Is Vancomycin MIC “Creep” Method Dependent? Analysis of Methicillin-Resistant Staphylococcus aureus Susceptibility Trends in Blood Isolates from North East Scotland from 2006 to 2010

This study investigated "creep" in vancomycin and daptomycin MICs among methicillin-resistant Staphylococcus aureus (MRSA) isolates from blood cultures over a 5-year period in a hospital in the United Kingdom, using different susceptibility testing methods. Trends in vancomycin and daptomycin susceptibility were evaluated by using Etest performed prospectively on isolates in routine clinical practice from December 2007 to December 2010 (n = 102). Comparison was made to results from prospective testing of subcultures at the Scottish MRSA Reference Laboratory, using an automated system (Vitek 2) and retrospective testing (Etest and CLSI reference broth microdilution [BMD] method) of stored isolates from 2006 to 2010 (n = 208). Spearman's rank correlations revealed a significant increase in vancomycin MIC (P = 0.012) and a significant decrease in daptomycin MIC (P = 0.03) by year of study for Etest results from the time of isolation. However, neither trend was replicated in MICs from automated or retrospective testing. The Friedman test revealed a significant difference between vancomycin MICs obtained from the same samples by different testing methods (χ(2) [3 degrees of freedom] = 97; P < 0.001). MICs from automated testing and Etest analysis of stored isolates were significantly lower than those from Etest analysis at the time of isolation for both antibiotics (P < 0.001). Effects of storage on the MIC appeared within the first 6 months of storage. Inconsistent evidence on vancomycin MIC creep and the relevance of the MIC to clinical outcome may arise from differences in susceptibility testing methods, including storage of isolates. There is a need to standardize and streamline susceptibility testing of vancomycin against MRSA.     

Trends in the susceptibility of methicillin-resistant Staphylococcus aureus to nine antimicrobial agents,including ceftobiprole,nemonoxacin, and tyrothricin: results from the Tigecycline In Vitro Surveillance in Taiwan (TIST) study, 2006–2010

This study investigated the in vitro susceptibilities of methicillin-resistant Staphylococcus aureus (MRSA) to nine antimicrobial agents in Taiwan. A total of 1,725 isolates were obtained from 20 hospitals throughout Taiwan from 2006 to 2010. The minimum inhibitory concentrations (MICs) of the nine agents were determined by the agar dilution method. The MICs of mupirocin and tyrothricin were determined for 223 MRSA isolates collected from 2009 to 2010. For vancomycin, 99.7 % were susceptible; however, 30.0 % (n?=?517) exhibited MICs of 2 μg/ml and 0.3 % (n?=?6) demonstrated intermediate susceptibility (MICs of 4 μg/ml). Nearly all isolates (≥99.9 %) were susceptible to teicoplanin, linezolid, and daptomycin. The MIC₉₀ values were 2 μg/ml for ceftobiprole and 1 μg/ml for nemonoxacin. The MIC₉₀ values of mupirocin and tyrothricin were 0.12 and 4 μg/ml, respectively. MIC creep was noted for daptomycin during this period, but not for vancomycin, teicoplanin, linezolid, or tigecycline. For isolates with vancomycin MICs of 2 μg/ml, the MIC₉₀ values were 2 μg/ml for teicoplanin, 0.5 μg/ml for daptomycin, and 0.5 μg/ml for tigecycline. Those values were four- to eight-fold higher than those among isolates with vancomycin MICs of 0.5 μg/ml (2, 0.06, and 0.12 μg/ml, respectively). Of the nine MRSA isolates exhibiting non-susceptibility to vancomycin (n?=?6), teicoplanin (n?=?1), daptomycin (n?=?2), or tigecycline (n?=?1), all had different pulsotypes, indicating the absence of intra-hospital or inter-hospital spread. The presence of a high proportion of MRSA isolates with elevated MICs (2 μg/ml) and MIC creep of daptomycin might alert clinicians on the therapy for serious MRSA infections in Taiwan.     

