Detection of methicillin and mupirocin resistance in staphylococcal hospital isolates with a touchdown multiplex polymerase chain reaction

Staphylococcal hospital isolates (n = 166) were tested in a touchdown multiplex-polymerase chain reaction assay for the identification of methicillin and mupirocin resistance and discrimination of S. aureus (femA gene) from coagulase negative staphylococci and other bacteria. All isolates harbored the 16SrDNA (Staphylococcus genus specific internal control) gene, and 130 (78 %) the mecA (methicillin resistance) gene. Fifty-seven (44 %) of these were determined as methicillin-resistant S. aureus, while the remaining 73 (56 %) were methicillin-resistant coagulase-negative staphylococci. Seventy-five (45 %) isolates harbored the ileS-2 (high-level mupirocin resistance) gene and were determined as mupirocin-resistant. This assay represents a simple, rapid, reliable approach for the detection and discrimination of methicillin-and mupirocin-resistant staphylococci.

The emergence of mupirocin resistance among clinical isolates of methicillin-resistant Staphylococcus aureus in Trinidad: a first report

The objective of the study was to investigate the trend of mupirocin resistance among methicillin-resistant Staphylococcus aureus (MRSA) in Trinidad. No premarketing susceptibility surveillance was ever done following the introduction of mupirocin in 1986. A total of 188 MRSA strains recovered over a 2-year period from various body sites were tested for mupirocin resistance via the disc diffusion method. The major sources of MRSA were surgical site infections (74.0%) and bloodstream infections (8.0%). High-level and low-level mupirocin resistance were detected in 26.1 and 44.1% of MRSA stains, respectively. Resistances to other non-beta-lactam antibiotics were also high. Ninety-eight percent of all MRSA were resistant to erythromycin. This was followed by resistance rates of 96.8, 95.2, 94.1, 93.6, and 93.1%, for gentamicin, ciprofloxacin, amikacin and tobramycin, co-trimoxazole, and tetracycline, respectively. No MRSA strains were found to be resistant to vancomycin, linezolid, and quinupristin-dalfopristin. The study showed that mupirocin resistance among Trinidadian MRSA strains was relatively high compared to that seen in other countries. Because of the increasing prevalence of MRSA at the San Fernando General Hospital (SFGH) and the apparently increasing resistance to mupirocin, frequent monitoring of MRSA susceptibility patterns and infection control initiatives may be helpful in reducing the incidence of MRSA with a concomitant decrease in mupirocin resistance. This report is the first after 20 years of continuous use of the drug at the SFGH.

Multiple mechanisms to ameliorate the fitness burden of mupirocin resistance in Salmonella typhimurium

We examined how the fitness costs of mupirocin resistance caused by mutations in the chromosomal isoleucyl-tRNA synthetase gene (ileS) can be ameliorated. Mupirocin-resistant mutants were isolated and four different, resistance-conferring point mutations in the chromosomal ileS gene were identified. Fifty independent lineages of the low-fitness, resistant mutants were serially passaged to evolve compensated mutants with increased fitness. In 34/50 of the evolved lineages, the increase in fitness resulted from additional point mutations in isoleucine tRNA synthetase (IleRS). Measurements in vitro of the kinetics of aminoacylation of wild-type and mutant enzymes showed that resistant IleRS had a reduced rate of aminoacylation due to altered interactions with both tRNAIle and ATP. The intragenic compensatory mutations improved IleRS kinetics towards the wild-type enzyme, thereby restoring bacterial fitness. Seven of the 16 lineages that lacked second-site compensatory mutations in ileS, showed an increase in ileS gene dosage, suggesting that an increased level of defective IleRS compensate for the decrease in aminoacylation activity. Our findings show that the fitness costs of ileS mutations conferring mupirocin resistance can be reduced by several types of mechanisms that may contribute to the stability of mupirocin resistance in clinical settings.

