Evaluation of the IDI-MRSA assay on the SmartCycler real-time PCR platform for rapid detection of MRSA from screening specimens

Comparative evaluation of two fully-automated real-time PCR methods for MRSA admission screening in a tertiary-care hospital

We evaluated two fully-automated real-time PCR systems, the novel QIAGEN artus MRSA/SA QS-RGQ and the widely used BD MAX MRSA assay, for their diagnostic performance in MRSA admission screening in a tertiary-care university hospital. Two hundred sixteen clinical swabs were analyzed for MRSA DNA using the BD MAX MRSA assay. In parallel, the same specimens were tested with the QIAGEN artus MRSA/SA QS-RGQ. Automated steps included lysis of bacteria, DNA extraction, real-time PCR and interpretation of results. MRSA culture was additionally performed as a reference method for MRSA detection. Sensitivity values were similar for both assays (80 %), while the QIAGEN artus MRSA/SA QS-RGQ reached a slightly higher specificity (95.8 % versus 90.0 %). Positive (PPVs) and negative predictive values (NPVs) were 17.4 % and 99.4 % for the BD MAX MRSA assay and 33.3 % and 99.5 % for the QIAGEN artus MRSA/SA QS-RGQ, respectively. Total turn-around time (TAT) for 24 samples was 3.5 hours for both assays. In conclusion, both assays represent reliable diagnostic tools due to their high negative predictive values, especially for the rapid identification of MRSA negative patients in a low prevalence MRSA area.

Active Surveillance for Methicillin-Resistant Staphylococcus aureus in the Neonatal Intensive Care Unit

Background.

We describe our experience using a real-time polymerase chain reaction (PCR) assay for methicillin-resistant Staphylococcus aureus (MRSA) during a period of active surveillance in the neonatal intensive care unit (NICU) from March 2007 until November 2007.

Objective.

TO compare PCR with bacterial culture methods and find the screening algorithm that most successfully ensures appropriate isolation of colonized patients.

Methods.

Patients in the NICU were screened for MRSA on admission and weekly thereafter until discharge. Healthcare workers (HCWs) were also screened as part of an outbreak investigation. A total of 599 individuals were screened for MRSA with both a PCR assay and selective bacterial culture. Strain typing was performed on all MRSA isolates to determine clonal relatedness.

Results.

Twenty-one of 435 infants (4.8%) screened positive for MRSA with the PCR assay. Only 11 patients (52.4%) had concomitant bacterial cultures positive for MRSA. Compared to bacterial culture, the PCR assay had a sensitivity of 100% and a specificity of 97.6%, with a positive predictive value (PPV) of 52.4%. Infants that tested positive for MRSA by both culture and PCR were more likely to have a positive PCR assay result when retested than were those who tested positive by PCR alone (80% vs 20%; P = .02). Strain typing of MRSA isolates identified a common clone in only 2 colonized infants.

Conclusion.

Our data show that, in our neonatal population, the reproducibility of PCR assay results for culture-negative patients was low compared with the reproducibility of results for culture-positive Patients. Furthermore, the low PPV suggests that for nearly half of individuals who were PCR-positive, the result was falsely positive, which argues against the use of PCR assays alone for MRSA screening in the NICU.

The Effect of Rapid Screening for Methicillin-ResistantStaphylococcus aureus(MRSA) on the Identification and Earlier Isolation of MRSA-Positive Patients

Objectives.

(1) To determine whether rapid screening with polymerase chain reaction (PCR) assays leads to the earlier isolation of patients at risk for methicillin-resistantStaphylococcus aureus(MRSA) colonization, (2) to assess compliance with routine MRSA screening protocols, (3) to confirm the diagnostic accuracy of the Xpert MRSA real-time PCR assay (Cepheid) by comparison with culture, and (4) to compare turnaround times for PCR assay results with those for culture results.

Design.

Before-and-after study conducted in a 700-bed acute tertiary care referral hospital. Study periods were (1) a 5-week period before PCR testing began, (2) a 10-week period when the PCR assay was used, and (3) a 5-week period after PCR testing was discontinued.

Results.

Among 489 at-risk patients, MRSA was isolated from 20 (33%) of 60 patients during period 1, 77 (22%) of 349 patients during period 2, and 18 (23%) of 80 patients during period 3. Twenty-two (27%) of 82 at-risk patients were not screened during period 1, compared with 40 (10%) of 389 at-risk patients not screened during period 2 (P< .001). More MRSA-positive patients were preemptively isolated during periods 1 and 3 compared with period 2 (34 [24%] of 140 vs 28 [8%] of 389;P< .001); however, more MRSA-positive patients were isolated after notification of MRSA-positive results during period 2 (47 [13%] of 349) compared with periods 1 and 3 (2 [1%] of 140;P< .001). The sensitivity, specificity, positive predictive value, and negative predictive value of the PCR assay were 95%, 97%, 82%, and 99%, respectively. The mean turnaround time from receipt of specimens in the laboratory to PCR assay result was 2.6 hours.

Conclusions.

Rapid screening with the Xpert MRSA PCR assay facilitated compliance with screening policies and the earlier isolation of MRSA-positive Patients. Discrepant results confirm that PCR testing should be used as a screening tool rather than as a diagnostic tool.

Bacterial nanoscale cultures for phenotypic multiplexed antibiotic susceptibility testing

An optimal antimicrobial drug regimen is the key to successful clinical outcomes of bacterial infections. To direct the choice of antibiotic, access to fast and precise antibiotic susceptibility profiling of the infecting bacteria is critical. We have developed a high-throughput nanowell antibiotic susceptibility testing (AST) device for direct, multiplexed analysis. By processing in real time the optical recordings of nanoscale cultures of reference and clinical uropathogenic Escherichia coli strains with a mathematical algorithm, the time point when growth shifts from lag phase to early logarithmic phase (Tlag) was identified for each of the several hundreds of cultures tested. Based on Tlag, the MIC could be defined within 4 h. Heatmap presentation of data from this high-throughput analysis allowed multiple resistance patterns to be differentiated at a glance. With a possibility to enhance multiplexing capacity, this device serves as a high-throughput diagnostic tool that rapidly aids clinicians in prescribing the optimal antibiotic therapy.

Recombinations in staphylococcal cassette chromosome mec elements compromise the molecular detection of methicillin resistance in Staphylococcus aureus

Clinical laboratories are increasingly using molecular tests for methicillin-resistant Staphylococcus aureus (MRSA) screening. However, primers have to be targeted to a variable chromosomal region, the staphylococcal cassette chromosome mec (SCCmec). We initially screened 726 MRSA isolates from a single UK hospital trust by recombinase polymerase amplification (RPA), a novel, isothermal alternative to PCR. Undetected isolates were further characterised using multilocus sequence, spa typing and whole genome sequencing. 96% of our tested phenotypically MRSA isolates contained one of the six orfX-SCCmec junctions our RPA test and commercially available molecular tests target. However 30 isolates could not be detected. Sequencing of 24 of these isolates demonstrated recombinations within the SCCmec element with novel insertions that interfered with the RPA, preventing identification as MRSA. This result suggests that clinical laboratories cannot rely solely upon molecular assays to reliably detect all methicillin-resistance. The presence of significant recombinations in the SCCmec element, where the majority of assays target their primers, suggests that there will continue to be isolates that escape identification. We caution that dependence on amplification-based molecular assays will continue to result in failure to diagnose a small proportion (∼4%) of MRSA isolates, unless the true level of SCCmec natural diversity is determined by whole genome sequencing of a large collection of MRSA isolates.