Evaluation of vancomycin MIC creep in methicillin-resistant Staphylococcus aureus infections—a systematic review and meta-analysis

ObjectivesVancomycin is currently the primary option treatment for methicillin-resistant Staphylococcus aureus (MRSA). However, an increasing number of MRSA isolates with high MICs, within the susceptible range (vancomycin MIC creep), are being reported worldwide. Resorting to a meta-analysis approach, this study aims to assess the evidence of vancomycin MIC creep.MethodsWe searched for studies in the PubMed database. The inclusion criteria for study eligibility included the possibility of retrieving the reported data values of vancomycin MIC and information concerning the applied MIC methodology.ResultsThe mean values of vancomycin MICs, of all 29 234 S. aureus isolates reported in the 55 studies included in the meta-analysis, were 1.23 mg/L (95% CI 1.13–1.33) and 1.20 mg/L (95% CI 1.13–1.28) determined by Etest and broth microdilution method, respectively. No significant differences were observed between these two methodologies. We found negative correlation between pooled mean/pooled proportion and time strata.ConclusionsWe have found no evidence of the MIC creep phenomenon.     

Prevalence and Genetic Mechanisms of Antimicrobial Resistance in Staphylococcus Species: A Multicentre Report of the Indian Council of Medical Research Antimicrobial Resistance Surveillance Network

Purpose: Routine surveillance of antimicrobial resistance (AMR) is an essential component of measures aimed to tackle the growing threat of resistant microbes in public health. This study presents a 1-year multicentre report on AMR in Staphylococcus species as part of Indian Council of Medical Research-AMR surveillance network. Materials and Methods: Staphylococcus species was routinely collected in the nodal and regional centres of the network and antimicrobial susceptibility testing was performed against a panel of antimicrobials. Minimum inhibitory concentration (MIC) values of vancomycin (VAN), daptomycin, tigecycline and linezolid (LNZ) against selected methicillin-resistant Staphylococcus aureus (MRSA) isolates were determined by E-test and MIC creep, if any, was determined. Resistant genotypes were determined by polymerase chain reaction for those isolates showing phenotypic resistance. Results: The prevalence of MRSA was found to be range from moderate (21%) to high (45%) among the centres with an overall prevalence of 37.3%. High prevalence of resistance was observed with commonly used antimicrobials such as ciprofloxacin and erythromycin in all the centres. Resistance to LNZ was not encountered except for a single case. Full-blown resistance to VAN in S. aureus was not observed; however, a few VAN-intermediate S. aureus isolates were documented. The most common species of coagulase negative staphylococci (CoNS) identified was Staphylococcus haemolyticus and Staphylococcus epidermidis. Resistance among CoNS was relatively higher than S. aureus. Most phenotypically resistant organisms possessed the corresponding resistance genes. Conclusion: There were localised differences in the prevalence of resistance between the centres. The efficacy of the anti-MRSA antimicrobials was very high; however, almost all these antimicrobials showed evidence of creeping MIC.     

Vancomycin MIC creep in MRSA blood culture isolates from Germany: a regional problem?

The aim of this study was to assess the vancomycin MIC distribution for MRSA blood culture isolates over a period of six years in Germany. The study examined 287 MRSA isolates from blood cultures collected at several hospitals in two German cities between 2004 and 2009. The vancomycin MIC was determined by Etest. Genotypic features of the MRSA strains with vancomycin MIC ≥ 1 mg/L were determined by semiautomated repetitive-sequence-based polymerase chain reaction. The range of vancomycin MIC as determined by Etest was 0.25 to 2.0 mg/L. The geometric mean MIC increased by 1.34-fold in city A over the study period (p < 0.05), but there was no meaningful change in city B (a 1.09-fold increase, p > 0.05). Furthermore, in city A a shift in vancomycin MICs occurred as an increase in the percentage of isolates with MIC ≥ 1 mg/L from period one (2004–2006) to period two (2007–2009) (p < 0.0001). Typing results showed that in city A a single clone was predominant (55% of the creep isolates). In this study, the creep phenomenon seems to be a regional problem. We suggest that all hospitals should monitor their local status of elevated vancomycin MICs in invasive MRSA isolates.     