Thematic review series: skin lipids. Antimicrobial lipids at the skin surface

The skin surface represents our interface with the external environment, and as such, is our first line of defense against microbial colonization and infection. Lipids at the skin surface are thought to underlie at least part of an antimicrobial barrier. Some of these lipids are synthesized in the epidermis and are carried to the surface as cells differentiate, whereas others are secreted onto the surface from the sebaceous glands. One such group, free sphingoid bases, are known to have broad antimicrobial activity, and our previous studies demonstrate their presence at the skin surface. Free sphingoid bases may be generated by enzymatic hydrolysis of epidermal ceramides. In addition, our preliminary results demonstrate potent antibacterial activity associated with two specific fatty acids derived from sebaceous triglycerides. Most remarkably, one of these fatty acids (sapienic acid, C16:1Delta6), in combination with a low concentration of ethanol, is very effective against methicillin-resistant Staphylococcus aureus (MRSA). In fact, this combination was far more effective than mupirocin with or without ethanol. Mupirocin is a "gold standard" for activity against MRSA.

Interpretive criteria for mupirocin susceptibility testing of Staphylococcus spp. using CLSI guidelines

Mupirocin is an antimicrobial agent commonly used to treat staphylococcal infection or to eliminate persistent carriage. To date, interpretive criteria have not been established to define susceptibility or resistance when performing mupirocin susceptibility testing. In this evaluation, using CLSI guidelines, a total of 502 staphylococci comprising 219 methicillin-sensitive Staphylococcus aureus, 222 methicillin-resistant S. aureus and 61 coagulase-negative staphylococci are tested by broth microdilution, disc diffusion and E-test. Disc diffusion using 5 microg mupirocin discs was found to be a reliable method to distinguish susceptible and resistant strains. Minimum inhibitory concentration (MIC) determination was required to differentiate low-level and high-level resistance to mupirocin. E-test was found to be an accurate alternative to broth microdilution for the routine determination of MIC values of staphylococci to mupirocin. Broth microdilution and disc-diffusion results were plotted on a scattergram, and error rates were calculated. No errors were found using susceptibility criteria of < 4 microg/mL (MIC) and > 19 mm (zone diameter).

Mupirocin-resistant, methicillin-resistant Staphylococcus aureus strains in Canadian hospitals

Mupirocin resistance in Staphylococcus aureus is increasingly being reported in many parts of the world. This study describes the epidemiology and laboratory characterization of mupirocin-resistant methicillin-resistant S. aureus (MRSA) strains in Canadian hospitals. Broth microdilution susceptibility testing of 4,980 MRSA isolates obtained between 1995 and 2004 from 32 Canadian hospitals was done in accordance with CLSI guidelines. The clinical and epidemiologic characteristics of strains with high-level mupirocin resistance (HLMup(r)) were compared with those of mupirocin-susceptible (Mup(s)) strains. MRSA strains were characterized by pulsed-field gel electrophoresis (PFGE) and typing of the staphylococcal chromosomal cassette mec. PCR was done to detect the presence of the mupA gene. For strains with mupA, plasmid DNA was extracted and subjected to Southern blot hybridization. A total of 198 (4.0%) HLMup(r) MRSA isolates were identified. The proportion of MRSA strains with HLMup(r) increased from 1.6% in the first 5 years of surveillance (1995 to 1999) to 7.0% from 2000 to 2004 (P < 0.001). Patients with HLMup(r) MRSA strains were more likely to have been aboriginal (odds ratio [OR], 3.7; 95% confidence interval [CI], 1.5 to 9.4; P = 0.006), to have had community-associated MRSA (OR, 2.2; 95% CI, 1.0 to 5.0; P = 0.05), and to have been colonized with MRSA (OR, 1.7; 95% CI, 1.0 to 3.0; P = 0.04). HLMup(r) MRSA strains were also more likely to be resistant to fusidic acid (21% versus 4% for mupirocin-susceptible strains; P < 0.001). All HLMup(r) MRSA strains had a plasmid-associated mupA gene, most often associated with a 9-kb HindIII fragment. PFGE typing and analysis of the plasmid profiles indicate that both plasmid transmission and the clonal spread of HLMup(r) MRSA have occurred in Canadian hospitals. These results indicate that the incidence of HLMup(r) is increasing among Canadian strains of MRSA and that HLMup(r) MRSA is recovered from patients with distinct clinical and epidemiologic characteristics compared to the characteristics of patents with Mup(s) MRSA strains.