Medical and household characteristics associated with methicillin resistant Staphylococcus aureus nasal carriage among patients admitted to a rural tertiary care hospital

BACKGROUND

Methicillin resistant Staphylococcus aureus (MRSA) poses a threat to patient safety and public health. Understanding how MRSA is acquired is important for prevention efforts. This study investigates risk factors for MRSA nasal carriage among patients at an eastern North Carolina hospital in 2011.

METHODS

Using a case-control design, hospitalized patients ages 18 - 65 years were enrolled between July 25, 2011 and December 15, 2011 at Vidant Medical Center, a tertiary care hospital that screens all admitted patients for nasal MRSA carriage. Cases, defined as MRSA nasal carriers, were age and gender matched to controls, non-MRSA carriers. In-hospital interviews were conducted, and medical records were reviewed to obtain information on medical and household exposures. Multivariable conditional logistic regression was used to derive odds ratio (OR) estimates of association between MRSA carriage and medical and household exposures.

RESULTS

In total, 117 cases and 119 controls were recruited to participate. Risk factors for MRSA carriage included having household members who took antibiotics or were hospitalized (OR: 3.27; 95% Confidence Interval (CI): 1.24-8.57) and prior hospitalization with a positive MRSA screen (OR: 3.21; 95% CI: 1.12-9.23). A lower proportion of cases than controls were previously hospitalized without a past positive MRSA screen (OR: 0.40; 95% CI: 0.19-0.87).

CONCLUSION

These findings suggest that household exposures are important determinants of MRSA nasal carriage in hospitalized patients screened at admission.

Multiplex real-time PCR assay for detection of methicillin-resistant Staphylococcus aureus (MRSA) strains suitable in regions of high MRSA endemicity

A multiplex real-time PCR assay that simultaneously detects the mecA, staphylococcal cassette chromosome (SCCmec)-open reading frame X (orfX) junction, and staphylococcal 16S rRNA genes was developed and evaluated using 444 staphylococcal strains. We demonstrated that this assay resulted in fewer false-positive results than a single-locus real-time PCR assay that amplified the SCCmec-orfX junction. This assay would be useful in a clinical laboratory in a region of high endemicity for methicillin-resistant Staphylococcus aureus (MRSA) infections.

Methicillin-resistant Staphylococcus aureus in nasal surveillance swabs at an intensive care unit: an evaluation of the LightCycler MRSA advanced test

Background

We compared the LightCycler MRSA advanced test (Roche Diagnostics, Germany) with enrichment culture methods to evaluate the relative diagnostic performance of the LightCycler MRSA advanced test for active surveillance in a high-prevalence setting.

Methods

A total of 342 nasal swab specimens were obtained from patients in the intensive care unit at admission and on the seventh day for follow-up. The results of LightCycler MRSA advanced test were compared to those of the enrichment culture. For discrepant results, mecA gene PCR was performed.

Results

For the detection of methicillin-resistant Staphylococcus aureus (MRSA), the LightCycler MRSA advanced test showed 98.5% sensitivity and 78.6% specificity and had positive and negative predictive values of 75.0% and 98.8%, respectively. A total of 46 samples had discrepant results between the LightCycler MRSA advanced test and enrichment culture. Of the 44 specimens that were positive in the LightCycler MRSA advanced test but negative by enrichment culture, mecA genes were detected in 37 specimens. In addition, of the original 44 cases, 21 patients had a history of MRSA colonization or infection within the last month; of those 21 specimens, 20 were positive for mecA gene as shown by PCR. Seven mecA-negative discrepant specimens comprised 3 methicillin-sensitive S. aureus-culture positive and only 2 patients had MRSA infections.

Conclusions

Despite its low specificity and positive predictive value, the LightCycler MRSA advanced test could serve as a rapid test for patients colonized with MRSA.

Direct detection of nasal Staphylococcus aureus carriage via helicase-dependent isothermal amplification and chip hybridization

BACKGROUND

The bacterium Staphylococcus aureus constitutes one of the most important causes of nosocomial infections. One out of every three individuals naturally carries S. aureus in their anterior nares, and nasal carriage is associated with a significantly higher infection rate in hospital settings. Nasal carriage can be either persistent or intermittent, and it is the persistent carriers who, as a group, are at the highest risk of infection and who have the highest nasal S. aureus cell counts. Prophylactic decolonization of S. aureus from patients' noses is known to reduce the incidence of postsurgical infections, and there is a clear rationale for rapid identification of nasal S. aureus carriers among hospital patients.

FINDINGS

A molecular diagnostic assay was developed which is based on helicase-dependent target amplification and amplicon detection by chip hybridization to a chip surface, producing a visible readout. Nasal swabs from 70 subjects were used to compare the molecular assay against culturing on "CHROMagar Staph aureus" agar plates. The overall relative sensitivity was 89%, and the relative specificity was 94%. The sensitivity rose to 100% when excluding low-count subjects (<100 S. aureus colony-forming units per swab).

CONCLUSIONS

This molecular assay is much faster than direct culture and has sensitivity that is appropriate for identification of high-count (>100 S. aureus colony-forming units per swab) nasal S. aureus carriers who are at greatest risk for nosocomial infections.

Prevalence of methicillin-resistant Staphylococcus aureus based on culture and PCR in inpatients at a tertiary care center in Tokyo, Japan

We investigated active screening for colonization with methicillin-resistant Staphylococcus aureus (MRSA) on admission and weekly follow-up surveillance after admission to a tertiary care center (TCC) between June 2007 and 31 December 2007. Eleven percent (30/267) of patients were found to be positive for MRSA by polymerase chain reaction (PCR) and/or culture on admission; 5% (12/267) became positive during the TCC stay. The major primary diagnoses in MRSA-positive patients were pneumonia and cerebrovascular diseases. Twenty-two (52%) of 42 patients were found to be MRSA positive by both PCR and culture, compared with 19 (45%) of 42 who were PCR positive and culture negative. These findings suggest that active surveillance with PCR is highly sensitive and useful for the detection of MRSA colonization. To our knowledge, this is the first report of active surveillance of MRSA by PCR and bacterial culture in critically ill inpatients in Japan.