Nasal carriage of Methicillin-resistant Staphylococcus aureus among healthy population of Kashmir,India

Background: Nasal colonisation with community acquired methicillin resistant Staphylococcus aureus (CA-MRSA) is being increasingly reported, especially in places where people are in close contact and where hygiene is compromised. The aim of this study was to find out prevalence of methicillin resistant S.aureus (MRSA) colonising anterior nares of healthy subjects. Materials and Methods: Nasal swabs of healthy subjects were collected aseptically and cultured using standard microbiological protocols. Antibiotic susceptibility was done by Kirby-Bauer disc diffusion method according to CLSI guidelines. Methicillin resistance was detected by cefoxitin disc diffusion method and confirmed by minimum inhibitory concentration (MIC) and amplification of mecA gene by PCR. Strain typing of MRSA strains was done by PFGE. Results: Out of 820 samples, S.aureus was isolated from 229 (27.92%) subjects. Of the 229 isolates, 15 were methicillin resistant. All S. aureus isolates were susceptible to vancomycin. Nasal carriage of MRSA was found to be 1.83% among healthy population. The isolates were found to be polyclonal by PFGE analysis. Conclusion: High prevalence of MRSA is a cause of concern and strategies to interrupt transmission should be implemented.     

Phenotypic changes of methicillin-resistant <Emphasis Type="Italic">Staphylococcus aureus</Emphasis> during vancomycin therapy for persistent bacteraemia and related clinical outcome

Persistent bacteraemia (PB) due to methicillin-resistant Staphylococcus aureus (MRSA) that fails to respond to glycopeptide therapy is a well-documented clinical problem. There are limited data on changes in agr functionality, vancomycin susceptibility and heteroresistance during MRSA PB. Thus, the frequency of these changes and their clinical significance remain unclear. Only patients with MRSA PB (≥7 days) from a prospective cohort of S. aureus bacteraemia were included. We collected isogenic paired strains and compared vancomycin MIC, vancomycin heteroresistance, and agr functionality between initial and final blood isolates. We also assessed the clinical outcome. A total of 49 patients had MRSA PB over 22 months. Bacteraemia persisted for a median of 13 days and most patients (98%) received glycopeptide as initial therapy. Among 49 isogenic pairs, only one pair showed a vancomycin MIC increase ≥2-fold by broth microdilution method, and only seven (14%) by E-test. Significant portions of initial isolates had vancomycin heteroresistance (49%) and agr dysfunction (76%). Development of vancomycin heteroresistance during PB occurred in four (16%) among 25 initial vancomycin-susceptible isolates, and acquisition of agr dysfunction occurred in two (16%) among 12 initial agr-functional isolates. Changes in the opposite direction occasionally occurred. These phenotypic changes during PB were not associated with mortality, whereas agr dysfunction of the initial isolates was significantly associated with mortality. During MRSA PB, phenotypic changes of MRSA isolates occurred occasionally under prolonged vancomycin exposure but were not significantly associated with clinical outcome. In contrast, initial agr dysfunction could be a predictor for mortality in MRSA PB.     

Differences in vancomycin MIC among MRSA isolates by agar dilution and E test method

In this study, the correlation between vancomycin minimum inhibitory concentration (MIC) obtained by the E test technique and the Clinical And Laboratory Standards Institute (CLSI) agar dilution method was evaluated. A total of 53 Methicillin Resistant Staphylococcus aureus (MRSA) strains were tested by both the methods in the present study. MICs of vancomycin obtained by the E test method were consistently higher (+0.5 to 2 log2 dilutions) than those obtained by the agar dilution method. Out of 53 MRSA isolates, 49 isolates showed higher MIC results by E test than by agar dilution method. Three isolates showed same MIC result by both methods. Since many studies have demonstrated increased clinical failure with MRSA isolates for which vancomycin MICs are increased (>1 µg/ml) but still within the susceptibility range (≤ 2 µg/ml), our findings suggest the requirement to re-look into the breakpoints for vancomycin for determining sensitivity of MRSA isolates. Guidelines should also specify the method to be used for determining the MIC.     

Prevalence and antimicrobial susceptibility pattern of methicillin resistant Staphylococcus aureus: a multicentre study