Mupirocin resistance in patients colonized with methicillin-resistant Staphylococcus aureus in a surgical intensive care unit

BACKGROUND

Nasal colonization with methicillin-resistant Staphylococcus aureus (MRSA) can be a precursor to serious infection, and decolonization with topical mupirocin has been studied as a means of preventing clinical infection. Mupirocin resistance in patients with MRSA has been reported, usually in the context of widespread mupirocin use.

METHODS

Patients admitted to a surgical intensive care unit (SICU) had nasal swab cultures for MRSA performed at admission, weekly, and at discharge in an active surveillance program. Collected MRSA isolates were tested for mupirocin resistance, and molecular analysis was performed. Clinical data on the characteristics and outcomes of the patients who stayed in the SICU for >48 h were collected prospectively.

RESULTS

Of the 302 MRSA isolates available for testing, 13.2% were resistant to mupirocin, with 8.6% having high-level resistance (minimum inhibitory concentration, >or=512 microg/mL) and 4.6% having low-level resistance (minimum inhibitory concentration, 8-256 microg/mL). Patients admitted to the SICU for >48 h who were colonized with mupirocin-resistant MRSA were more likely to have been admitted to our hospital during the previous year (P=.016), were older (P=.009), and had higher in-hospital mortality (16% vs. 33%; P=.027), compared with patients colonized with mupirocin-susceptible MRSA. Molecular analysis of the mupirocin-resistant isolates revealed that 72.5% of isolates contained staphylococcal cassette chromosome mec II. Repetitive sequence polymerase chain reaction typing revealed that high-level mupirocin resistance was present in multiple clonal groups. The rate of mupirocin use hospital-wide during the study period was 6.08 treatment-days per 1000 patient-days.

CONCLUSIONS

We documented a high rate of mupirocin resistance in MRSA isolates from SICU patients, despite low levels of in-hospital mupirocin use.

In vivo activity of a novel polymeric guanidine in experimental skin infection with methicillin-resistant Staphylococcus aureus

The in vivo efficacy of the novel polymeric guanidine AKACID Plus was evaluated in a guinea pig model of experimental skin infection with methicillin-resistant Staphylococcus aureus (MRSA). Topical application of AKACID Plus at concentrations of > or =0.5% was as effective as mupirocin 2% cream in the treatment of superficial skin infection with MRSA.

Current options for the treatment of impetigo in children

Impetigo contagiosa is a common, superficial, bacterial infection of the skin characterised by an inflamed and infected epidermis caused by Staphylococcus aureus, Streptococcus pyogenes or both. The less common bullous impetigo is characterised by fragile fluid-filled vesicles and flaccid blisters, and is invariably caused by pathogenic strains of S. aureus. In bullous impetigo, exfoliative toxins are produced, although these are restricted to the area of infection and bacteria can be cultured from the blister contents. In the rare variant, staphylococcal scalded skin syndrome, the exfoliative toxins are spread haematogenously from a localised source causing widespread epidermal damage at distant sites.

[Mupirocin resistance in methicillin-resistant Staphylococcus aureus strains isolated from wound infections]

Methicillin-resistant Staphylococcus aureus (MRSA) has emerged as an important pathogen in community-acquired and nosocomial infections. The unique bactericidal action of mupirocin makes it one of the few antibiotics still effective against MRSA. The purpose of this study was to investigate the mupirocin resistance in MRSA strains isolated from wound infections of in- and out-patients of two distinct hospitals located in Ankara and Istanbul. A total of 143 MRSA strains were included in the study. Mupirocin resistance was investigated by Kirby-Bauer disk diffusion method and the results were confirmed by determination of the MIC values by E-test strips. Among 143 MRSA isolates, mupirocin resistance was detected by none of the methods, and overall mupirocin sensitivity was detected as 100 percent. The majority of mupirocin resistant MRSA is isolated from wound infections. The aetiology mostly depends on the increased topical use of the agent. The method used in the detection of mupirocin resistance and interpretation of the results are important parameters in the determination of mupirocin resistance in MRSA strains. Since there was no resistant strain among 143 clinical isolates obtained from two different hospitals, it was concluded that, mupirocin resistance is not an important problem in these regions currently, and mupirocin may be safely used in treating wound infections.