BD GeneOhm-MRSA assay for detection of methicillin-resistant Staphylococcus aureus directly in nasal and non-nasal swab specimens from haematologic patients

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are responsible for rising health care costs and have a high attribution to mortality. Reliable and rapid detection of MRSA carriage is essential. Real-time PCR allows an early detection of MRSA colonization within 2 h. By using the BD GeneOhm-MRSA assay we analysed directly swabs of different sampling sites and compared the assay with culture method. One thousand one hundred and sixty samples from 129 patients in Magdeburg were examined. Of the samples, 8 (0.69%) or 1117 (96.3%) were tested equally positive or negative by both methods whereas 16 (1.38%) specimens were MRSA positive only by the GeneOhm-MRSA assay and 6 (0.52%) were MRSA positive only by culture method. Thirteen samples (1.12%), which are culture negative, were unresolved by the GeneOhm-MRSA. With regard to the patients, seven were detected as MRSA carriers only by the GeneOhm-MRSA while one patient was tested positive for MRSA only by culture. Assuming 100% correct results by the culture method, sensitivity and specificity of GeneOhm-MRSA assay could be calculated as 84.4% and 96.1% for nasal swabs, 78.7% and 96.9% for all swabs under study, and 94.8% and 99.5% when focussed on patients. PPV and NPV were 70.3% and 98% for all specimens together, respectively. BD GeneOhm-MRSA assay is a sensitive test for the detection of MRSA colonization from swab specimens without the need for an initial culture, but should always be performed in parallel to the culture method for comparison reasons. Furthermore, our results indicate that in addition swabs taken from different body sites were successfully analysed by the BD GeneOhm-MRSA assay. However, we conclude that the PCR assay might not be a preferred tool for screening in haematologic patients with low MRSA rate; for screening haematologic patients, the culture method is sufficient enough.

Screening, isolation, and decolonisation strategies in the control of meticillin resistant Staphylococcus aureus in intensive care units: cost effectiveness evaluation

OBJECTIVE

To assess the cost effectiveness of screening, isolation, and decolonisation strategies in the control of meticillin resistant Staphylococcus aureus (MRSA) in intensive care units.

DESIGN

Economic evaluation based on a dynamic transmission model.

SETTING

England and Wales. Population Theoretical population of patients on an intensive care unit.

MAIN OUTCOME MEASURES

Infections, deaths, costs, quality adjusted life years (QALYs), incremental cost effectiveness ratios for alternative strategies, and net monetary benefits.

RESULTS

All decolonisation strategies improved health outcomes and reduced costs. Although universal decolonisation (regardless of MRSA status) was the most cost effective in the short term, strategies using screening to target MRSA carriers may be preferred owing to the reduced risk of selecting for resistance. Among such targeted strategies, universal admission and weekly screening with polymerase chain reaction coupled with decolonisation using nasal mupirocin was the most cost effective. This finding was robust to the size of intensive care units, prevalence of MRSA on admission, proportion of patients classified as high risk, and precise value of willingness to pay for health benefits. All strategies using isolation but not decolonisation improved health outcomes but costs were increased. When the prevalence of MRSA on admission to the intensive care unit was 5% and the willingness to pay per QALY gained was between £20,000 (€23,000; $32,000) and £30,000, the best such strategy was to isolate only those patients at high risk of carrying MRSA (either pre-emptively or after identification by admission and weekly screening for MRSA using chromogenic agar). Universal admission and weekly screening using polymerase chain reaction based detection of MRSA coupled with isolation was unlikely to be cost effective unless prevalence was high (10% of patients colonised with MRSA on admission).

CONCLUSIONS

MRSA control strategies that use decolonisation are likely to be cost saving in an intensive care unit setting provided resistance is lacking, and combining universal screening using polymerase chain reaction with decolonisation is likely to represent good value for money if untargeted decolonisation is considered unacceptable. In intensive care units where decolonisation is not implemented, evidence is insufficient to support universal screening for MRSA outside high prevalence settings.

Performance of the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) PCR assay for detecting MRSA nasal colonization in Taiwanese adults

BACKGROUND/PURPOSE

A rapid diagnostic method for methicillin-resistant Staphylococcus aureus (MRSA) has been implemented for surveillance of the at-risk population, but its performance in those without traditional risk factors is not clear. The objective of this study was to evaluate MRSA colonization status by comparing the performance of the BD GeneOhm MRSA polymerase chain reaction (PCR) assay with that of conventional culture during a 3-month active surveillance of Taiwanese adults in the community.

METHODS

From 1 October 2007 to 28 December 2007, adults (≥ 18 years old) attending a mandatory health examination arranged by their employers as a part of the workplace health promotion program at three medical centers in northern Taiwan were enrolled in the study. No healthcare workers were included. A total of 498 paired nasal swabs were prospectively obtained and used for both the BD GeneOhm MRSA PCR assay and conventional culture.

RESULTS

Of the 498 paired nasal swabs, 14 (2.8%) were positive for MRSA by conventional culture and 34 (6.8%) were positive by the BD GeneOhm MRSA PCR assay (p < 0.005). Thirteen specimens were both culture- and PCR-positive, and 463 samples were both culture- and PCR-negative. There were two discordant results: 21 specimens were culture-negative/PCR-positive, and one was culture-positive/PCR-negative. The simple kappa coefficient for measuring the agreement between conventional culture and the MRSA PCR assay was 0.52.

CONCLUSION

This study demonstrates the feasibility of using both the MRSA PCR assay and conventional culture as surveillance tools. Also, the MRSA-positive rate detected by MRSA PCR assay was significantly higher than that of conventional culture.

Diagnostic accuracy of culture-based and PCR-based detection tests for methicillin-resistant Staphylococcus aureus: a meta-analysis

A systematic review and meta-analysis were performed to determine and compare the sensitivity and specificity of PCR-based and culture-based diagnostic tests for methicillin-resistant Staphylococcus aureus (MRSA). Our analysis included 74 accuracy measurements from 29 publications. Nine tests were evaluated: the PCR-based Genotype MRSA Direct and IDI-MRSA, the chromogenic media CHROMagar, Chromogenic MRSA Medium, MRSA ID, MRSA Select and ORSAB, and the nonchromogenic culture media MSA-Cefoxitin and MSA-Oxacillin. For four chromogenic media, incubation periods of 18-24 and 48 h were evaluated. Considerable heterogeneity was detected in most analyses. A significantly higher sensitivity was found for the overall PCR pooled estimate (92.5; 95% CI 87.4-95.9) and the chromogenic media after 48 h of incubation (87.6; 95% CI 82.1-91.6) compared to the overall sensitivity of chromogenic media after 18-24 h (78.3; 95% CI 71.0-84.1). The specificity of chromogenic media after 18-24 h (98.6; 95% CI 97.7-99.1) was higher than the specificity of PCR (97.0; 95% CI 94.5-98.4) but declined after 48 h of incubation (94.7; 95% CI 91.6-96.8).The most sensitive chromogenic medium after 18-24 h of incubation was Chromogenic MRSA Medium (sensitivity: 89.3; 95% CI 72.8-96.3), whereas the most specific chromogenic medium after 18-24 h of incubation was MRSA Select (specificity: 99.4; 95% CI 98.6-99.7). After 48 h of incubation, MRSA Select had the highest sensitivity (93.2; 95% CI 83.5-97.0), whereas CHROMagar had the highest specificity (96.4; 95% CI 91.3-98.5). This meta-analysis showed statistically significant differences in diagnostic accuracy between several of the tests and the test methods evaluated. A reduction of the incubation time of chromogenic media (from 48 to 18-24 h) increases specificity but reduces sensitivity.