Purpose: Methicillin resistant Staphylococcus aureus (MRSA) is an important nosocomial pathogen. We report the prevalence and antibiotic susceptibility pattern of MRSA in major southern districts of Tamilnadu. Methods: A total of 7172 clinical specimens and 1725 carrier screening samples were collected from different centers and subjected to MRSA screening using conventional microbiological methods. Subsequently the antibiotic sensitivity test was performed for the confirmed MRSA isolates. Results: Out of 906 strains of S. aureus isolated from clinical and carrier samples, 250 (31.1%) and 39 (37.9%) were found to be methicillin resistant respectively. Almost all clinical MRSA strains (99.6%) were resistant to penicillin, 93.6% to ampicillin, and 63.2% towards gentamicin, co-trimoxazole, cephalexin, erythromycin, and cephotaxime. All MRSA strains (100%) of carrier screening samples had resistance to penicillin and about 71.8% and 35.9% were resistant to ampicillin and co-trimoxazole respectively. Multidrug resistance was observed among 63.6% of clinical and 23% of carrier MRSA isolates. However, all strains of clinical and carrier subjects were sensitive to vancomycin. Conclusion: The determination of prevalence and antibiotic sensitivity pattern of MRSA will help the treating clinicians for first line treatment in referral hospitals.     

Vancomycin susceptibility trends and prevalence of heterogeneous vancomycin-intermediate Staphylococcus aureus in clinical methicillin-resistant S. aureus isolates

Due to the rise in methicillin-resistant Staphylococcus aureus (MRSA) infections and widespread use of vancomycin, MRSA isolates with reduced susceptibility to vancomycin are emerging (i.e., MIC creep). However, the prevalence of heterogeneous vancomycin-intermediate S. aureus (hVISA) is unknown due to the difficulty in detecting this phenotype. Recently, Etest glycopeptide resistance detection (GRD) strips have been developed to detect hVISA. This study assessed vancomycin susceptibility in MRSA isolates and determined the prevalence of hVISA by Etest GRD and population analysis profile-area under the curve ratio (PAP-AUC). The genetic backgrounds of 167 MRSA isolates collected from 2000 to 2008 were identified by pulsed-field gel electrophoresis. Vancomycin MICs were determined using Etest and two broth microdilution assays, MicroScan and Sensititre. Etest GRD was performed on all isolates, and those exhibiting a hVISA phenotype were further tested by PAP-AUC. The vancomycin MIC modes remained consistent at 1 μg/ml, as assessed by Sensititre and MicroScan. Etest reported a significant increase (mode MIC = 1.5 μg/ml) in the MIC between 2000 and 2008 (P < 0.01); however, this increase did not reflect a ≥ 2-fold change. In addition, the slight MIC increase did not increase linearly from 2000 to 2008, suggesting biological fluctuation, and is inconsistent with the concept of MIC creep. Etest GRD identified six hVISA isolates, two of which were confirmed to be hVISA by PAP-AUC. In conclusion, reduced vancomycin susceptibility was not detected in our hospital over a 9-year period using three different MIC methodologies, and the hVISA incidence was 1.2%, as determined by Etest GRD and PAP-AUC.     

Impact of psm-mec in the mobile genetic element on the clinical characteristics and outcome of SCCmec-II methicillin-resistant Staphylococcus aureus bacteraemia in Japan

Over-expression of alpha-phenol-soluble modulins (PSMs) results in high virulence of community-associated methicillin-resistant Staphylococcus aureus (MRSA). The psm-mec gene, located in the mobile genetic element SCCmec-II, suppresses PSMαs production. Fifty-two patients with MRSA bacteraemia were enrolled. MRSA isolates were evaluated with regard to the psm-mec gene sequence, bacterial virulence, and the minimum inhibitory concentration (MIC) of vancomycin and teicoplanin. Fifty-one MRSA isolates were classified as SCCmec-II, and 10 had one point mutation in the psm-mec promoter. We compared clinical characteristics and outcomes between mutant MRSA and wild-type MRSA. Production of PSMα3 in mutant MRSA was significantly increased, but biofilm formation was suppressed. Wild-type MRSA caused more catheter-related bloodstream infections (30/41 vs. 3/10, p 0.0028), whereas mutant MRSA formed more deep abscesses (4/10 vs. 3/41, p 0.035). Bacteraemia caused by mutant MRSA was associated with reduced 30-day mortality (1/10 vs. 13/41, p 0.25), although this difference was not significant. The MIC₉₀ of teicoplanin was higher for wild-type MRSA (1.5 mg/L vs. 1 mg/L), but the MIC of vancomycin was not different between the two groups. The 30-day mortality of MRSA with a high MIC of teicoplanin (≥1.5 mg/L) was higher than that of strains with a lower MIC (≤0.75 mg/L) (6/10 vs. 6/33, p 0.017). Mutation of the psm-mec promoter contributes to virulence of SCCmec-II MRSA, and the product of psm-mec may determine the clinical characteristics of bacteraemia caused by SCCmec-II MRSA, but it does not affect mortality.     

