The antimicrobial activity of mupirocin--an update on resistance

Evidence-Based Recommendations for Antimicrobial Nasal Washes in Chronic Rhinosinusitis

Background

Chronic rhinosinusitis (CRS) refractory to medical and surgical therapy is a difficult problem for patients and physicians. Topical antimicrobial nasal irrigations are commonly used for treatment with great variation in methodology and without clear scientific support for current treatment formulations. The purpose of this study was to develop a scientific rationale for creating standardized recommendations for clinical practice in the use of topical antimicrobial washes for CRS.

Methods

An extensive review of basic science and clinical literature on the treatment of CRS with topical antimicrobial washes was completed. Pharmacokinetics of and organism susceptibility to appropriate topically applied antimicrobial agents were reviewed.

Results

The most common organisms associated with CRS were identified. The relevant pharmacokinetics of drugs targeted at these organisms are presented. Susceptibility breakpoints set by the National Committee for Clinical Laboratory Standards are identified to help establish the most effective concentration of the identified drugs. Recommendations for agent selection, agent concentration, length of treatment, dosing schedule, and methods of irrigation are presented.

Conclusion

Antimicrobial nasal washes provide a potentially effective treatment for the growing population of patients who remain symptomatic after appropriate medical and surgical intervention. This study establishes the basic principles supporting this treatment option and offers rational, evidence-based treatment guidelines. The study has identified additional areas that need to be investigated before prospective clinical trials can be effectively undertaken.

Prevention of Catheter Related Infections in Patients on CAPD

Catheter-related infections remain a serious problem for patients on peritoneal dialysis. Such infections can be reduced by careful patient selection and training, by the use of the best connection technology and screening and treating nasal carriage. To date, treatment is less than optimal and therefore, the primary goal should be prevention of catheter-related infections. Prevention is based on improving catheter design and implantation technique, while providing careful exit-site care. Regardless of how it is implemented, we must aggressively pursue the prevention of catheter-related infections by eradicating S.aureus exit-site carriage in PD patients. Based on its effectiveness in adult PD patients, its low rate of adverse effects, and its reasonable cost-effectiveness, application of mupirocin ointment at the exit-site is the current method of choice for preventing PD catheter infections caused byS. aureus. In addition to reducing S. aureus exit-site infections, mupirocin seems to reduce the rates of staphylococcal peritonitis and PD catheter loss. Whether the ointment should be applied in the nares, to the exit-site or both, and whether it should be used only in staphylococcal nasal carriers or all PD patients requires further study.

Bacterial Infections in Neonates Following Mupirocin-Based MRSA Decolonization: A Multicenter Cohort Study

OBJECTIVE To characterize the risk of infection after MRSA decolonization with intranasal mupirocin. DESIGN Multicenter, retrospective cohort study. SETTING Tertiary care neonatal intensive care units (NICUs) from 3 urban hospitals in the United States ranging in size from 45 to 100 beds. METHODS MRSA-colonized neonates were identified from NICU admissions occurring from January 2007 to December 2014, during which a targeted decolonization strategy was used for MRSA control. In 2 time-to-event analyses, MRSA-colonized neonates were observed from the date of the first MRSA-positive surveillance screen until (1) the first occurrence of novel gram-positive cocci in sterile culture or discharge or (2) the first occurrence of novel gram-negative bacilli in sterile culture or discharge. Mupirocin exposure was treated as time varying. RESULTS A total of 522 MRSA-colonized neonates were identified from 16,144 neonates admitted to site NICUs. Of the MRSA-colonized neonates, 384 (74%) received mupirocin. Average time from positive culture to mupirocin treatment was 3.5 days (standard deviation, 7.2 days). The adjusted hazard of gram-positive cocci infection was 64% lower among mupirocin-exposed versus mupirocin-unexposed neonates (hazard ratio, 0.36; 95% confidence interval [CI], 0.17-0.76), whereas the adjusted hazard ratio of gram-negative bacilli infection comparing mupirocin-exposed and -unexposed neonates was 1.05 (95% CI, 0.42-2.62). CONCLUSIONS In this multicentered cohort of MRSA-colonized neonates, mupirocin-based decolonization treatment appeared to decrease the risk of infection with select gram-positive organisms as intended, and the treatment was not significantly associated with risk of subsequent infections with organisms not covered by mupirocin's spectrum of activity. Infect Control Hosp Epidemiol 2017;38:930-936.