Genotypic and phenotypic characterization of methicillin-susceptible Staphylococcus aureus isolates misidentified as methicillin-resistant Staphylococcus aureus by the BD GeneOhm MRSA assay

Twenty-three nasal swab samples that tested positive for methicillin-resistant Staphylococcus aureus (MRSA) on initial testing by the BD GeneOhm MRSA assay (BD-MRSA PCR; BD GeneOhm, San Diego, CA) were culture positive only for methicillin-susceptible S. aureus (MSSA) from an enrichment broth. The 23 recovered isolates were confirmed as MSSA by a variety of phenotypic methods, including the BD Phoenix automated microbiology system (BD Diagnostics, Sparks, MD), oxacillin screening agar (BD Diagnostics), BBL CHROMagar MRSA (BD Diagnostics), and a PBP2' assay (Denka Seiken Co., Tokyo, Japan); susceptibilities were determined by using Mueller-Hinton agar with oxacillin. All were positive by nuc PCR, specific for S. aureus, but negative for mecA with one exception. Isolates were characterized by using multiplex PCR methodology to determine structural types and variants (SCCmec typing); additional PCRs were performed for the detection of the ccr and mec complexes, the junkyard regions as well as the Panton-Valentine leukocidin. Pulsed-field gel electrophoresis was used to determine clonality. One phenotypic MSSA isolate contained an intact SCCmec. Twelve MSSA isolates tested positive for MRSA by the BD-MRSA PCR because of amplification of the mec priming site flanking the SCC insertion point, although these isolates lacked mecA. The 10 remaining isolates were not MRSA and tested as MSSA by phenotypic and genotypic assays. In our patient population, diagnostic and surveillance testing and subsequent infection control practices may be impacted by the frequency of these excision events when using the BD-MRSA PCR for MRSA detection.

Evaluation of the BD GeneOhm StaphSR assay for detection of Staphylococcus aureus in patients in intensive care units

BACKGROUND

Staphylococcus aureus is the major nosocomial pathogen and rapid detection of colonized patients with subsequent precaution is needed to prevent transmission. A new assay, the BD GeneOhm™ SaphSR assay (BD GeneOhm™, San Diego, CA, USA), is a multiplex real-time polymerase chain reaction (PCR) for rapid detection of both methicillin-sensitive S aureus (MSSA) as well as methicillin-resistant S aureus (MRSA).

METHODS

Anterior nasal swab specimens of 273 pediatric and adult patients hospitalized in intensive care units at Chang Gung Memorial Hospital were collected for this assay in parallel with conventional cultures as standard.

RESULTS

Overall, 71 (26.0%) patients were colonized with S aureus by conventional culture and MRSA accounted for 67.6% of all isolates. For the detection of MRSA, 79 patients (28.9%) were positive by PCR and 48 (17.6%) were positive by conventional cultures. The sensitivity, specificity, and positive and negative predictive values were 95.9%, 85.3%, 58.5%, 99.0%, respectively. For the detection of MSSA, 48 patients (17.6%) were positive by PCR and 23 (8.4%) were positive by conventional culture. The sensitivity, specificity, and positive and negative predictive values were 91.3%, 89.2%, 43.8%, and 99.1%, respectively.

CONCLUSION

As a screening method, the BD GeneOhm™ StaphSR assay could rapidly detect and differentiate between MRSA and MSSA colonization. A negative result of the assay could almost exclude S aureus colonization.

Evaluation of the BD GeneOhm assay using the rotor-gene 6000 platform for rapid detection of methicillin-resistant Staphylococcus aureus from pooled screening swabs

As health services move toward universal methicillin-resistant Staphylococcus aureus (MRSA) screening for hospital admissions, the most cost-effective approach is yet to be defined. In this study, one of the largest to date, we evaluated the performance of the BD GeneOhm MRSA assay on the Rotor-Gene 6000 thermal cycler, using samples taken directly from pooled MRSA screens. Results were compared with the same assay performed on the Smart-Cycler II platform and overnight broth culture. Samples yielding discrepant results were subjected to detailed analysis with an in-house PCR and patient note review. A total of 1,428 pooled MRSA screens were tested. Sensitivities and specificities of 85.3% and 95.8% for the Rotor-Gene and 81% and 95.7% for the Smart-Cycler were obtained, compared with broth enrichment. The sensitivity of the BD GeneOhm assay was increased to 100% when the results of in-house PCR and patient note review were taken into account. This study demonstrates that the Rotor-Gene 6000 thermal cycler is a reliable platform for use with the BD GeneOhm assay. It also proves that commercial PCR can be performed direct on pooled samples in selective broth, without the need for overnight incubation.

Evaluation of molecular assays for rapid detection of methicillin-resistant Staphylococcus aureus

The diagnostic sensitivities of the BD GeneOhm and Cepheid Xpert assays were compared using culture on log-serial dilutions of well-characterized methicillin-resistant Staphylococcus aureus (MRSA) and non-MRSA strains and on nasal and groin swabs from patients with histories of MRSA carriage. The sensitivities of GeneOhm and Xpert were high at 10(3)-CFU/ml MRSA concentrations (92.3% and 96.3%, respectively) although decreased considerably (<35%) at a 1-log-lower concentration. Unexpectedly, both assays also detected select coagulase-negative staphylococci, which requires further evaluation.

Detection of methicillin-resistant Staphylococcus aureus using double duplex real-time PCR and dye Syto 9

A screening method for methicillin-resistant Staphylococcus aureus (MRSA) using real-time polymerase chain reaction (PCR) and dye Syto 9 was developed and evaluated. The assay was based on the two duplex reactions run simultaneously. The detection reaction amplified staphylococcal cassette chromosome mec (SCCmec) right extremity sequences and S. aureus-specific 442-bp DNA (Sa442). The control reaction amplified S. aureus-specific nuclease gene nuc and a marker of methicillin resistance, mecA. The method was evaluated by analyzing 214 clinical S. aureus isolates yielding 98.7 % sensitivity, 100 % specificity, 100 % positive predictive value and 96.6 % negative predictive value for detection of MRSA. The detection limit was determined to be 15-80 genome copies per real-time PCR. It was able to discriminate between MRSA, methicillin resistant coagulase negative staphylococci and methicillin susceptible S. aureus (MSSA) isolates containing only small fragments of the right extremity of the SCCmec (MSSA revertants).

Reliability of the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) assay in detecting MRSA isolates with a variety of genotypes from the United States and Taiwan

The BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) assay is a molecular screening test for detection of MRSA in nasal colonization. This assay coamplifies the extremity of staphylococcal chromosome cassette mec (SCCmec) and adjacent chromosomal DNA at the SCCmec insertion site. Increasing reports of novel SCCmec types and the diverse genetic backgrounds of MRSA strains prompted us to test the accuracy of the BD GeneOhm MRSA kit with 914 MRSA isolates with a variety of SCCmec types harbored in 21 genetic backgrounds, as determined by the multilocus sequence type (ST). The BD GeneOhm MRSA assay was performed on colony lysates; purified genomic DNA (0.2 pg/μl and 0.2 ng/μl) was tested to confirm negative results from lysates. Of 914 MRSA isolates tested, 911 tested positive (detection rate, 99.7%). The SCCmec types carried by assay-positive isolates were I, II, III, IV, V, V(5C2&5), VI, and VIII and SCCmec composite islands with mec class A and ccr complexes 2 and 4. One of the assay-negative isolates had a community-associated genotype: ST8, SCCmec type IV. However, this was an outlier among the 99.8% (434/435) ST8, SCCmec type IV-containing isolates that tested positive. The two other assay-negative isolates had a health care-associated genotype (ST5); both carried a distinct, uncommon, composite SCCmec type. In summary, the BD GeneOhm MRSA assay had a high rate of detection of MRSA isolates harboring common and uncommon SCCmec types from the United States and Taiwan.