Nationwide Survey Shows that Methicillin-Resistant Staphylococcus aureus Strains Heterogeneously and Intermediately Resistant to Vancomycin Are Not Disseminated throughout Japanese Hospitals

A total of 6,625 methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates obtained from 278 hospitals throughout Japan were obtained between November and December 1997 and were examined for their sensitivities to vancomycin using Mueller Hinton (MH), brain heart infusion (BHI), agar plates, or the broth microdilution method. A concentrated inoculum of an MRSA strain or the use of highly enriched medium, such as BHI medium, allows an individual cell to grow on agar plates containing a vancomycin concentration greater than the MIC for the parent strain. However, cells of the colonies which grew on BHI agar plates containing the higher vancomycin concentrations did not acquire a level of vancomycin resistance greater than that of the parent strain and were not subpopulations of heterogeneously vancomycin-resistant MRSA. There was no significance in the fact that these colonies grew on the higher concentration of vancomycin: none showed stable resistance to vancomycin at a concentration above the MIC for the parent strain, and no cell from these colonies showed a relationship between the MIC and the ability of these colonies to grow on higher concentrations of vancomycin. The vancomycin MIC was not above 2 microg/ml for any of the cells originating from these colonies. No Mu3-type heterogeneously resistant MRSA strains, which constitutively produce subpopulations from MRSA clinical isolates with intermediate vancomycin resistance at a high frequency, were detected. There was a unipolar distribution of the MICs ranging from 0.25 to 2 microg of vancomycin/ml among the 6,625 MRSA clinical isolates, indicating that there was no Mu50-type intermediately vancomycin-resistant MRSA (MIC, 8 microg/ml by National Committee for Clinical Laboratory Standards criteria) among the clinical isolates, and there was no evidence of dissemination of Mu3-type MRSA heteroresistant to vancomycin.     

Risk factors for treatment failure in orthopedic device-related methicillin-resistant Staphylococcus aureus infection

The purpose of this study was to determine the clinical and microbiological risk factors for treatment failure of methicillin-resistant Staphylococcus aureus (MRSA) orthopedic device-related infection (ODRI). A retrospective cohort study of patients with MRSA ODRI who were treated at Geneva University Hospitals between 2000 and 2008 was undertaken. Stored MRSA isolates were retrieved for genetic characterization and determination of the vancomycin minimum inhibitory concentration (MIC). Fifty-two patients were included, of whom 23 (44%) had joint arthroplasty and 29 (56%) had osteosynthesis. All 41 of the retrieved MRSA isolates were susceptible to vancomycin (MIC?≤?2 mg/L) and 35 (85%) shared genetic characteristics of the South German clone (ST228). During a median follow-up of 391 days (range, 4–2,922 days), 18 patients (35%) experienced treatment failure involving MRSA persistence or recurrence. Microbiological factors such as infection with the predominant clone and a vancomycin MIC of 2 mg/L were not associated with treatment failure. Using a Cox proportional hazards model, implant retention (hazard ratio [HR], 4.9; 95% confidence interval [CI], 1.3–18.2; P?=?0.017) and single-agent antimicrobial therapy (HR, 4.4; 95% CI, 1.2–16.3; P?=?0.025) were independent predictors of treatment failure after debridement. Therapy using a combination of antimicrobials should be considered for patients with MRSA ODRI, especially when implant removal is not feasible.     

Increased vancomycin MICs for Staphylococcus aureus clinical isolates from a university hospital during a 5-year period

Staphylococcus aureus is one of the most commonly isolated organisms in nosocomial infections. While the prevalence of methicillin-resistant S. aureus (MRSA) continues to increase worldwide, there is concern about an increase in vancomycin MICs among S. aureus strains. The prevalence of MRSA and vancomycin MIC trends in S. aureus from patients in a university hospital were analyzed. Clinical Laboratory Standards Institute (CLSI, formerly NCCLS) reference broth microdilution MIC testing was performed on all clinically relevant S. aureus isolates from January 2000 through December 2004. A total of 6,003 S. aureus isolates were analyzed. No vancomycin-resistant S. aureus isolates were detected. One MRSA isolate had a vancomycin MIC of 8 mug/ml and was confirmed as vancomycin-intermediate S. aureus. Among the 6,002 remaining isolates, a shift in vancomycin MICs from 相似文献   