The Role of Colonization in the Pathogenesis of Nosocomial Infections

The close relationship between colonization and the development of nosocomial infections has been demonstrated. Patient-related factors, such as underlying illness for all major sites of infection and advanced age, and pathogen-related factors, such as the ability of bacteria to adhere to epithelial cells, play the major roles in the pathogenesis of colonization. However, exact mechanisms of colonization have not been elucidated, and modulation of bacterial adherence as a method of infection prevention remains experimental. Current methods of infection prevention, therefore, focus either on preventing growth of colonizing microorganisms or on preventing patient-pathogen contact. Topical antibiotics have been used as a method of colonization prevention. However, their effectiveness may be limited by increases in antibiotic resistance; moreover, the effects on patient outcome are controversial. Maintenance of the physiologic mucosal environment using nonantimicrobial agents seems a promising approach, but only a few studies demonstrating efficacy have been published. Prevention of colonization still must rely heavily on basic infection control measures to prevent contact between patient and pathogen.

Mupirocin: biosynthesis, special features and applications of an antibiotic from a gram-negative bacterium

Mupirocin is a polyketide antibiotic produced by Pseudomonas fluorescens. The biosynthetic cluster encodes 6 type I polyketide synthase multifunctional proteins and 29 single function proteins. The biosynthetic pathway belongs to the trans-AT group in which acyltransferase activity is provided by a separate polypeptide rather than in-cis as found in the original type I polyketide synthases. Special features of this group are in-cis methyltransferase domains and a trans-acting HMG-CoA synthase-cassette which insert α- and β- methyl groups respectively while enoyl reductase domains are absent from the condensing modules. In addition, for the mupirocin system, there is no obvious loading mechanism for initiation of the polyketide chain and many aspects of the pathway remain to be elucidated. Mupirocin inhibits isoleucyl-tRNA synthetase and has been used since 1985 to help prevent infection by methicillin-resistant Staphylococcus aureus, particularly within hospitals. Resistance to mupirocin was first detected in 1987 and high-level resistance in S. aureus is due to a plasmid-encoded second isoleucyl-tRNA synthetase, a more eukaryotic-like enzyme. Recent analysis of the biosynthetic pathway for thiomarinols from marine bacteria opens up possibilities to modify mupirocin so as to overcome this resistance.

Nanofibers offer alternative ways to the treatment of skin infections

Injury to the skin causes a breach in the protective layer surrounding the body. Many pathogens are resistant to antibiotics, rendering conventional treatment less effective. This led to the use of alternative antimicrobial compounds, such as silver ions, in skin treatment. In this review nanofibers, and the incorporation of natural antimicrobial compounds in these scaffolds, are discussed as an alternative way to control skin infections. Electrospinning as a technique to prepare nanofibers is discussed. The possibility of using these structures as drug delivery systems is investigated.

Multicenter study to determine disk diffusion and broth microdilution criteria for prediction of high- and low-level mupirocin resistance in Staphylococcus aureus

Mupirocin susceptibility testing of Staphylococcus aureus has become more important as mupirocin is used more widely to suppress or eliminate S. aureus colonization and prevent subsequent health care- and community-associated infections. The present multicenter study evaluated two susceptibility testing screening methods to detect mupirocin high-level resistance (HLR), broth microdilution (BMD) MICs of >or=512 microg/ml, and a 6-mm zone diameter for a disk diffusion (DD) test with a 200-microg disk. Initial testing indicated that with Clinical and Laboratory Standards Institute methods for BMD and DD testing, the optimal conditions for the detection of mupirocin HLR were 24 h of incubation and reading of the DD zone diameters with transmitted light. Using the presence or absence of mupA as the "gold standard" for HLR, the sensitivity and specificity of a single-well 256 microg/ml BMD test were 97 and 99%, respectively, and those for the 200-microg disk test were 98 and 99%, respectively. Testing with two disks, 200 microg and 5 microg, was evaluated for its ability to distinguish HLR isolates (MICs >or= 512 microg/ml), low-level-resistant (LLR) isolates (MICs = 8 to 256 microg/ml), and susceptible isolates (MICs <or= 4 microg/ml). Using no zone with both disks as an indication of HLR and no zone with the 5-microg disk plus any zone with the 200-microg disk as LLR, only 3 of the 340 isolates were misclassified, with 3 susceptible isolates being classified as LLR. Use of standardized MIC or disk tests could enable the detection of emerging high- and low-level mupirocin resistance in S. aureus.

