Comparison of the Velogene Rapid MRSA Identification Assay, Denka MRSA-Screen Assay, and BBL Crystal MRSA ID System for rapid identification of methicillin-resistant Staphylococcus aureus

Colorimetric method for rapid detection of Oxacillin resistance in Staphylococcus aureus and its comparison with PCR for mec A gene

Rapid and accurate detection of Methicillin Resistant Staphylococcus aureus (MRSA) is an important role of clinical microbiology laboratories to avoid treatment failure. The detection of MRSA is based on phenotypic assays which require at least 24 h to perform. Detection of the mecA gene or of PBP 2a is the "gold standard", but not always available. The aim of this study was to evaluate a rapid method for detection of MRSA by using 3 (4, 5 dimethyl thiazole -2-yl) -2, 5 diphenyl tetrazolium bromide (MTT). Total 126 isolates of MRSA were collected from tertiary healthcare center and were confirmed by oxacillin screening agar test as per CLSI guidelines. Amplification of mecA gene was performed by using PCR. MTT assay was carried out for all the isolates in 96 well Microtitre plate and compared with standard methods of CLSI. Out of 126 isolates, 98 were found to be mecA positive. MTT method was found to be 98.98% sensitive and 96.43% specific. The MTT based colorimetric method is rapid and simple test for screening of oxacillin resistance in Staphylococcus aureus. It significantly shortens the time to just 7 h required to obtained a drug susceptibility test and could be useful to screen MRSA.

Evaluation of three methods for detection of methicillin-resistant Staphylococcus aureus

AIM: To evaluate GenoType methicillin-resistant Staphylococcus aureus (MRSA) Direct assay and cultivation for the identification of MRSA by using mecA polymerase chain reaction (PCR) as the “gold standard” assay.

METHODS: In total of 61 nasal specimens from patients at the intensive care unit were studied by GenoType MRSA Direct test, conventional culture method and automated bacterial identification system. The results of GenoType MRSA Direct assay were compared to conventional culture method the identification of MRSA and mecA gene PCR as the “gold standard” method. The sensitivity, specificity, positive predictive value and negative predictive value were calculated.

RESULTS: In total, 61 specimens were studied. Fifty-four specimens (88.5%) were negative by all three methods. Six swabs (9.8%) were found positive by GenoType MRSA Direct test, conventional culture method and automated bacterial identification system. The presence of mecA in these strains was confirmed by PCR. One swab sample was negative for culture methods but MRSA and mecA gene were detected by GenoType MRSA Direct test and mecA PCR respectively. GenoType MRSA Direct test had a sensitivity of 100% (6/6) and a specificity of 100% (55/55), with a positive predictive value of 100% and a negative predictive value of 98%. Culture method of MRSA had a sensitivity of 83.3% (5/6) and a specificity of 98.2% (55/56).

CONCLUSION: It was found that the GenoType MRSA Direct assay, which is a rapid and accurate test, is of the same sensitivity and specificity with mecA PCR. The GenoType MRSA Direct assay can be a better tool for rapid and accurate detection of MRSA in diagnostic laboratories.

Direct detection of methicillin-resistant Staphylococcus aureus from blood cultures using an immunochromatographic immunoassay-based MRSA rapid kit for the detection of penicillin-binding protein 2a

Using an EZ-Step MRSA rapid kit, a novel screening test for methicillin-resistant Staphylococcus aureus (MRSA) that detects penicillin-binding protein 2a, 34 of 36 MRSA-positive clinical blood culture samples were positive on direct testing (sensitivity, 94.4%), whereas 21 of 21 methicillin-susceptible S. aureus-positive samples were negative (specificity, 100%).

Diagnosis and management of community-associated MRSA infections in children

The history of antibiotic resistance in Staphylococcus aureus spans more than half a century. Methicillin-resistant S. aureus (MRSA) has emerged as an almost ubiquitous pathogen in both the community and hospital settings. The predominant clone responsible for community-associated MRSA, USA300, is a highly successful pathogen, as demonstrated by its rapid global spread and associated morbidity and mortality. The management of MRSA infections in pediatric patients is complicated by the limited number of effective antibiotics that have been well-studied in children. The gold standard antimicrobial, vancomycin, has several shortcomings that have prompted the development of newer agents for the treatment of MRSA disease. Moreover, the emergence of vancomycin-intermediate or -resistant S. aureus, while uncommon, portends a potential new era of resistance that will require research and development of the next generation of antibiotics that act by novel mechanisms.

