Prevalence of Mupirocin Resistance Among Staphylococci, its Clinical Significance and Relationship to Clinical Use

INTRODUCTION

Mupirocin (pseudomonic acid A) an analogue of amino acid isoleucine is derived from Pseudomonas fluorescens. It competes with isoleucine for binding sites of bacterial isoleucyl transfer RNA (tRNA) synthetase and thereby inhibits protein synthesis.[1] The drug is widely used as a topical antimicrobial agent to treat Staphylococcus aureus infections and carrier status. Widespread usage in clinical practice and over-the-counter availability has led to the development of resistance to this drug.

Mupirocin susceptibility is categorized into three types: Mupirocin susceptible with minimum inhibitory concentration (MIC) of ≤4 µg/ml (Mup^S), low level resistance (Mup^RL) with MIC of ≥8–256µg/ml and high level resistance (Mup^RH) with MIC of ≥ 512 µg/ml. Some authors describe low-level resistance as MIC ≥ 8–64 µg/ml because isolates with MIC of 128 and 256 µg/ml are uncommon.[2] The resistance can be detected in Kirby–Bauer disc diffusion testing by using 5 µg and 200 µg discs. The dilution method is considered the gold standard for the determination of mupirocin-resistance levels.[3]

Coagulase-negative staphylococci (CoNS) are considered to be a part of the normal human flora. However in recent days, CoNS have emerged as a major cause of device associated infections, infections among immunocompromised and cancer patients. Multidrug-resistant phenotypes are more common among CoNS when compared to S. aureus. Many resistances have been proved to originate from CoNS and spread to S. aureus by horizontal gene transfer. Being the reservoir of conjugative plasmids CoNS might have transferred high-level mupirocin resistance to S. aureus.[4]

Mupirocin is mainly used for decolonization of methicillin-resistant S. aureus (MRSA) among patients and health care workers. This selective use has made a higher prevalence of mupirocin resistance among MRSA when compared to methicillin-sensitive S. aureus (MSSA). Higher prevalence of Mup^RH may also be attributed to selective reports from MRSA outbreaks from hospitals. Studies defining the overall prevalence of mupirocin resistance among staphylococcal isolates are limited.[4]

The present study aimed to determine the overall prevalence of high-and low-level mupirocin resistance among S. aureus and CoNS and correlated with its clinical use.

METHODS

Disc diffusion method

Pure form of mupirocin was purchased from HiMedia laboratories Pvt Ltd (Mumbai, India). Mupirocin disc of 5 µg and 200 µg strength were prepared in house. The discs once prepared were used within a week. Both discs were included in the routine sensitivity testing and plates were incubated for 24 h at 35 ± 2°C. The zone diameters were carefully examined with transmitted light for light growth within the zone of inhibition. Isolates with no zone of inhibition were interpreted as mupirocin resistant. Isolate resistant to 5 µg disc and any zone for 200 µg disc was considered Mup^RL. Isolates resistant for both the discs were considered high-level mupirocin resistant[3] [Figure 1].

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Figure 1

High level mupirocin resistant methicillin sensitive Staphylococcus aureus showing inducible clindamycin resistance phenotype

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RESULTS

A total of 98 (68.5%) S. aureus and 45 (31.5%) CoNS were grown. Among the forty-five CoNS spp., 39 were S epidermidis, 4 were S haemolyticus and 2 were S lugdanensis. A total of 82 staphylococci were isolated from samples of outpatients, 56 from inpatients and 5 from ICU.

Methicillin resistance was found in 22.4% of S. aureus and 20% of CoNS. Methicillin resistance among staphylococci isolated from outpatients and in patients was 19.5% and 24.6%, respectively. iMLSB and cMLSB were found in 19% and 1% of S. aureus. CoNS had higher proportion of cMLSB (22%) when compared to iMLSB (12%).

Overall prevalence of Mup^RL and Mup^RH among staphylococci was found to be 14.7% (n = 21) and 10.5% (n = 15), respectively [Table 1]. Fifteen percent of CoNS had Mup^RH which is very high when compared to the resistance in S. aureus (8.1%). Higher proportion of S. aureus (17.3%) had Mup^RL resistance when compared to CoNS spp. (8.9%). Among the coagulase negative species, Mup^RH was found only in S. epidermidis. Of the 7 Mup^RH S. epidermidis, 5 were methicillin resistant, and four were cMLSB [Table 2].

Table 1 Distribution of Mup^R among S. aureus and CoNS

Table 2 Distribution of mupirocin and methicillin resistance among S. aureus and CoNS

Among the 15 Mup^RH Staphylococcus spp., 5 showed iMLSB and 4 showed cMLSB [Table 3]. Occurrence of Mup^RH and MLSB [Figure 1] phenotype in same isolate may indicate carriage of both genes on same plasmids. The susceptibility of various antibiotics classes with mupirocin resistance is shown in Table 4.