Activities of clindamycin, synercid, telithromycin, linezolid, and mupirocin against gram-positive coccal strains resistant to erythromycin in korea

The antibacterial activities of clindamycin, synercid, telithromycin, linezolid and mupirocin were evaluated against erythromycin-resistant Gram-positive coccal clinical isolates collected in Korean hospitals. In Staphylococcus aureus, synercid, linezolid and mupirocin were the most active agents. Against coagulase-negative staphylococci (CNS), synercid, linezolid and mupirocin were also active. Telithromycin and synercid resistance was common against enterococci, only linezolid and mupirocin were active. The reason of low activity of telithromycin against staphylococci and enterococci is because most of the isolates were constitutively resistant to erythromycin. Synercid, telithromycin, linezolid and mupirocin were active against streptococci.

Interpretive criteria to differentiate low- and high-level mupirocin resistance in Staphylococcus aureus

Meticillin-resistant Staphylococcus aureus isolates were classified into three mupirocin susceptibility groups by the disc diffusion method using 5 and 200 μg mupirocin discs. The zone diameter observed for a 5 μg disc distinguished MupS from the resistant strains (either MupRL or MupRH). On the other hand, a 200 μg disc distinguished the high-resistance MupRH strains from the other two (MupS or MupRL). Thus, the concomitant use of 5 and 200 μg mupirocin discs allowed the clear distinction among the three mupirocin susceptibility groups, MupS, MupRL or MupRH.

Aminoacyl-tRNA synthetases: essential and still promising targets for new anti-infective agents

The emergence of resistance to existing antibiotics demands the development of novel antimicrobial agents directed against novel targets. Historically, bacterial cell wall synthesis, protein, and DNA and RNA synthesis have been major targets of very successful classes of antibiotics such as beta-lactams, glycopeptides, macrolides, aminoglycosides, tetracyclines, rifampicins and quinolones. Recently, efforts have been made to develop novel agents against validated targets in these pathways but also against new, previously unexploited targets. The era of genomics has provided insights into novel targets in microbial pathogens. Among the less exploited--but still promising--targets is the family of 20 aminoacyl-tRNA synthetases (aaRSs), which are essential for protein synthesis. These targets have been validated in nature as aaRS inhibition has been shown as the specific mode of action for many natural antimicrobial agents synthesized by bacteria and fungi. Therefore, aaRSs have the potential to be targeted by novel agents either from synthetic or natural sources to yield specific and selective anti-infectives. Numerous high-throughput screening programs aimed at identifying aaRS inhibitors have been performed over the last 20 years. A large number of promising lead compounds have been identified but only a few agents have moved forward into clinical development. This review provides an update on the present strategies to develop novel aaRS inhibitors as anti-infective drugs.

Mupirocin resistance in Staphylococcus aureus in an Indian hospital

Two hundred Staphylococcus aureus strains collected from an Indian hospital were tested for mupirocin susceptibility using disc diffusion method and E-test. High-level and low-level mupirocin resistance was detected in 10 (5%) and 2 (1%) S. aureus strains, respectively. Pulsed-field gel electrophoresis analysis of the high-level mupirocin-resistant methicillin-resistant S. aureus isolates revealed the presence of 2 clones with the majority of strains belonging to 1 clone, suggesting clonal dissemination.

Transient loss of high-level mupirocin resistance in Staphylococcus aureus due to MupA polymorphism

Spontaneous loss of MupA-mediated high-level mupirocin resistance was observed in Staphylococcus aureus, although the isolate gave a PCR-positive test result for mupA. Sequencing of the mupA gene identified a single base-pair deletion that resulted in a frameshift mutation and loss of functional protein. Reversion to the wild-type allele and restoration of high-level resistance occurred with high frequency (>10(-6)), indicating the transient nature of MupA polymorphism.