Antibacterial susceptibility testing in the clinical laboratory

This article familiarizes the clinician with the principles of bacterial susceptibility testing and reporting to facilitate communication with the clinical microbiology laboratory. As resistance continues to emerge among a wide range of clinically relevant bacteria, the complexity of this communication increases. This updated version provides an overview of the important susceptibility concerns for most commonly isolated bacterial pathogens.

Molecular Diagnostics and Comparative Genomics in Clinical Microbiology

Initially, the availability of molecular diagnostics was considered a panacea, but replacement of conventional tests for detection and identification of microorganisms by molecular procedures eventually gathered momentum. This chapter describes current state-of-the-art molecular diagnostics and comparative genomics in medical microbiology to provide an understanding of infectious disease over the coming years. Nucleic acid-based tests are being introduced with increasing speed into routine clinical microbiology laboratories. Some of the problems remaining to be solved prior to general acceptance of nucleic acid-mediated detection and identification of microbial pathogens are reviewed. Historic objections are slowly being taken apart, and an accelerated introduction of molecular diagnostics is being pursued in many cases. Clear improvement in clinical testing is achieved by introducing molecular tests. Therefore, swift introduction of such tests into clinical practice is important to be pursued. Several PCR tests show increased sensitivity, excellent specificity, and cost effectiveness highlighting the success of the novel applications in the field of bacterial infections. Finally, some of the problems remaining to be solved prior to general acceptation of nucleic acid-mediated detection and identification of microbial pathogens are also reviewed.

High density whole genome fingerprinting of methicillin-resistant and -susceptible strains of Staphylococcus aureus in search of phenotype-specific molecular determinants

AFLP is a selective restriction fragment amplification method generating DNA fingerprints for microbial isolates. We present high-throughput AFLP (htAFLP) to characterize molecular markers associated with bacterial phenotypes. Methicillin-resistant and -susceptible isolates of Staphylococcus aureus have been used for this model study in conjunction with the available S. aureus genome sequences. This facilitates the calculation of theoretical AFLP fingerprints, comparison of these fingerprints with genuine experimental fingerprints, and the subsequent identification of polymorphic AFLP markers without sequence analysis. Analysis of 46 MRSA and 46 MSSA strains by 39 different AFLP reactions generated more than 2500 fragments per strain and an overall number of 6180 scorable markers within all strains. We successfully identify MRSA specific markers and elaborate on the general applicability of the htAFLP approach. This method can be applied to any microbial species for which at least one full-genome sequence is available.

Control and prevention of MRSA infections

Methicillin-resistant Staphylococcus aureus (MRSA) has posed an immense problem for clinicians in the hospital setting for years, emerging as the most frequent nosocomial infection. To deal with this problem pathogen and others, infectious disease specialists have developed a variety of procedures for their control and prevention, involving options from preventative measures such as decolonization and isolation of MRSA-confirmed patients, to the more simple procedures of hand washing, expanding glove use, and reducing time in the hospital. With the realization that MRSA is now a community problem, there are expanded efforts toward more direct intervention, such as the use of anti-MRSA antibacterials and vaccines, in an attempt to reduce the overall burden of MRSA.

Evaluation of rapid screening and pre-emptive contact isolation for detecting and controlling methicillin-resistant Staphylococcus aureus in critical care: an interventional cohort study

INTRODUCTION

Rapid diagnostic tests may allow early identification of previously unknown methicillin-resistant Staphylococcus aureus (MRSA) carriers at intensive care unit (ICU) admission. The aim of this study was twofold: first, to assess whether a new molecular MRSA screening test can substantially decrease the time between ICU admission and identification of MRSA carriers; and, second, to examine the combined effect of rapid testing and pre-emptive contact isolation on MRSA infections.

METHOD

Since November 2003, patients admitted for longer than 24 hours to two adult ICUs were screened systematically on admission using quick, multiplex immunocapture-coupled PCR (qMRSA). Median time intervals from admission to notification of test results were calculated for a five-month intervention phase (November 2003-March 2004) and compared with a historical control period (April 2003-October 2003) by nonparametric tests. ICU-acquired MRSA infection rates were determined for an extended surveillance period (January 2003 through August 2005) and analyzed by Poisson regression methods.

RESULTS

During the intervention phase, 97% (450/462) of patients admitted to the surgical ICU and 80% (470/591) of patients admitted to the medical ICU were screened. On-admission screening identified the prevalence of MRSA to be 6.7% (71/1053). Without admission screening, 55 previously unknown MRSA carriers would have been missed in both ICUs. Median time from ICU admission to notification of test results decreased from 87 to 21 hours in the surgical ICU (P < 0.001) and from 106 to 23 hours in the medical ICU (P < 0.001). In the surgical ICU, 1,227 pre-emptive isolation days for 245 MRSA-negative patients were saved by using the qMRSA test. After adjusting for colonization pressure, the systematic on-admission screening and pre-emptive isolation policy was associated with a reduction in medical ICU acquired MRSA infections (relative risk 0.3, 95% confidence interval 0.1-0.7) but had no effect in the surgical ICU (relative risk 1.0, 95% confidence interval 0.6-1.7).