Effects of Aggregate and Individual Antibiotic Exposure on Vancomycin MICs for Staphylococcus aureus Isolates Recovered from Pediatric Patients

We evaluated the evolution of vancomycin MICs for Staphylococcus aureus and their relationship with vancomycin use among hospitalized children. S. aureus isolates recovered from sterile sites were prospectively tested for vancomycin susceptibility using the Etest between 1 April 2000 and 31 March 2008. Vancomycin MICs were grouped into three categories: ≤1, 1.5, and 2 μg/ml. The association between vancomycin MICs and aggregate vancomycin use and individual patient vancomycin exposure 6 months prior to the documented infection was assessed. The geometric mean values for vancomycin MICs for S. aureus fluctuated over time without a significant trend (P = 0.146). Of the 436 patients included in the study, 363 (83%) had methicillin-susceptible S. aureus (MSSA) and 73 (17%) had methicillin-resistant S. aureus (MRSA) infections. The rate of isolates with a vancomycin MIC of 2 μg/ml increased from 4% (2 of 46) in 2000 to 2001 to 24% (11 of 46) in 2007 to 2008, despite a decrease in vancomycin use (r = −0.11; P = 0.825). The percentage of isolates with a vancomycin MIC of 2 μg/ml was higher for MRSA (15%; 11 of 73) than for MSSA strains (5.2%; 19 of 363) (χ² = 9.2; P = 0.01). Individual patient vancomycin exposure was not associated with a higher vancomycin MIC. In the unadjusted model, in which we compared patients with S. aureus infections with MICs of ≤1 μg/ml, the odds ratios of exposure rates for patients with isolates with MICs of 1.5 μg/ml and 2 μg/ml were 1.02 (P = 0.929) and 1.13 (P = 0.767), respectively. In our experience, the geometric means of vancomycin MICs from S. aureus isolates recovered from hospitalized children oscillated over time and were not associated with previous individual patient vancomycin exposure or aggregate vancomycin use.     

Detection of the classical G2576U mutation in linezolid resistant Staphylococcus aureus along with isolation of linezolid resistant Enterococcus faecium from a patient on short-term linezolid therapy: First report from India

Purpose: Linezolid is an effective drug against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). We describe the emergence of linezolid resistance in MRSA and VRE from India. Material and Methods: One MRSA and two VRE strains were isolated from a patient on linezolid therapy of one week duration. All three isolates were resistant to linezolid with minimal inhibitory concentrations (MIC) ≥4 mg/L. The 746-bp region flanking the possible G2576U mutation on the corresponding DNA from the 23S rRNA was amplified by polymerase chain reaction (PCR) and amplicons were sequenced for all the three isolates. Conjugation experiments using the linezolid resistant MRSA (LRMRSA) and linezolid resistant VRE (LRVRE) isolates as donors and wild strains of corresponding genera as recipients were performed. Results: The MRSA isolate had the classical G2576U mutation. High quality value scores in the sequencing software validated the mutation. Conjugation studies did not indicate presence of transferable resistance for linezolid. Sequencing did not indicate presence of any mutation in the two LRVRE isolates. Conclusions: This is the first report from India citing resistance in Staphylococcus and Enterococcus against Linezolid.     

High prevalence of class 1 integrons in clinical isolates of methicillin-resistant Staphylococcus aureus from India

Introduction: Class1 integrons are one of the prevalent mechanisms of antibiotic resistance gene transfer in Gram-negative organisms, but their prevalence and role in the spread of antibiotic resistance genes in methicillin-resistant Staphylococcus aureus (MRSA) is unexplored. The purpose of this study was to investigate the prevalence of class 1 integrons in clinical isolates of MRSA. Materials and Methods: Total 143 MRSA isolates obtained from two different cities in India (Pune and Mumbai) were characterized by biochemical tests, and the antibiotic sensitivity was performed using the Clinical and Laboratory Standards Institute (CLSI) guidelines. The presence of class 1 integrons, sul1/qacEΔ1 region of class 1 integron and mecA gene among these isolates was determined by polymerase chain reaction (PCR). Results: All 143 isolates were mecA positive and coagulase-positive. Overall, 71% of the MRSA isolates carried class 1 integrons; 58% (45/77) of the isolates obtained from Mumbai and 85% (56/66) of the isolates from Pune carried class 1 integrons. In all, 39% of these isolates carried sul1/qacEΔ1 region, thus confirming the association of class 1 integrons with antibiotic resistance genes. Along with β-lactam antibiotics the MRSA isolates were resistant to several other antibiotics, with resistance to erythromycin, ciprofloxacin and trimethoprim-sulfamethoxazole being observed in 75%, 66% and 60% of the isolates, respectively. Conclusion: To the best of our knowledge, this is the first report of class 1 integrons in MRSA isolates from India. The study provides insights into the prevalence of a novel mechanism adapted by MRSA for the propagation of antibiotic resistance genes.     