Contemporary antimicrobial activity of triple antibiotic ointment: a multiphased study of recent clinical isolates in the United States and Australia

Triple antibiotic ointment (TAO) containing neomycin, polymyxin B, and bacitracin was launched in the 1950s in the United States (USA) as a prescription product and then was used over the counter (OTC) since the 1970s (USA) to prevent superficial wound infections. In Australia, TAO has been restricted to prescription use. This study 1) determined cross-resistance patterns of neomycin compared with other aminoglycosides; 2) determined the level and trend of resistance to TAO and individual components especially versus mupirocin-resistant strains (USA); and 3) established the baseline TAO activity level against pathogens from Australia. A total of 200 strains (> or =50% gentamicin-resistant) from the United States were used for the cross-resistance study including Staphylococcus aureus (110), coagulase-negative staphylococci (CoNS; 50), Pseudomonas aeruginosa (10), Escherichia coli (20), and other Enterobacteriaceae (10) tested against TAO, bacitracin, polymyxin B, neomycin, amikacin, gentamicin, streptomycin, tobramycin, and mupirocin. Fifty gentamicin-resistant isolates from each year (1997-2002) were used to determine the activity of TAO over time. Baseline resistance rates of TAO among 300 Australian isolates (AGARS Program, 2002-2003) were also studied. Reference broth microdilution methods were used in all phases of this study. At a 1:100 dilution of the ointment concentration, TAO inhibited all CoNS, Pseudomonas aeruginosa, and Enterobacteriaceae isolates, and resistance to TAO among Staphylococcus aureus at this concentration was only 5% in the cross-resistance study. Patterns of susceptibility in the United States did not significantly vary from 1997 to 2002. Australian pathogens showed that TAO was 98% active against methicillin-resistant Staphylococcus aureus and 100% for Enterobacteriaceae, methicillin-susceptible S. aureus, CoNS, and P. aeruginosa, the rates equivalent to those observed in the United States. Mupirocin-resistant S. aureus (5%) and CoNS (47%) were all TAO-susceptible. All Gram-negative species were also mupirocin-resistant, but inhibited by neomycin and/or polymyxin B components of TAO. In conclusion, aminoglycoside resistance patterns differ significantly, and none of the commonly tested agents could accurately predict neomycin resistance. TAO resistance was rare in the United States after extensive OTC use and was not adversely influenced by decades of parenteral aminoglycoside use. Australian surveillance showed high levels of TAO susceptibility in sampled isolates as a baseline for possible OTC availability. TAO maintains a wider spectrum of activity compared with mupirocin and was usable against mupirocin-resistant Gram-positive strains.

Simultaneous detection of the mecA and ileS-2 genes in coagulase-negative staphylococci isolated from Brazilian hospitals by multiplex PCR

Coagulase-negative Staphylococcus spp. (CNS) has been associated with primary bloodstream infections and implanted medical devices. Its importance is increasing due to the acquisition of resistance to oxacillin (Oxa) and, recently, resistance to mupirocin (Mup). Mupirocin, a topical antimicrobial, has been used in the prevention of staphylococci catheter colonization. Susceptibility to Oxa and Mup was analyzed by different testing methods in clinical CNS isolates. Among 112 CNS strains, 69 (61.6%) were Oxa(R) by the disk diffusion (DD) method and 72 (64.2%) grew on the oxacillin agar screen plate. S. epidermidis and S. haemolyticus presented high rates of oxacillin resistance, 75.4% and 96.1%, respectively. Twenty four (21.4%) strains were Mup(R) by the DD test and 21 of them (87.5%) were identified as S. epidermidis. The detection of the mecA and ileS-2 genes, determined by multiplex-PCR, showed that 72 (64.2%) CNS strains possessed the mecA gene, while 16 (14.3%) possessed the ileS-2 gene. Fifteen of these strains presented the two resistance genes simultaneously. The isolates containing the ileS-2 gene presented a minimum inhibitory concentration (MIC) >1024 microg/mL in the E-test, while low-level mupirocin resistance (MICs of 12-16 microg/mL) was observed in those strains without ileS-2. The resistances to high and low levels of mupirocin could not be distinguished when the DD test was used. The analysis of the Mup(R) S. epidermidis strains by Pulsed Field Gel Electrophoresis showed that 17 (80.9%) strains belonged to one of two patterns (A and B), which have been shown to be prevalent in hospitals in Rio de Janeiro. This report showed that the PCR method for detection of oxacillin and mupirocin resistance in CNS is necessary to determine accurate rates of these resistance, and will can help in the staphylococcal infections prevention and control policies in Brazil.