Detection of methicillin-resistant Staphylococcus aureus (MRSA) in specimens from various body sites: performance characteristics of the BD GeneOhm MRSA assay, the Xpert MRSA assay, and broth-enriched culture in an area with a low prevalence of MRSA infections

Universal surveillance upon patient admission is important in reducing the transmission of methicillin-resistant Staphylococcus aureus (MRSA) and associated disease in hospitals. High costs for the health care system in conjunction with MRSA have promoted the development of rapid screening methods to detect MRSA carriers. This study compared two real-time PCR methods, the BD GeneOhm MRSA assay (BDGO) and the Xpert MRSA assay, with broth-enriched culture to define their performance characteristics and rapidity in an area with low MRSA prevalence. In total, 414 swabs from the nose and 389 swabs from the groin from 425 patients were tested. Of those 425 patients, 378 had swabs from both the nose and groin in parallel. Two hundred thirty-one and 194 patients were randomly assigned to the BDGO group and the Xpert MRSA group, respectively. In general, sensitivity, specificity, and negative predictive value (NPV) were high for the BDGO (100%, 98.5%, and 100%, respectively) and the Xpert MRSA (100%, 98.2%, and 100%, respectively), irrespective of whether or not nasal and inguinal specimens were considered alone or combined. In contrast, the positive predictive value (PPV) was lower: before the resolution of discrepant results, the PPVs for nasal and inguinal specimens alone and combined were 87.5%, 86.7%, and 82.4% for the BDGO and 91.7%, 66.7%, and 92.9% for the Xpert MRSA, respectively. After the resolution of discrepant results, PPVs were 93.8%, 93.3% and 94.1% for the BDGO and 91.7%, 88.9% and 92.9% for the Xpert MRSA, respectively. With the BDGO, 4 of 16 carriers were each identified by nasal or inguinal swabs alone, whereas in the Xpert MRSA group, 4 of 13 carriers were exclusively identified by nasal swabs and 2 of 13 were identified by inguinal swabs alone. Both PCR methods showed no significant difference in the number of discrepant results (odds ratio, 0.70 [P = 0.789]), but specimens from wounds and other body sites (axilla, vagina, and throat) produced discrepancies more often than nasal and groin specimens (odds ratios, 4.724 [P = 0.058] and 12.163 [P < 0.001], respectively). The facts that no false-negative PCR results were detected and increased PPVs were found after the resolution of discrepant results point to PCR as the actual gold standard. Since both sensitivity and NPV were exceptionally high for PCR, backup cultures may, therefore, be unnecessary in an area with low prevalence and with a preemptive isolation strategy but may still be useful for PCR-positive specimens because of the lower PPV for both methods and the possibility of susceptibility testing. The median time for analysis, including extraction, hands-on time, and actual PCR was 2 h 20 min for the Xpert MRSA versus 5 h 40 min for the BDGO. Concerning reporting time, including administration and specimen collection, the Xpert MRSA was faster than the BDGO (7 h 50 min versus 17 h).

The performance of the BD geneOhm MRSA™ assay for MRSA isolated from clinical patients in Japan, including the effects of specimen contamination and ways to improve it

The BD geneOhm MRSA™ assay has been increasingly used in recent years, and it is possible to use it to screen and detect methicillin-resistant Staphylococcus aureus (MRSA) from a specimen within 2 h. The purpose of the present study was to evaluate the performance, i.e., the specificity and sensitivity, of the BD geneOhm MRSA™ assay to detect MRSA. Its specificity was assessed to be 100% compared to bacterial culture methods, which are commonly used in medical laboratories. Its bacterial limit of detection was over 10 colony-forming units (cfu) per reaction, although MRSA was detected at a cfu below 10 per reaction in a few samples. Additionally, the effect of MRSA isolate contamination was examined. While contamination with protein or other bacteria did not affect the outcome, contamination with a high concentration of blood resulted in an unresolved outcome. To inactivate polymerase chain reaction (PCR) inhibitors, the DNA samples were freeze-thawed prior to the BD geneOhm MRSA™ assay, which led to the sensitivity of the assay increasing. In summary, the BD geneOhm MRSA™ assay is rapid and shows high specificity and sensitivity of cultured MRSA isolates. It will, therefore, be a valuable diagnostic tool for detecting MRSA in specimens from clinical patients.

Characterization of Staphylococcus aureus isolates with a partial or complete absence of staphylococcal cassette chromosome elements

Detection of methicillin-resistant Staphylococcus aureus (MRSA) by single-locus PCR assays that target the extremity of the staphylococcal cassette chromosome-mec (SCCmec) and part of the adjacent S. aureus-specific open reading frame gene (orfX) is a significant diagnostic advancement, since it provides real-time detection directly from screening specimens. However, isolates harboring mecA deletions within SCCmec may result in false-positive identification of MRSA in these assays. We characterized 24 methicillin-susceptible S. aureus (MSSA) isolates that tested positive in one such assay to investigate this phenomenon. Seven isolates resembled USA100 and carried SCCmec II elements with mecA deletions that spanned 20 to 46 kbp. The mecA excisions in USA100-resembling isolates appeared to be linked with IS431 transposable elements present in SCCmec II. For 17 isolates that resembled USA400 and/or MSSA476, the identity and possible excision of SCC elements could not be confirmed. The downstream common sequence (dcs) shared by SCCmec I, II, and IV elements was detected in these isolates. Sequence analysis of the chromosomal regions flanking the missing SCC element revealed an intact SCC integration site, a duplicate dcs, and the enterotoxin gene cluster downstream of orfX. An annealing sequence for one of the SCCmec-specific primers (mecii574) in the single-locus PCR assay was identified in the duplicate dcs. In the absence of SCC, a 176-bp amplicon can be generated from this mecii574 annealing sequence to yield a false-positive result. In conclusion, partial SCCmec II excisions via IS431 elements in strains that resembled USA100 and the presence of a duplicate mecii574 annealing sequence in strains that resembled USA400/MSSA476 were identified as causes for false-positive results in a single-locus PCR assay that targets the SCCmec/orfX junction.

[Performance evaluation of BD GeneOhm MRSA PCR assay for detection of nasal colonization of methicillin-resistant Staphylococcus aureus at endemic intensive care units]

BACKGROUND

The BD GeneOhm MRSA PCR assay (Becton Dickinson, USA) is a qualitative real-time PCR test for rapid detection of nasal colonization of methicillin-resistant Staphylococcus aureus (MRSA). We evaluated the performance of BD GeneOhm MRSA PCR assay versus MRSASelect (Bio-Rad, France) and broth enrichment cultures for detection of MRSA from nasal swabs.

METHODS

From August 2008 to January 2009, 295 nasal swabs were taken from patients in intensive care units and transported to the laboratory with BD CultureSwab Liquid Stuart Single Swab (Becton Dickinson, USA). The swabs were inoculated onto MRSASelect first and then suspended into GeneOhm sample buffer: 100 microL of the suspension was inoculated into 6.5% NaCl-tryptic soy broth (Becton Dickinson, USA), which was subcultured on MRSASelect after overnight incubation (TSBS). Performances of GeneOhm MRSA and MRSASelect were compared to TSBS.