CONCLUSION

The qMRSA test decreased median time to notification from four days to one day and helped to identify previously unknown MRSA carriers rapidly. A strategy linking the rapid screening test to pre-emptive isolation and cohorting of MRSA patients substantially reduced MRSA cross-infections in the medical but not in the surgical ICU.

Two new colorimetric methods for early detection of vancomycin and oxacillin resistance in Staphylococcus aureus

We developed two colorimetric methods for the detection of vancomycin- and oxacillin-resistant Staphylococcus aureus in </=6 h: (i) a nitrate reductase assay and (ii) a resazurin microplate method. MICs agreed with results obtained by CLSI methods for oxacillin. However, detection of vancomycin resistance required a larger inoculum. These methods may be recommended for the detection of vancomycin- and oxacillin-resistant S. aureus.

Diagnostic PCR analysis of the occurrence of methicillin and tetracycline resistance genes among Staphylococcus aureus isolates from phase 3 clinical trials of tigecycline for complicated skin and skin structure infections

Diagnostic PCR assays were developed to track common genetic determinants of oxacillin resistance as well as resistance to classical tetracyclines in Staphylococcus aureus isolates from the recently completed worldwide phase 3 clinical trials of tigecycline. A total of 503 unique S. aureus strains isolated from complicated skin and skin structure infections were analyzed. The mecA gene was amplified from 120 strains (23.9%) determined to be resistant to oxacillin (MICs > or = 4 microg/ml). The prevalence of the mecA gene was found to vary regionally from 6.5% to 50.9% among isolates originating in Eastern Europe and North America, respectively. The presence of a tetracycline resistance determinant, tet(M) or tet(K), among methicillin-resistant S. aureus (MRSA) isolates also varied regionally, with a range of 11.9% to 46.2% among isolates tested from North America and Eastern Europe, respectively. The occurrence of a tetracycline resistance marker in methicillin-susceptible S. aureus (MSSA) strains varied from 2.5 to 16.1% among the isolates tested across the regions of study. The presence of tet(M) or tet(K) had no discernible effect on the tigecycline MICs for either MRSA or MSSA strains, which is consistent with the ability of the glycylcyclines to retain activity in the presence of both the ribosomal protection and efflux mechanisms of resistance to the tetracyclines.

Effective communication of antimicrobial susceptibility data by pathologists to clinicians

As antimicrobial-resistant gram-positive and gram-negative bacterial infections continue to emerge over time, influenced by selective pressure, and the costs of treating these infections increase, the ability of microbiology laboratories to accurately detect drug resistant bacteria and communicate susceptibility test results effectively to clinicians in a timely manner is more important than ever. Ongoing cooperation and coordination of activities among clinical pathologists, technical personnel, pharmacy, clinical services, and infection control using up-to-date laboratory methodologies and interpretive criteria to develop laboratory reports that are transmitted efficiently through computerized communication systems for use within each particular institution are essential to provide optimum patient care, control resistance, and conserve resources.

Guidelines for the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA)

These evidence-based guidelines have been produced after a literature review of the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA). We have considered the detection of MRSA in screening samples and the detection of reduced susceptibility to glycopeptides in S. aureus. Recommendations are given for the identification of S. aureus and for suitable methods of susceptibility testing and screening for MRSA and for S. aureus with reduced susceptibility to glycopeptides. These guidelines indicate what tests should be used but not when the tests are applicable, as aspects of this are dealt with in guidelines on control of MRSA. There are currently several developments in screening media and molecular methods. It is likely that some of our recommendations will require modification as the new methods become available.

Evaluation of the Vitek 2 system for identification and antimicrobial susceptibility testing of Staphylococcus spp

The Vitek 2 system was assessed against reference methods with 197 methicillin-resistant Staphylococcus aureus from Belgian hospitals and 121 clinically significant blood culture isolates of Staphylococcus spp. Vitek 2 identified 95% of staphylococcal isolates correctly, detected oxacillin resistance with a sensitivity/specificity of 99/96%, and showed acceptable accuracy for susceptibility testing of five of eight other evaluable antibiotics. The median time for reporting results was 2 h 45 min for identification and 7 h for susceptibility tests.