Opportunities for mupirocin calcium cream in the emergency department

Mupirocin calcium cream is a newly reformulated topical antibiotic with a bactericidal spectrum specific for the pathogens that frequently cause secondary infections in superficial wounds. Both the calcium cream and ointment formulations have demonstrated efficacy in the treatment of secondarily infected traumatic lesions and dermatoses, including eczema, burns, wounds, bites, and ulcers. Mupirocin has a low risk of systemic and topical complications. To date, antimicrobial resistance is rare among target pathogens. The use of mupirocin to treat secondary wound infection has a profile of high efficacy and does not impair the normal healing in traumatized skin.

Characterization of isoleucyl-tRNA synthetase from Staphylococcus aureus. II. Mechanism of inhibition by reaction intermediate and pseudomonic acid analogues studied using transient and steady-state kinetics

The interactions of isoleucyl-tRNA synthetase (IleRS, E) from Staphylococcus aureus with both intermediate analogues and pseudomonic acid (PS-A) have been investigated using transient and steady-state techniques. Non-hydrolyzable analogues of isoleucyl-AMP (I) were simple competitive inhibitors (Ile-ol-AMP, Ki = 50 nM and Ile-NHSO2-AMP, Ki = 1 nM;). PS-A (J) inhibits IleRS via a slow-tight binding competitive mechanism where E.J (Kj = approximately 2 nM), undergoes an isomerization to form a stabilized E*.J complex (K*j = 50 pM). To overcome tight-binding artifacts when K*j << [E], K*j values were estimated from PPi/ATP exchange where [S] >> Km, thus raising K*j,app well above [E]. Using [3H]PS-A, it was confirmed that binding occurs with 1:1 stoichiometry and is reversible. Formation of inhibitor complexes was monitored directly through changes in enzyme tryptophan fluorescence. For Ile-ol-AMP and Ile-NHSO2-AMP, the fluorescence intensity of E.I was identical to that when E.Ile-AMP forms catalytically. Binding of PS-A induced only a small change in IleRS fluorescence that was characterized using transient kinetic competition. SB-205952, a PS-A analogue, produced a 37% quenching of IleRS fluorescence upon binding as a result of radiationless energy transfer. Inhibitor reversal rates were obtained by measuring relaxation between spectroscopically different complexes. Together, these data represent a comprehensive solution to the kinetics of inhibition by these compounds.

Methicillin-resistant Staphylococcus aureus versus the burn patient

Methicillin-resistant Staphylococcus aureus (MRSA) has become a frequent cause of nosocomial infection, its increasing prevalence posing serious therapeutic and infection control problems within the hospital environment. MRSA is a major challenge to the burn patient, with potential to cause significant morbidity and mortality. Burn patients have been shown to become colonised and infected more readily than other patient groups. Extensive burn injuries are particularly susceptible to infection as a result of the disruption of the normal skin barrier and accompanying depression of immune responses. Extended hospitalisation and antibiotic therapy have been identified as additional risk factors for MRSA carriage and infection. Microbial surveillance, epidemiological studies and the introduction of strict infection control regimes can reduce the prevalence of MRSA but may be insufficient for eradication or prevention of outbreak situations. Recognition of the clinical importance of MRSA to the burn patient highlights the need to take appropriate measures to minimise transmission and infection in this vulnerable group of patients.

Infection with methicillin-resistant Staphylococcus aureus after carbon dioxide resurfacing of the face. Successful treatment with minocycline, rifampin, and mupiricin ointment

BACKGROUND

The incidence of cutaneous bacterial infection after carbon dioxide (CO2 laser resurfacing is increasing. Patients with staphylococcal colonization of their anterior nares may be at greater risk for postoperative cutaneous colonization and/or infection, which can potentially cause scarring.

OBJECTIVE

We present a case report of methicillin-resistant Staphylococcus aureus secondary infection of the skin after CO2 laser resurfacing. We discuss the possible etiologies of this patient's infection, her postoperative management, and preoperative suggestions for possibly preventing infection.

METHODS

A 49-year-old woman was treated with CO2 laser resurfacing for moderate actinic damage and facial rhytides. She developed a cutaneous infection with methicillin-resistant S. aureus, which caused diffuse linear scarring on her cheeks and upper lip.