RESULTS

With GeneOhm MRSA, 125 swabs (44.6%) were positive for MRSA, 13 (4.4%) were unresolved, and 2 were not determined. With MRSASelect and TSBS 86 (29.4%) and 106 swabs (36.2%), respectively, were positive. The sensitivity, specificity, and positive and negative predictive value of GeneOhm MRSA were 85.8%, 77.5%, and 72.8% and 93.5%, respectively, and corresponding values for MRSASelect were 78.3%, 94.8%, and 96.5% and 88.9%. Of the 33 patients whose 34 specimens were found false positive in GeneOhm MRSA, 23 patients were MRSA-positive either previously or subsequently to this study. All of the 10 patients with false-negative specimens in GeneOhm MRSA PCR assay were previously MRSA or methicilln-resistant coagulase negative staphylococci (MRCNS)-positive and were treated for MRSA, but they became MRSA-positive after 1 to 4 negative surveillance cultures.

CONCLUSIONS

GeneOhm MRSA PCR assay showed a relatively high negative predictive value. However, its low specificity and frequent occurrence of unresolved results would be problematic in the endemic areas with a high prevalence of MRSA.

Significantly larger numbers of methicillin-resistant Staphylococcus aureus bacteria are recovered from polymicrobial respiratory and wound sites by use of chromogenic primary media than by use of conventional culture

Previous studies have validated the properties and documented the utility of chromogenic agar for surveillance of methicillin-resistant Staphylococcus aureus (MRSA). In this study, we used one of the chromagars, MRSASelect (Bio-Rad), as one of the primary isolation media for selected wound and respiratory clinical specimens which, in our institution, were typically polymicrobial. We examined a total of 638 specimens; 142 (22%) MRSA isolates were recovered. Twenty-six of these isolates were recovered only on the MRSASelect plate, representing a 28% (15/54) increase for endotracheal aspirates/sputa and a 15% increase for superficial wounds/ulcers (11/73) compared to the results with conventional culture. One isolate (1 CFU) was recovered by conventional medium alone. MRSASelect has generally been used for surveillance cultures; however, we document that an additional 21% of MRSA isolates would have gone unreported in these selected clinical specimens using only standard culture media. For 40% (6/15) of inpatients, MRSA isolated from the MRSASelect plate was the sole indicator of MRSA. Although these isolates can represent either colonization or infection, they are a potential reservoir of infection and nosocomial transmission. Our data support the focused use of chromogenic selective media for the increased detection of MRSA in polymicrobial wound and respiratory specimens, which could have an impact on both clinical treatment and infection control.

Comparison of the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) PCR assay to culture by use of BBL CHROMagar MRSA for detection of MRSA in nasal surveillance cultures from intensive care unit patients

This study compared the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) real-time PCR assay to culture by the use of BBL CHROMagar MRSA for the detection of MRSA in 627 nasal surveillance specimens collected from intensive care unit (ICU) patients. The PCR assay had a sensitivity, specificity, positive predictive value, and negative predictive value of 100%, 96.7%, 70.3%, and 100%, respectively. Nine of 19 false-positive PCR specimens grew methicillin-susceptible S. aureus (MSSA) from broth enrichment culture, of which two demonstrated evidence of mecA gene dropout. Compared to culture by the use of BBL CHROMagar MRSA, the BD GeneOhm MRSA PCR assay demonstrated sensitivity and specificity above 95% for the detection of MRSA nasal colonization and provided shorter turnaround time in generating positive and negative final results.

Rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) in diverse clinical specimens by the BD GeneOhm MRSA assay and comparison with culture

The efficacy of the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) assay was assessed by analyzing nasal swabs and swabs from other body sites for the presence of MRSA in a low-prevalence area. From 681 patients with a high risk for MRSA carriage, 1,601 specimens were collected and transported in Amies agar. After discordant analysis, the sensitivity, specificity, positive predictive value, and negative predictive value of the BD GeneOhm MRSA assay were 84.3%, 99.2%, 88.4%, and 98.9%, respectively, compared to culture.

MRSA screening: can one swab be used for both culture and rapid testing? An evaluation of chromogenic culture and subsequent Hain GenoQuick PCR amplification/detection

The use of a single swab for both MRSA culture and for rapid testing by PCR was evaluated, using the Hain GenoQuick (GQM) methicillin resistant Staphylococcus aureus (MRSA) assay for the rapid detection of MRSA, as a single swab would be the preferred option for routine diagnostic testing. GQM detected current prevalent Irish MRSA strains incorporating all known SSCmec types, including Panton-Valentine leukocidin-positive strains. Using the GQM method, all methicillin-resistant coagulase-negative staphylococci tested were confirmed to be negative, although three of seven gentamicin-resistant methicillin-sensitive Staphylococcus aureus strains tested were identified as MRSA. The theoretical ex-vivo limit of detection of the assay was 704 CFU per GQM assay reaction (1.7 × 10(4) CFU/mL) when MRSA suspensions were used for DNA extraction, or 1.4 × 10(3) CFU/swab (1.4 × 10(4) CFU/mL) using MRSA absorbed onto Copan screening swabs. Swab processing on chromogenic agar prior to PCR resulted in some inhibition of the PCR reaction, increasing the limit of detection of the assay by a factor of four. Based on 540 single swab screening specimens (nasal and groin) processed first for culture assay, then by GQM, the specificity and positive predictive value were both 100%, the negative predictive value was 92%, and the sensitivity was 57%. Culture followed by PCR from a single specimen is not optimal for the rapid detection of MRSA. Further laboratory validation of the GQM assay is required to determine the true diagnostic sensitivity and value of this kit in routine microbiology laboratories, modifying the protocol for single specimens, or using two specimens.

Clinical application of real-time PCR to screening critically ill and emergency-care surgical patients for methicillin-resistant Staphylococcus aureus: a quantitative analytical study

The clinical utility of real-time PCR screening assays for methicillin (methicillin)-resistant Staphylococcus aureus (MRSA) colonization is constrained by the predictive values of their results: as MRSA prevalence falls, the assay's positive predictive value (PPV) drops, and a rising proportion of positive PCR assays will not be confirmed by culture. We provide a quantitative analysis of universal PCR screening of critical care and emergency surgical patients using the BD GeneOhm MRSA PCR system, involving 3,294 assays over six months. A total of 248 PCR assays (7.7%) were positive; however, 88 failed to be confirmed by culture, giving a PPV of 65%. Multivariate analysis was performed to compare PCR-positive culture-positive (P+C+) and PCR-positive culture-negative (P+C-) assays. P+C- results were positively associated with a history of methicillin-sensitive Staphylococcus aureus infection or colonization (odds ratio [OR], 3.15; 95% confidence interval [CI], 1.32 to 7.54) and high PCR thresholds of signal intensity, indicative of a low concentration of target DNA (OR, 1.19 per cycle; 95% CI, 1.11 to 1.26). P+C- results were negatively associated with a history of MRSA infection or colonization (OR, 0.19; 95% CI, 0.09 to 0.42) and male sex (OR, 0.40; 95% CI, 0.20 to 0.81). P+C+ patients were significantly more likely to have subsequent positive MRSA culture assays and microbiological evidence of clinical MRSA infection. The risk of subsequent MRSA infection in P+C- patients was not significantly different from that in case-matched PCR-negative controls. We conclude that, given the low PPV and poor correlation between a PCR-positive assay and the clinical outcome, it would be prudent to await culture confirmation before altering infection control measures on the basis of a positive PCR result.