In vitro activity of lysostaphin, mupirocin, and tea tree oil against clinical methicillin-resistant Staphylococcus aureus

Colonization of methicillin-resistant Staphylococcus aureus (MRSA) commonly leads to infection by the same strain. We examined the activity of lysostaphin, mupirocin, and tea tree oil against clinical MRSA (n = 98) isolates. MIC(50) (range) were as follows: lysostaphin, 0.125 mg/L (0.125-0.25); mupirocin, 0.5 mg/L (0.19-1024); tea tree oil, 1024 mg/L (512-2048). High- and low-level mupirocin resistance was noted in 9.2% of our MRSA isolates. Time kill results indicate MRSA activity at 24 h was lysostaphin = gentamicin = vancomycin (P <or= .001) > mupirocin > tea tree oil (P >or= .05). Checkerboard testing indicated a synergistic relationship between lysostaphin and mupirocin in combination with gentamicin. Antagonism was observed with the combination of vancomycin and tea tree oil; time kill studies confirmed this result. Decolonization options are limited and resistance to mupirocin exists. Lysostaphin and tea tree oil may offer additional therapeutic options for the decolonization of MRSA where current treatment alternatives are limited.

High-level mupirocin resistance within methicillin-resistant Staphylococcus aureus pandemic lineages

The methicillin-resistant Staphylococcus aureus (MRSA) population in the Hospital Universitario Nuestra Señora de Candelaria over a 5-year period (1998 to 2002) was marked by shifts in the circulation of pandemic clones. Here, we investigated the emergence of high-level mupirocin resistance (Hi-Mup(r)). In addition to clonal spread, transfer of ileS2-carrying plasmids played a significant role in the dissemination of Hi-Mup(r) among pandemic MRSA lineages.

[Positive screening for MRSA--clinical consequences?]

BACKGROUND

The worldwide rise of MRSA is equivalent to an increase of nasal colonization with MRSA. The objectives of this study were to investigate the rate of occult nasal MRSA colonization in trauma patients, to elucidate the role of MRSA carriers for endogenous infection (nose --> wound) and to check the efficiency of mupirocin therapy.

PATIENTS AND METHODS

A total of 643 consecutive trauma patients underwent MRSA screening (nasal swabs) on admission. At the same time all MRSA wound infections were registered and all isolates were analysed with PFGE (pulsed-field gel electrophoresis) to detect cross-infection between individuals.

RESULTS

In 13 patients (2.0%) we found MRSA in the nose and limited isolation as well as therapy with mupirocin were performed. No endogenous transmission of MRSA from the nose to the wound could be seen, and no cross-infection to other patients could be detected.

CONCLUSION

Our findings suggest that in our patients with nasal colonization the risk of intra- and interindividual transmission of MRSA is very small. Therefore, in trauma patients screening on admission does not seem to be absolutely necessary either for clinical or for epidemiological reasons.

Selection of retapamulin, a novel pleuromutilin for topical use

The in vitro activity of retapamulin was determined and compared to that of topical and community antibiotics. The MIC(90)s of retapamulin against Staphylococcus aureus and Streptococcus pyogenes were 0.12 microg/ml and 0.016 microg/ml, respectively. Retapamulin has a low propensity to select resistance and produces an in vitro postantibiotic effect.

Molecular analysis of isoleucyl-tRNA synthetase mutations in clinical isolates of methicillin-resistant Staphylococcus aureus with low-level mupirocin resistance

Emergence and spread of low-level mupirocin resistance in staphylococci have been increasingly reported in recent years. The aim of this study was to characterize missense mutations within the chromosomal isoleucyl-tRNA synthetase gene (ileS) among clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) with low-level mupirocin resistance. A total of 20 isolates of MRSA with low-level mupirocin resistance (minimal inhibitory concentration, 16-64 microg/mL) were collected from 79 patients in intensive care units for six months. The isolates were analyzed for isoleucyl-tRNA synthetase (IleS) mutations that might affect the binding of mupirocin to the three-dimensional structure of the S. aureus IleS enzyme. All isolates with low-level mupirocin resistance contained the known V588F mutation affecting the Rossman fold, and some of them additionally had previously unidentified mutations such as P187F, K226T, F227L, Q612H, or V767D. Interestingly, Q612H was a novel mutation that was involved in stabilizing the conformation of the catalytic loop containing the KMSKS motif. In conclusion, this study confirms that molecular heterogeneity in ileS gene is common among clinical MRSA isolates with low-level mupirocin resistance, and further study on clinical mutants is needed to understand the structural basis of low-level mupirocin resistance.

