coagulase-negative Staphylococcus spp. (MRCoNS). MuL strains may be still treated with mupirocin, while MuH strains require other treatment options for eradication, making prior screening and differentiation important.Keywords: High-level and low-level mupirocin resistance, MuH, MuL Introduction Methicillin-resistant Staphylococcus aureus (MRSA) is a problem in hospitals worldwide and is increasingly recovered from nursing homes and the community. [1] Indiscriminate use of antibiotics, prolonged hospital stay, intravenous drug use, carriage of MRSA in nose, axilla, perineum are important risk factors for MRSA acquisition.[2] The commonly used antibiotic for treatment of MRSA infection is vancomycin or linezolid, while mupirocin (derived from Pseudomonas fluorescens) is an effective topical antibiotic for the elimination of MRSA in carriers.[3] Mupirocin (pseudomonic acid A) specifically binds to bacterial isoleucyl-tRNA synthetase (IRS) and inhibits protein synthesis. [4] The increased use of this antibiotic has been accompanied by outbreaks of MRSA resistant to mupirocin, although the frequency of resistance is still low.[5] Nasal application of mupirocin at clinically effective concentrations may result in the presence of low levels of the antibiotic in the pharynx, which could induce or select for the emergence of mupirocin-resistant MRSA. [6] Traditionally, susceptible strains are defined as those with a minimum inhibitory concentration (MIC) of ≤2 μg/ml while those having a MIC of ≥4 μg/ml were considered resistant, and for disk diffusion zone diameter of ≥14 mm with a 5-μg disc was considered as susceptible while zones of ≤13 mm as resistant. [7] However, recently mupirocin-resistant strains are grouped into two distinct categories: low level (MuL) with MICs of 8-256 μg/ml, and high level (MuH) with MICs ≥512 μg/ml, while mupirocin-sensitive isolates were those with MIC of ≤4 μg/ml. [8] With the previously used 5 μg mupirocin disc, one cannot differentiate between MuL and MuH strains. The concomitant use of 5 and 200 μg mupirocin discs can easily differentiate them.[9] High-level resistant strains are more likely to be associated with clinical and microbiological failure. [10],[11] Studies suggest that mupA gene which is known to encode for mupirocin resistance is transferred from Staphylococcus epidermidis to MRSA during mupirocin prophylaxis. [12] It has been suggested that mupirocin-resistant coagulase-negative Staphylococcus spp. (CoNS) might be an important source of the mupA determinant in MRSA. The increasing prevalence of transferable mupirocin resistance among CoNS species could be an important threat to the future use of mupirocin against MRSA. [3] Though mupirocin resistance is often associated with methicillin resistance, the true extent of mupirocin-resistant methicillin-resistant Staphylococcus spp. in our country is unknown. Thus, this study was carried out to primarily determine the rates of high-level and low-level mupirocin resistance in S. aureus and coagulase-negative Staphylococcus spp. by disc diffusion and MIC methods and secondarily to compare its association with methicillin-resistant isolates. Materials and Methods Clinical samples such as pus, blood, central venous catheters tips, tracheal aspirates, urine submitted to the microbiology laboratory in a tertiary care hospital in South India for culture and sensitivity during a 3-month period were screened for the growth of staphylococci. Isolates were identified as S. aureus or coagulase-negative Staphylococcus spp. by routine microbiological procedures. [13] Detection of methicillin resistance was carried out by using cefoxitin (30 μg) discs as per Clinical Standards Laboratory Institute (CLSI) 2008 guidelines.[14] Non-duplicate staphylococcal isolates (methicillin resistant and sensitive) were included in the study. Antimicrobial susceptibility testing was done according to the CLSI standards [14] by disc diffusion method for the following antibiotics: ciprofloxacin (5 μg), clindamycin (2 μg), cotrimoxazole (1.25/23.75 μg), doxycycline (30 μg), erythromycin (15 μg), gentamicin (10 μg), linezolid (30 μg), mupirocin (5 and 200 μg), penicillin (10 units), rifampicin (5 μg) and vancomycin (30 μg). D zone test was done do determine inducible resistance to clindamycin.[14] Vancomycin susceptibility was confirmed by agar dilution method. [14] Mupirocin-resistant strains were tested for fusidic acid (Hi Media, Mumbai, India) susceptibility by MIC method. [15] A zone diameter of greater than or equal to 14 mm for both 5 and 200 μg discs was considered to be susceptible for mupirocin. Isolates that showed zone diameters less than 14 mm in the 5 μg disc but more than or equal to 14 mm in the 200 μg disc were considered to be MuL strains. All isolates with zone diameters less than 14 mm for both 5 and 200 μg were considered to be MuH strains. [9] MIC for mupirocin was determined by a standardised agar dilution method [14] on Mueller-Hinton agar with final mupirocin (HiMedia) concentrations that ranged from 0.016 to 1024 μg/ml. Staphylococci requiring concentrations of mupirocin of ≤4 μg/ml for growth inhibition were considered as "susceptible," those requiring concentrations of 8-256 μg/ml for inhibition as with "low level resistance," and those requiring concentrations of ≥512 μg/ml as with "high level resistance". [9] S. aureus ATCC 25923 was used as the control. Acceptable range for ATCC