Control of MRSA infection and colonisation in an intensive care unit by GeneOhm MRSA assay and culture methods

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the major nosocomial pathogens. Due to the diffusion of MRSA strains in both hospital and community settings, prevention and control strategies are receiving increased attention. Approximately 25% to 30% of the population is colonised with S. aureus and 0.2% to 7% with MRSA. The BD GeneOhm MRSA real-time PCR assay offers quicker identification of MRSA-colonised patients than do culture methods.

METHODS

Ninety-five patients admitted to the Intensive Care Unit of IRCCS Policlinico San Matteo of Pavia (Italy) for a period > 24 h were screened for MRSA colonisation with both the culture method and the GeneOhm assay.

RESULTS

Of the 246 nasal swabs collected from 95 patients, 36 samples were found to be positive by both methods (true-positive). 30% of colonised patients had developed the MRSA infection.

CONCLUSION

Our results show that the GeneOhm MRSA assay is a valuable diagnostic tool for detecting MRSA quickly in nasal swabs. This study confirms that colonisation represents a high risk factor for MRSA infection, and that good MRSA surveillance in an Intensive Care Unit is therefore an excellent way to prevent MRSA infection.

Cost-effectiveness of rapid MRSA screening in surgical patients

This study investigates the effectiveness of a same-day polymerase chain reaction (PCR) test for the rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) in a general screening of patients admitted to the trauma surgery and heart surgery department in a German university hospital. A total of 442 patients were screened over a 4-month period by using a PCR assay, compared to culture methods, for specimens from the nose and throat. The MRSA carriage rate on admission was 3.85% during the study period. The PCR results of 1,680 swabs showed a sensitivity of 85% and a specificity of 99.39% for swabs from the nares and for the throat 42.11% and 98.78%, respectively. A combination of specimens from the nose and throat from the same patient led to a sensitivity of 100% with a specificity of 98.29%. Cost calculation under the circumstances of a diagnosis-related groups (DRG) payment system found that the eight MRSA-positive patients created costs of 38,472 euros, i.e. 4,809 euros per patient, facing screening costs of 36.62 euros per sample. Screening patients by using the rapid PCR assay for a combination of specimens from the nose and throat would offer a safe and cost-effective way of MRSA screening on admission.

Quantitative reverse transcription-PCR assay for the rapid detection of methicillin-resistant Staphylococcus aureus

AIM

To evaluate a new quantitative reverse transcription-PCR (qRT-PCR) assay for the rapid detection of methicillin-resistant Staphylococcus aureus (MRSA).

METHODS AND RESULTS

Primers for Staphylococcus-specific regions of 16S rRNA gene, spa gene and mecA gene were newly designed. RNAs extracted from broth-cultured strains were tested by qRT-PCR targeting each primer, and the bacterial counts obtained correlated well with those counted by the plating method with detection limits of 10(0), 10(1) and 10(2) CFU. The qRT-PCR assay targeting the 16S rRNA was 6430-fold or more sensitive than qPCR assay. All Staph. aureus strains tested were detected and none of the other Staphylococcus species and genus strains tested cross-reacted with the assay targeting the spa gene. All MRSAs tested were detected by the assay targeting the mecA gene. Clinical samples, faecal material and bronchial washout solutions were tested by our assay, and MRSAs were detected with a high sensitivity within 6 h.

CONCLUSION

Our qRT-PCR assay targeting three new primers to the target genes is a rapid and sensitive tool for the detection of MRSA directly from clinical samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

Because of its sensitivity and rapidity, our qRT-PCR assay is considered to be a valuable tool for clinical management.

Current applications and future trends of molecular diagnostics in clinical bacteriology

Molecular diagnostics of infectious diseases, in particular, nucleic-acid-based methods, are the fastest growing field in clinical laboratory diagnostics. These applications are stepwise replacing or complementing culture-based, biochemical, and immunological assays in microbiology laboratories. The first-generation nucleic acid assays were monoparametric such as conventional tests, determining only a single parameter. Improvements and new approaches in technology now open the possibility for the development of multiparameter assays using microarrays, multiplex nucleic acid amplification techniques, or mass spectrometry, while the introduction of closed-tube systems has resulted in rapid microbial diagnostics with a subsequently reduced contamination risk. Whereas the first assays were focused on the detection and identification of microbial pathogens, these new technologies paved the way for the parallel determination of multiple antibiotic resistance determinants or to perform microbial epidemiology and surveillance on a genetic level.

A common variant of staphylococcal cassette chromosome mec type IVa in isolates from Copenhagen, Denmark, is not detected by the BD GeneOhm methicillin-resistant Staphylococcus aureus assay

Rapid tests for detection of methicillin-resistant Staphylococcus aureus (MRSA) carriage are important to limit the transmission of MRSA in the health care setting. We evaluated the performance of the BD GeneOhm MRSA real-time PCR assay using a diverse collection of MRSA isolates, mainly from Copenhagen, Denmark, but also including international isolates, e.g., USA100-1100. Pure cultures of 349 MRSA isolates representing variants of staphylococcal cassette chromosome mec (SCCmec) types I to V and 103 different staphylococcal protein A (spa) types were tested. In addition, 53 methicillin-susceptible Staphylococcus aureus isolates were included as negative controls. Forty-four MRSA isolates were undetectable; of these, 95% harbored SCCmec type IVa, and these included the most-common clone in Copenhagen, spa t024-sequence type 8-IVa. The false-negative MRSA isolates were tested with new primers (analyte-specific reagent [ASR] BD GeneOhm MRSA assay) supplied by Becton Dickinson (BD). The ASR BD GeneOhm MRSA assay detected 42 of the 44 isolates that were false negative in the BD GeneOhm MRSA assay. Combining the BD GeneOhm MRSA assay with the ASR BD GeneOhm MRSA assay greatly improved the results, with only two MRSA isolates being false negative. The BD GeneOhm MRSA assay alone is not adequate for MRSA detection in Copenhagen, Denmark, as more than one-third of our MRSA isolates would not be detected. We recommend that the BD GeneOhm MRSA assay be evaluated against the local MRSA diversity before being established as a standard assay, and due to the constant evolution of SCCmec cassettes, a continuous global surveillance is advisable in order to update the assay as necessary.

One-year experience with modified BD GeneOhm MRSA assay for detection of methicillin-resistant Staphylococcus aureus from pooled nasal, skin, and throat samples

We report our 1-year experience with modified GeneOhm MRSA assay (formerly IDI-MRSA) for pooled surveillance specimens in low methicillin-resistant Staphylococcus aureus (MRSA) prevalence clinical setting. We have successfully modified the GeneOhm MRSA assay protocol during the specimen preparation step by adding an extra washing step followed by pooling of up to 3 samples per patient (nose, skin, with or without throat) at the lysis step. The sensitivity of the modified assay compared with conventional cultivation was 94.3%, specificity 99.2%, negative predictive value 99.2%, and positive predictive value 94.3%. The modified test is reliable and performed well compared with conventional culture methods in our clinical setting with low-level prevalence of MRSA colonization. Our findings support the use of pooling of the patients samples as a cost-effective way of screening for MRSA colonization.