Risk factors for isolation of low-level mupirocin-resistant versus -susceptible methicillin-resistant Staphylococcus aureus from patients in intensive care units

OBJECTIVES

The aim of this study was to determine the risk factors for the recovery of low-level mupirocin-resistant (mup(r)) or -susceptible (mup(s)) MRSA from patients in intensive care units (ICUs).

METHODS

A case-case-control study was conducted from November 2003 to April 2004. Two case groups consisted of patients with low-level mup(r) MRSA and mup(s) MRSA. A control group was frequency matched.

RESULTS

Mup(r) MRSA and mup(s) MRSA were isolated from 20 to 51 patients, respectively, during a six-month period. Risk factors identified for mup(r) MRSA were as follows: exposure to piperacillin-tazobactam (odds ratio [OR] 13.8; 95% confidence intervals [CI], 1.8-105.0), third-generation cephalosporins (OR, 5.0; 95% CI, 1.6-15.5) and quinolones (OR, 3.4; 95% CI, 1.1-10.7). Risk factors identified for mup(s) MRSA were as follows: length of ICU stay (OR, 1.1; 95% CI, 1.0-1.1), surgery (OR, 3.7; 95% CI, 1.5-9.0), exposure to third-generation cephalosporins (OR, 8.4; 95% CI, 3.3-21.7) and quinolones (OR, 7.7; 95% CI, 2.8-21.3).

CONCLUSIONS

Our results suggest that nosocomial isolation of low-level mup(r) MRSA may be affected by piperacillin-tazobactam.

Spotlight on targeting aminoacyl-tRNA synthetases for the treatment of fungal infections

This month's Spotlight on... focuses on targeting aminoacyl-tRNA synthetases for the treatment of fungal infections. The emergence of drug-resistant strains of bacterial and fungal pathogens has led to the development of new antibiotic strategies. Aminoacyl-tRNA synthetases, crucial for protein synthesis, represent a new target for the treatment of different infectious diseases.

Antimicrobial and toxicological profile of the new biocide Akacid plus(R)

OBJECTIVES

Akacid plus is a new member of the polymeric guanidine family of disinfectants. It was especially developed to enhance the antimicrobial activity of this class with significantly less toxicity. The in vitro activity of Akacid plus compared with chlorhexidine digluconate and mupirocin was tested against a total of 369 recent clinical isolates.

METHODS

The organisms tested by CLSI reference methods included the following: Staphylococcus aureus (98), Staphylococcus epidermidis (9), Bacillus spp. (2), Enterococcus faecalis (32), Klebsiella spp. (45), Enterobacter spp. (20), Escherichia coli (65), Salmonella spp. (6), Shigella spp. (2), Yersinia enterocolitica (1), Acinetobacter spp. (4), Proteus spp. (7), Pseudomonas aeruginosa (59), Stenotrophomonas maltophilia (4), Candida spp. (10) and Aspergillus spp. (7). In vitro selection of resistance to Akacid plus was carried out on 24 strains. Toxicological analyses were also performed.

RESULTS

All tested agents were more effective against Staphylococcus spp. and Bacillus spp. than against E. faecalis and Gram-negative bacteria. The MIC90s of chlorhexidine and mupirocin showed a 4-fold and 32-fold increase for methicillin-resistant S. aureus in comparison with methicillin-susceptible strains, while MIC values of Akacid plus were similar for antibiotic-susceptible and multiresistant strains. Bactericidal action of Akacid plus was observed at 1-2x MIC. The in vitro selection of resistance test showed no increase in MIC values of Akacid plus for any isolate after 30 passages. In addition, Akacid plus showed low oral and dermal toxicity.

CONCLUSIONS

These preliminary results demonstrate the broad antimicrobial properties of Akacid plus, which makes it a promising tool for topical application in the prophylaxis and treatment of bacterial and fungal infections.