RESULTS

The patient was successfully treated with oral minocycline, rifampin, and topical mupiricin ointment to her cutaneous erosions.

CONCLUSIONS

We propose that it would be helpful for patients undergoing CO2 laser resurfacing to have their nares cultured to see if they are staphylococcal carriers. If a patient is found to be a carrier, mupiricin ointment can be used preoperatively treat to the nares, to help decrease the risk of infection of the skin from this potential source.

Mupirocin resistance and methicillin-resistant Staphylococcus aureus (MRSA)

Mupirocin has become the topical agent of choice for the elimination of methicillin-resistant Staphylococcus aureus (MRSA) carriage. The increased use of this antibiotic has been followed by reports of outbreaks due to MRSA with both low- and high-level resistance. Whilst low-level resistance is becoming more widespread, it is unlikely to have a major impact upon current practice. High-level resistance is plasmid borne, and although uncommon, can lead to problems with elimination especially in an outbreak situation. Alternatives are available but uncertainty exists as to their efficacy and safety. Any strategy to limit the increase of mupirocin resistance in MRSA should emphasize the importance of controlled antibiotic use both for mupirocin and other agents.

Identification of chromosomal location of mupA gene, encoding low-level mupirocin resistance in staphylococcal isolates

Low- and high-level mupirocin resistance have been reported in Staphylococcus aureus. The expression of plasmid-encoded mupA is responsible for high-level mupirocin resistance. Low-level mupirocin-resistant strains do not contain plasmid-encoded mupA, and a chromosomal location for this gene has not previously been reported. We examined high- and low-level mupirocin-resistant S. aureus strains to determine if mupA was present on the chromosome of low-level-resistant isolates. Southern blot analysis of DNA from four mupirocin-resistant strains identified mupA in both high- and low-level mupirocin-resistant strains. Low-level mupirocin-resistant strains contained a copy of mupA on the chromosome, while the high-level mupirocin-resistant isolate contained a copy of the gene on the plasmid. PCR amplification of genomic DNA from each mupirocin-resistant strain resulted in a 1.65-kb fragment, the predicted product from the intragenic mupA primers. This is the first report of a chromosomal location for the mupA gene conferring low-level mupirocin resistance.

The chemistry of pseudomonic acid. 17. Dual-action C-1 oxazole derivatives of pseudomonic acid having an extended spectrum of antibacterial activity

A series of C-1 oxazole isosteres of pseudomonic acid A (mupirocin) bearing a nitroheterocycle have been synthesized, and significant differences in both spectrum of activity and potency were found between these derivatives and mupirocin. Additionally, the antibacterial potency of two members of this class of compounds against mupirocin-resistant staphylococci could not be accounted for solely by inhibition of the target enzyme isoleucyl-tRNA synthetase (IRS), indicating an additional mode of action. The most potent compound, the nitrofuran 3f (SB 205952), was the most electron affinic derivative prepared and was transformed by NAD(P)H-dependent bacterial reductases at a rate similar to that for nitrofurantoin. The second mode of action of this compound may therefore arise from its reduction to a species with cellular targets other than IRS. In in vivo studies, 3f was shown to be a very effective agent by both the subcutaneous and oral routes of administration.

An open study of mupirocin in Libyan patients with skin infections

In an open study in Libya 48 patients suffering from skin infections, usually caused by staphylococci and streptococci, were treated with a new topical antibiotic, mupirocin, applied three times daily for 7 days. By day 7 of treatment 93.5% of patients were asymptomatic and were considered cured, and 100% were considered cured 1 week later. In total, 50 bacterial strains were isolated, of which, 86% were staphylococci and 14% were streptococci. Neither adverse events nor cutaneous reactions were reported. Mupirocin 2% ointment proved to be effective and safe in the treatment of primary and secondary skin infections.

Managing Staphylococcus aureus catheter infection in continuous ambulatory peritoneal dialysis patients

A case of Staphylococcus aureus catheter infection in a patient on peritoneal dialysis is presented and discussed by nephrologists, a social worker, a nurse specializing in the care of peritoneal dialysis patients, and the patient involved. The focus of the multidisciplinary case discussion concerns the management of S aureus catheter infections, including catheter removal, psychosocial issues and the patient's response to the need for catheter removal, the risk factors and prevention of S aureus catheter infections in peritoneal dialysis patients, and exit site care practices.