Table 3 Distribution of Mup^R among inducible and constitutive clindamycin resistant staphylococci

Table 4 Comparison of antibiotic susceptibility with mupirocin susceptibility

DISCUSSION

Topical preparations of mupirocin first became available in 1985, since then it is widely used for management of infection and colonization of MRSA in both patients and medical personnel. The first report of mupirocin resistant S. aureus came shortly after its introduction (1987) from UK. In recent days, there is a worldwide increase in mupirocin resistance among S. aureus. Genetic basis of mupirocin resistance is defined. Low-level mupirocin resistance is due to point mutations in native isoleucyl-tRNA synthetase (IRS) gene (ileS). High-level resistance is due to a plasmid-mediated gene, mupA (ileS2), which encodes an additional modified IRS, which has less affinity for mupirocin. Recently, a novel mupB gene is also identified for high-level mupirocin resistance.[8]

The present study showed Mup^RH was more frequent in MSSA compared to MRSA (P < 0.05) and was more common among isolates from out-patients when compared to in-patients. This is attributed to the over-the-counter availability of mupirocin in community settings and more frequent usage in the general population for skin infections than for eradicating carriage or treating MRSA outbreaks in hospitals.[9] In our hospital, since last 5 years, mupirocin is the only topical preparation available for treatment of suspected staphylococcal infections. Our hospital utilized 20,000 mupirocin ointments (5 g) in 2013 accounting to the usage of 100 kg of mupirocin. The ointment is easily available over-the-counter from a private pharmacy. This increased usage of mupirocin has led to the development of resistance. It is interesting to note the widespread usage of mupirocin ointment for skin and soft tissue infection may be the reason for higher resistance rates among isolates obtained from pus samples. Higher prevalence of Mup^RL among S. aureus is clinically less significant as mupirocin ointment contains 20,000 mg/L (2%), which can effectively clear such strains.[4]

Abimanyu et al. from South India showed 32% MRSA ST239 were associated with Mup^RH and iMLSB.[10] The present study showed three isolates of MSSA associated with Mup^RH and iMLSB. The prevalence of Mup^RH in India among MRSA varied from 0% to 38.46% and among MSSA from 0% to 1.5%. Prevalence of mupirocin resistance among staphylococci from various parts of India has been tabulated and compared with the present study [Table 5]. Most of the studies are focused on MRSA than MSSA.

Table 5 Caparison of Mup^R from various studies from India

High-level mupirocin resistant S. aureus were more likely to be susceptible to cotrimoxazole, gentamycin and tetracycline and were more likely to be resistant to penicillin, ciprofloxacin and amoxicillin-clavulanic acid. Resistance to vancomycin, teicoplanin, and linezolid was not found. Hence, the judicial use of these antibiotics must be carefully employed.

The Mup^RH CoNS showed resistance to penicillin (100%), erythromycin (100%), ciprofloxacin (71%), clindamycin (57%) and amoxicillin-clavulanic acid (57%). The co-occurrence of mupirocin resistance and resistance to other antibiotics may indicate the organism carrying plasmid with multiple resistance genes. A higher rate of mupirocin resistance among MR-CoNS is a concern because; they can serve as a reservoir for horizontal transmission to S. aureus.[1718] Similar study by Teo et al. found mupirocin-resistant CoNS were concomitantly methicillin resistant.[19]

Cadilla et al. in their study found a strong association of HL mupirocin resistance with resistance to ≥ 4 non-β-lactam antimicrobial classes among S. aureus.[18] In the present study, association was found between Mup^HR and ciprofloxacin among S. aureus. Resistance in CoNS showed similar findings as that of Cadilla et al.[18]

Occurrence of methicillin resistance and mupirocin resistance in the same isolates is a cause for concern considering the role of mupirocin as a topical agent for MRSA elimination and may select the MRSA infection. Simor et al. in their study found the co-existence of high-level mupirocin resistance and fusidic acid resistance in same isolates.[20] Fusidic acid can be used both topically and orally. Use of fusidic acid for treatment of patients infected Mup^RH staphylococci can result in the selection of strains, which are multidrug resistant. As an alternative to topical antibiotic preparations, a hydrogen peroxide cream has been developed and used successfully.[21]

The present study correlates the prevalence of mupirocin resistance and its usage in clinical practice. Most of the previous studies have focused on MRSA outbreaks; very few define overall mupirocin resistance rates among Staphylococcus spp. Ours being a tertiary care hospital caters services to patients from the different geographical area. The information we report here may also apply to other hospitals in our city as well as in other cities where data are lacking. A Limitation of this study was the relatively small sample size.

CONCLUSION

The results of this study indicate that the rate of mupirocin resistance has been increased. Higher prevalence of mupirocin resistance was noticed among community-acquired than hospital-acquired strains. Continued surveillance for mupirocin resistance is important in order to retain the usefulness of this agent for the treatment and prevention of staphylococcal infections. Judicial prescription of mupirocin after knowing the susceptibility report should become standard practice by clinicians. Prolonged or widespread use of mupirocin in hospital or community must be stopped. Infection control and antibiotic policies have to be developed, audited, and reviewed regularly.