Antibacterial susceptibility testing in the clinical laboratory

This article familiarizes the clinician with the principles of bacterial susceptibility testing and reporting to facilitate communication with the clinical microbiology laboratory. The emergence of resistance in nearly all commonly isolated bacterial organisms has highlighted the need for ongoing dialogue between the laboratory and those who use its services.

Community-acquired methicillin-resistant Staphylococcus aureus infections

Staphylococcus aureus causes a variety of minor diseases but also is responsible for staphylococcal pneumonia and sepsis, both of which can be fatal. It is thought to be responsible for many of the pneumonia deaths associated with the influenza pandemics of the 20th century. The introduction of penicillin in the 1940s greatly improved the prognosis for patients with severe staphylococcal infections. However, after a few years of clinical use, most staphylococcal strains were able to hydrolyze penicillin by producing b-lactamases, making penicillin a useless antibiotic to treat staphylococcal infections caused by b-lactamase-producing S aureus. Methicillin, a semisynthetic penicillin introduced in 1959, was specifically designed to be resistant to b-lactamase degradation, but resistance developed soon after its introduction into clinical practice. Methicillin-resistant S aureus (MRSA) was first reported in the United Kingdom in 1961, followed by reports from other European countries, Japan, and Australia. The first reported case of MRSA in the United States was in 1968. Currently, MRSA is an important pathogen in nosocomial infections and is a problem in hospitals worldwide, and it is increasingly recovered from nursing home residents with established risk factors. More recently, community acquired MRSA infections have been documented among healthy individuals with no recognizable risk factors, and it seems clear that community-acquired MRSA (CA-MRSA) strains are epidemiologically and clonally unrelated to hospital-acquired strains. This review focuses on the epidemiology, clinical significance, and virulence markers of CA-MRSA infections.

Identification of methicillin-resistant isolates of Staphylococcus aureus and coagulase-negative staphylococci responsible for bloodstream infections with the Phoenix™ system

We evaluated the reliability of the new Phoenix system (Becton Dickinson Microbiology Systems, Sparks, Md.) in species-level identification and detection of oxacillin (methicillin) resistance among 493 staphylococcal isolates (Staphylococcus aureus, n = 223; coagulase-negative staphylococci, CoNS, n = 270) recovered from patients with bacteremia. Identification results were concordant with those of the ID 32 STAPH system (bioMérieux, Marcy l'Etoile, France) for 100% of S. aureus (223/223) and 97.4% (263/270) of CoNS isolates. For S. aureus isolates, Phoenix oxacillin-susceptibility results fully concurred with those of mecA polymerase chain reaction (PCR) (reference method): 96 mecA-positive isolates identified as resistant, 127 mecA-negative strains as susceptible. Two of the 210 mecA-positive CoNS isolates were misclassified as susceptible by the Phoenix (sensitivity 99%, positive predictive value 97.6%). Five of 60 mecA-negative CoNS isolates were classified as resistant by the Phoenix (specificity 91.7%; negative predictive value 96.5%). The Phoenix system can provide accurate and reliable identification of methicillin-resistant staphylococci responsible for bloodstream infections.

Evaluation of an isothermal signal amplification method for rapid detection of methicillin-resistant Staphylococcus aureus from patient-screening swabs

A new molecular assay (CytAMP) utilizing isothermal signal-mediated amplification of RNA was evaluated for rapid detection of methicillin (oxacillin)-resistant Staphylococcus aureus (MRSA). The assay targeted the coa (coagulase) and mecA genes, thereby simultaneously identifying S. aureus and methicillin (oxacillin) resistance. Results were obtained in approximately 3.5 h as a color signal in 96-well microtiter plates. The detection limit was between 2 x 10(5) and 10(6) CFU/assay, equivalent to 4 x 10(6) to 2 x 10(7) CFU/ml in an overnight broth. This level of growth was obtained with an initial inoculum of 10 to 50 CFU. The CytAMP assay and a mecA-femB PCR assay both detected 113 MRSA strains among 396 clinical isolates of bacteria (CytAMP sensitivity and specificity were both 100% relative to those of PCR). Conventional culture detected 109 MRSA strains, but with 19 false-positive and 23 false-negative results relative to both molecular methods. Discrepancies were also observed for 100 enrichment broths containing MRSA screening swabs, with 11 broths culture negative but PCR positive. CytAMP and PCR were more in agreement, but six broths were CytAMP negative and PCR positive. Five of these contained 10(2) to 10(5) CFU/assay (below the CytAMP detection limit of 2 x 10(5) CFU/assay), and the sixth contained 10(6) CFU/assay. Overall, culture and CytAMP had similar sensitivities and specificities relative to those of PCR, but the CytAMP assay enabled swabs to be analyzed as a batch following overnight incubation in enrichment broth, with results reported before 12 noon the next day.