The efficacy of intranasal mupirocin in the prevention of staphylococcal infections: a review of recent experience

Staphylococcal infections remain an important cause of morbidity and mortality. Methicillin-resistant Staphylococcus aureus (MRSA) present a particular problem because of the costs of treatment and containing outbreaks. The role of nasal carriage of staphylococci in the epidemiology of staphylococcal infection has been recognized for over 30 years. Until recently, eradication of nasal carriage of S. aureus has proved difficult, with a variety of topical and systemic agents yielding poor results with either little discernible effect on nasal carriage or rapid recolonization. Mupirocin is a novel topical antibiotic with excellent antibacterial activity against staphylococci, including MRSA. Intranasal administration of calcium mupirocin has achieved excellent results in the eradication of nasal carriage of S. aureus and producing an associated reduction in S. aureus infection in a variety of clinical settings, including MRSA outbreaks, neonatal nurseries, haemodialysis, cardiothoracic surgery and familial staphylococcal infections. This article reviews the efficacy and safety of intranasal mupirocin in the prevention of staphylococcal infections.

Treatment of Staphylococcus aureus nasal carriers in continuous ambulatory peritoneal dialysis with mupirocin: long-term results

We present the clinical results of a prospective protocol of the treatment of Staphylococcus aureus nasal carriers (SANCs) in our continuous ambulatory peritoneal dialysis unit with mupirocin (Bactroban, SmithKline Beecham Pharmaceuticals, Philadelphia, PA). We monitored the incidence of peritonitis and catheter exit-site infection, the rate of infection-related catheter loss, and the degree of association between SANC state and S aureus infection. The study group included 94 patients with a follow-up of 1,097 patient-months (phase B). The same information was retrospectively collected among 74 continuous ambulatory peritoneal dialysis patients treated during the 24 months preceding the study period (follow-up of 1,043 patient-months) (phase A). S aureus nasal carriage was observed in 47.5% of the patients. Mupirocin was very effective in eradicating S aureus from the nares, but most patients required periodic retreatment. The incidence of S aureus peritonitis decreased from 1 episode/58 patient-months in phase A to 1 episode/548 patient-months in phase B, and the incidence of exit-site infection decreased from one episode/55 patient-months in phase A to 1 episode/137 patient-months in phase B. However, there was a simultaneous increase in the incidence of infections by other gram-positive and -negative bacteria. The rate of catheter loss after peritonitis (P = not significant) or exit-site infection (P < 0.05) tended to decrease from phase A to phase B. Seventy-seven percent of the peritonitis infections and 74% of the exit-site infections by S aureus occurred in SANCs.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of a broad-spectrum antibiotic (mupirocin) in an in vitro model of infected skin

The microbicidal effect of mupirocin (Bactroban) was studied using an in vitro model of burn eschar in contact with this antimicrobial cream, to indicate its inhibiting action after 1, 2, 4, and 24 h. The microorganisms used were 20 isolates of Ps. aeruginosa, 20 isolates of MRSA, 10 isolates of Staph. epidermidis, 12 isolates of Enterobacteriaceae and eight isolates of Candida albicans. There was a marked effect on Gram-negative bacilli (< 0.1 per cent surviving) by 4 h, whereas on Gram-positive organisms its effect was slower, principally on MRSA with 2.2 per cent survivors by 24 h. The action on Candida was very slow initially, but after 24 h, the mupirocin's effect was marked (< 0.1 per cent survivors). The studies showed that mupirocin (a topical broad-spectrum antimicrobial agent) can be used in vitro against Gram-positive and Gram-negative bacteria and yeasts, which contaminate skin and mucosa. Further clinical experience is required before mupirocin can be used to treat colonized or infected wounds in burned patients.

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

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 < or = 4 micrograms/ml), four intermediate level-resistant (MICs between 8 and 256 micrograms/ml), and four highly resistant (MICs > or = 512 micrograms/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 mupirocin-resistant 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.

Postoperative complications due to methicillin-resistant staphylococcus aureus (MRSA) in an elderly patient: management and control of MRSA

An elderly lady was admitted to hospital for elective resection of an adenocarcinoma of the colon. Following an anastomotic leak she developed intra-abdominal sepsis and underwent abdominal drainage of pus. During recovery from her second operation, she developed pneumonia and a bacteraemia due to methicillin-resistant Staphylococcus aureus (MRSA). She was treated with vancomycin and co-trimoxazole and survived without further sequelae. Details of the development and treatment of this case are discussed. Procedures for the control and eradication of MRSA infections in hospitals are reviewed.