S. aureus[13] is from 00.12 to 0.5 μg/ml. Statistical methods Proportions of mupirocin-resistant and mupirocin-susceptible staphylococcal strains were calculated. All clinical variables were tabulated using frequency tables, along with pattern of susceptibility to commonly used antibiotics. Fisher's exact test was done to determine the statistical significance. A P value of <0.05 was considered as statistically significant. Results Among the 167 non-duplicate staphylococcal isolates included in the study, 48 (28.7%) were MRSA, 50 (29.9%) were methicillin-sensitive S. aureus (MSSA), 39 (23.3%) were methicillin-resistant CoNS (MRCoNS) and 30 (17.9%) were methicillin-sensitive CoNS (MSCoNS). Of these, 2.08% of MRSA and 28.2% of MRCoNS were mupirocin resistant. No resistance was detected in MSSA and MSCoNS. Amongst the mupirocin-resistant strains, the percentage of low-level resistant and high-level resistant S. aureus and CoNS isolates and its comparison with methicillin resistance are documented in [Table - 1]. For MRSA, this comparison was (P = 0.4898) not statistically significant, while for MRCoNS (P = 0.0016) it was found to be statistically significant. Linezolid and vancomycin were found to be the most sensitive antibiotics across all staphylococcal species. One isolate of vancomycin-intermediate coagulase-negative Staphylococcus spp. (MIC of 8 μg/ml) was detected by agar dilution. This isolate was sensitive to mupirocin and linezolid but was methicillin resistant by cefoxitin disc diffusion. Amongst the other antibiotics, percentage resistance was as follows: ciprofloxacin 44% , cotrimoxazole 56% , erythromycin 35% , clindamycin 28% , doxycycline 16% , gentamicin 4% and rifampicin 10%.. Of the 11 mupirocin-resistant strains, eight were sensitive to fusidic acid which included the single strain of mupirocin resistant MRSA. Discussion Twelve (7%) of the total 167 isolates were found to be resistant to both 5 and 200 μg mupirocin discs. These isolates were classified as high-level mupirocin resistant staphylococci (MuH). These included 11 strains (16%) of CoNS and one strain (1.02%) of S. aureus. Mupirocin resistance was found only in methicillin-resistant strains. Thus, 2% of MRSA and 28% of MRCoNS were mupirocin resistant in our study. The MIC of all the MuH isolates was found to be greater than 1024 μg/ml by agar dilution method, while the MIC values of mupirocin-sensitive isolates were between 0.5 and 1 μg/ml. No MuL staphylococci were detected either by disc diffusion (i.e. isolates that were resistant to 5 μg mupirocin disc but sensitive to 200 μg mupirocin disc) or by agar dilution method (MIC of 8-256 μg/ml). In a study that included staphylococci from 19 European hospitals, [5] the prevalence of high-level mupirocin resistance was found to be 1.6% in S. aureus and 5.6% in CoNS. In a Greek hospital in 2001, five of 250 (2%) S. aureus isolates exhibited high-level resistance to mupirocin.[4] In this aspect, our study is comparable, with the rate of mupirocin resistance in S. aureus as 1.02% but the rate of mupirocin resistance in CoNS (16%) in our study was considerably higher. There are not many studies documenting mupirocin resistance in CoNS, which may be the reservoir of mupirocin resistance genes. In a study which first documented the extent of mupirocin resistance in an Indian hospital, it was found that 1% of 200 S. aureus isolates (including 0.9% of MRSA and 1.1% of MSSA) showed low-level resistance and 5% showed high-level resistance (8.2% of MRSA and 1.1% of MSSA isolates).[16] This study, however, did not include the prevalence of mupirocin resistance in CoNS. Low-level mupirocin-resistant staphylococci were not detected in our study. Also, none of our isolates of MSSA or MSCoNS showed any resistance (either high or low level) to mupirocin. Mupirocin demonstrates superior efficacy with a significant duration of nasal clearance of MRSA in carriers. Resistance to mupirocin, especially high-level resistance, offers fewer topical treatment options. The presence of comparatively higher rates of mupirocin resistance in CoNS is also a cause for concern. Studies suggest that mupA gene which is known to encode for mupirocin resistance can be transferred from S. epidermidis to MRSA during mupirocin prophylaxis, which could be an important threat to the future use of mupirocin against MRSA. [3],[12] High-level mupirocin resistance has been associated with failure to clear the organism from patients on mupirocin therapy. However, it has been suggested that MuL nasal isolates can still be controlled with mupirocin therapy, as the ointment used contains a much higher mupirocin concentration (2000 μg/ml) than the MuL MICs.[17] Detection of MRSA in a healthcare worker usually leads to a 7-day chlorhexidine-based baths and topical 2% mupirocin ointment application along with absence or relocation from duty till two negative culture reports are documented. Hence, it would be advisable to screen all isolates obtained from nasal carriers with mupirocin (not only with 5 μg and but also with 200 μg discs), prior to start of therapy such that MuH strains may be treated with other alternatives like fusidic acid, neomycin, or perhaps the newer reptapamulin. References
Copyright 2010 - Indian Journal of Medical Microbiology The following images related to this document are available:Photo images[mb10109t1.jpg] |
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