Detection of various types of resistance patterns and their correlation with minimal inhibitory concentrations against clindamycin among methicillin-resistant Staphylococcus aureus isolates

Purpose: The macrolide lincosamide streptogramin B (MLS_B) family of antibiotics serves as an alternative for the treatment of skin and soft tissue infections caused by methicillin-resistant Staphylococcus aureus (MRSA). However, resistance to clindamycin too has emerged, which is of two types, inducible and constitutive. Therapeutic failure is common with inducible type of clindamycin resistance. This study was done to determine the various clindamycin resistance patterns in MRSA isolates and to compare them with minimal inhibitory concentration (MIC) of clindamycin. Materials and Methods: Fifty MRSA isolates were studied by disc approximation test (D test) to detect inducible iMLS_B resistance and MIC by agar dilution technique. Results: Of the 50 isolates, 34 were sensitive to both clindamycin and erythromycin. 16 isolates showed different sensitivity patterns; nine of these were positive for D zone indicating inducible iMLS_B resistance, five were positive for constitutive MLS_B resistance and two showed possible efflux mechanism for macrolide resistance. Out of the 34 sensitive isolates, 5 showed isolated colonies (subpopulation) inside the clindamycin-sensitive zone. When these sub-populations were tested further, two were constitutive MLS_B phenotypes, two were inducible iMLS_B and one was HD (hazy D zone), which is D⁺ with growth up to clindamycin disc (which is also considered as constitutive MLS_B phenotype). Seven isolates showed an MIC of ≥4 μg/ml to clindamycin in spite of being susceptible to both erythromycin and clindamycin by Kirby Bauer disc diffusion technique. Out of these seven isolates, five were those which grew as subpopulation inside the clindamycin-sensitive zone. Conclusion: Detection of iMLS_B resistance among MRSA helps to avoid treatment failure with clindamycin. Studying the subpopulation inside the clindamycin-sensitive zone raises the question of existence of hetero-resistance or some other mechanism, which needs further study.     

Prevalence and clinical implications of Staphylococcus aureus with a vancomycin MIC of 4 microg/ml in Korea

In addition to vancomycin-intermediate Staphylococcus aureus (VISA), S. aureus with a vancomycin MIC of 4 microg/ml has been reported to be the cause of therapeutic failure. This study was designed to determine the prevalence of methicillin-resistant S. aureus (MRSA) with a vancomycin MIC of 4 microg/ml and to clarify the clinical characteristics of infections caused by these isolates. During the 8-week period from April to May, 2001, 27 hospitals participated in a nationwide surveillance program for VISA and vancomycin-resistant S. aureus (VRSA) in Korea. After screening on brain-heart infusion agar containing 4 microg/ml of vancomycin as previously described, 100 isolates with confluent growth were tested. The medical records of the patients involved were reviewed. Even though VISA or VRSA was not detected among 3,756 MRSA isolates, 18 (0.5%) had a vancomycin MIC of 4 microg/ml. The infections in 12 of these patients, excluding 5 that were colonized, were 8 chronic osteomyelitis, 1 surgical site infection, 1 pneumonia, 1 intra-abdominal infection, and 1 catheter-related infection. Although 11 cases were exposed to glycopeptides for a long time (median 56 days), the site of infection became culture-negative in only 1 case. Two patients died of their S. aureus infections. MRSA with a vancomycin MIC of 4 microg/ml was rare. Chronic osteomyelitis was the most common type of infection, and prolonged exposure to glycopeptides was associated with reduced susceptibility to vancomycin.