Detection of staphylococcal cassette chromosome mec-associated DNA segments in multiresistant methicillin-susceptible Staphylococcus aureus (MSSA) and identification of Staphylococcus epidermidis ccrAB4 in both methicillin-resistant S. aureus and MSSA

Methicillin-susceptible Staphylococcus aureus (MSSA) can arise from methicillin-resistant S. aureus (MRSA) following partial or complete excision of staphylococcal cassette chromosome mec (SCCmec). This study investigated whether multiresistant MSSA isolates from Irish hospitals, where MRSA has been endemic for decades, harbor SCCmec DNA. Twenty-five multiresistant MSSA isolates recovered between 2002 and 2006 were tested for SCCmec DNA by PCR and were genotyped by multilocus sequence typing and spa typing. All isolates lacked mecA. Three isolates (12%) harbored SCCmec DNA; two of these (genotype ST8/t190) harbored a 26-kb SCCmec IID (II.3.1.2) remnant that lacked part of mecI and all of mecR1, mecA, and IS431; the third isolate (ST8/t3209) harbored the SCCmec region from dcs to orfX. All three isolates were detected as MRSA using the BD GeneOhm and Cepheid's Xpert MRSA real-time PCR assays. Six isolates (ST8/t190, n = 4; ST5/t088, n = 2), including both isolates with the SCCmec IID remnant, harbored ccrAB4 with 100% identity to ccrAB4 from the Staphylococcus epidermidis composite island SCC-CI. This ccrAB4 gene was also identified in 23 MRSA isolates representative of ST8/t190-MRSA with variant SCCmec II subtypes IIA to IIE, which predominated previously in Irish hospitals. ccrAB4 was located 5,549 bp upstream of the left SCCmec junction in both the MRSA and MSSA isolates with SCCmec elements and remnants and 5,549 bp upstream of orfX in the four MSSA isolates with ccrAB4 only on an SCC-CI homologous region. This is the first description of a large SCCmec remnant with ccr and partial mec genes in MSSA and of the S. epidermidis SCC-CI and ccrAB4 genes in S. aureus.

Evaluation of the Xpert methicillin-resistant Staphylococcus aureus (MRSA) assay using the GeneXpert real-time PCR platform for rapid detection of MRSA from screening specimens

The need for rapid methods to accurately detect methicillin-resistant Staphylococcus aureus (MRSA) is widely acknowledged, and a number of molecular assays are commercially available. This study evaluated the Xpert MRSA assay, which is run on the GeneXpert real-time PCR platform (Cepheid) for use in a clinical laboratory. The following parameters were investigated: (i) the limits of detection (LoDs) for four MRSA strains; (ii) the ability to detect isolates of MRSA from a collection representative of MRSA in Ireland since 1974 (n = 114) and the ability to detect control strains with staphylococcal cassette chromosome mec types IVa (IV.1.1.1), IVb (IV.2.1.1), IVc (IV.3.1.1), IVd (IV.4.1.1), V (V.1.1.1), V(T), and VI; and (iii) performance in a clinical trial with swabs from nose, throat, and groin/perineum sites from 204 patients, where results were compared with those obtained by direct and enrichment cultures. The average LoD of the four test strains was 610 CFU/ml (equivalent to 58 CFU/swab). All 114 MRSA isolates and 7 control strains tested were detected. Sensitivity, specificity, and positive and negative predictive values for clinical specimens from all sites investigated were 90%, 97%, 86%, and 98%, respectively, but throat specimens yielded poor sensitivity (75%). Sensitivity, specificity, and positive and negative predictive values for nasal specimens were 95%, 98%, 90%, and 99%, respectively. Overall, the assay was rapid and easy to perform, but performance might be enhanced by the inclusion of an equivocal interpretive category based on analysis of all available amplification data.

Rapid diagnostics for methicillin-resistant Staphylococcus aureus: current status

Methicillin-resistant Staphylococcus aureus (MRSA) is a significant cause of healthcare- and community-associated infections, and its prevalence continues to increase. These infections are associated with morbidity and excessive mortality compared with infections caused by methicillin-susceptible S. aureus (MSSA). Numerous studies have cited the increased healthcare costs associated with MRSA infections. Infection control guidelines that combine active surveillance with aggressive patient management, such as patient isolation, decontamination, and other strategies, have been shown to reduce transmission and subsequent infections. The availability of rapid molecular diagnostics has strengthened infection control programs by providing results in hours rather than days, as the time required for culture-based methods. This review summarizes the current status of rapid diagnostic methods available for MRSA detection from nasal surveillance specimens, and assays available for rapid identification of MRSA from positive blood cultures containing Gram-positive cocci in clusters. Both amplification- and probe-based assays are highlighted and discussed in detail. Future technological advances are likely to see real-time assays that combine multiple gene targets for assessment of microbial identification, virulence detection, and mechanisms of resistance beyond mecA.

Comparison of the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) PCR assay to culture by use of BBL CHROMagar MRSA for detection of MRSA in nasal surveillance cultures from an at-risk community population

We compared the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) PCR assay to culture with BBL CHROMagar MRSA for nasal surveillance among 602 arrestees from the Baltimore City Jail. The sensitivity and specificity were 88.5% and 91.0%, respectively, and after secondary analysis using enrichment broth, they were 89.0% and 91.7%, respectively. Twenty-three of 42 false-positive PCR lysates contained methicillin-susceptible S. aureus.

[Evaluation of IDI-MRDA assay on a collection of community-acquired methicillin-resistant Staphylococcus aureus isolates and on carriage specimens]

The efficacy of infection control measures against MRSA is linked to the rapid detection of MRSA. With the conventional diagnosis by culture the response delays vary from 48 to 72 hours. In contrast molecular techniques give results within hours.

OBJECTIVE

The objective of the present study is to perform the IDI-MRSA PCR test (BD Diagnostic GeneOhm) on a collection of characterized community-acquired MRSA (CA-MRSA) isolates and on carriage specimens. COLLECTION OF ISOLATES: Fifty-two isolates of CA-MRSA previously characterised by their toxinotype and SCCmec type cassette were analysed. All of them were identified as MRSA by the IDI-MRSA test.

SPECIMENS

Seventy screening specimens from 35 different patients were tested in comparison with the culture on specific media (MRSA ID, BioMérieux). Among those 70 specimens, 24 were from nose, 25 from cutaneous sites (axillar; groin) and 21 from other sites. Sensitivity and specificity were 86.4 and 91.3% respectively; positive and negative predictive values were 93.3 and 82.6% respectively.

RESULTS

Three of four false-positive results came from specimens collected during a decolonisation treatment. Without taking account those specimens, specificity and positive predictive reach 97.9 and 95% respectively. This study shows that IDI-MRSA is an interesting additional test for the diagnosis of MRSA carriage.

Comparison of BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) PCR versus the CHROMagar MRSA assay for screening patients for the presence of MRSA strains

We compared the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) real-time PCR assay with the CHROMagar MRSA assay for the detection of MRSA in 286 nasal surveillance specimens. Compared with the CHROMagar MRSA assay, PCR had sensitivity, specificity, positive predictive value, and negative predictive values of 100%, 98.6%, 95.8%, and 100%, respectively. The mean PCR turnaround time was 14.5 h.