Rapid Methods for Detection of MRSA in Clinical Specimens

Abstract

Antimicrobial susceptibility test methods such as disk diffusion, broth microdilution, and oxacillin screen plate require 24 h of incubation after having the organism growing in pure culture. Rapid and accurate identification of MRSA isolates is essential not only for patient care, but also for effective infection control programs to limit the spread of MRSA. In the last few years, several commercial rapid tests for detection of MRSA directly from nasal swabs and blood cultures have been developed for use in clinical laboratories. Real-time PCR and other molecular tests are gaining popularity as MRSA screening tests to identify patients who are candidates for contact precaution at the time of admission decreasing the risk for nosocomial transmission. These new methodologies have the advantage of a lower turnaround time than that of traditional culture and susceptibility testing and they are capable of detecting MRSA directly from nasal or wound swabs allowing rapid identification of colonized or infected patients. In addition, molecular methods able to detect and differentiate S. aureus and MRSA (SA/MRSA) directly from blood cultures are becoming a useful tool for rapid detection of bacteremia caused by MSSA and MRSA. This review focuses on the procedures for performing testing using rapid methods currently available for detection of MRSA directly from clinical specimens.

A rapid, low-cost, and microfluidic chip-based system for parallel identification of multiple pathogens related to clinical pneumonia

“…However, the delay between sample acquisition and results report, usually 48–96 h, makes MRSA cross-transmission a potential threat. In this respect, rapid tests for the detection of MRSA (generally based on nucleic acids tests) are a great supplement to conventional methods, especially for screening 24 . Due to their very low turnaround time, nucleic acid amplification tests are extremely suitable to identify patients who are candidates for contact precaution and to decrease nosocomial transmission 25 .…”

Contemporary genomic approaches in the diagnosis and typing of Staphylococcus aureus

“…PCR-based assays, such as the Cepheid Xpert MRSA/SA Blood Culture assay 24 and the BD GeneOhm StaphSR assay, 25,26 are commonly used molecular tests for direct detection of S. aureus from blood cultures. When compared to conventional culture methods, both assays are reported to have high sensitivity and specificity for the identification of methicillin-susceptible S. aureus (MSSA) and MRSA, with a manufacturer's report suggesting 98.8-100% sensitivity and 97.2-100% specificity for the BD GeneOhm StaphSR assay, 12 and similar high reported rates of sensitivity and specificity for the Cepheid Xpert MRSA/SA assay. 24,27,28 In order to differentiate between S. aureus and other staphylococcal species, primers for a S. aureus species-specific marker are generally incorporated into molecular assays.…”

“…These are generally based on identification of a S. aureus species-specific marker, with or without detection of mecA, the gene responsible for production of the atypical penicillin-binding protein (PBP2a) that confers methicillin resistance in staphylococcal species. [11][12][13] To date, the two most common clinical situations where these tests have been employed are: (i) in the rapid speciation and mecA profiling of S. aureus from blood cultures, and (ii) screening for the presence of S. aureus, particularly MRSA, in the context of infection prevention and control.…”

“…11,13 The most commonly utilised molecular tests in the rapid diagnosis of S. aureus bacteraemia are those employed when a blood culture bottle is found to contain GPC resembling staphylococci. 12 An aliquot of the blood culture broth is used for subsequent molecular testing, generally using PCR amplification, followed by a number of detection methods (e.g., fluorescent probes for real-time detection, or microarray hybridisation). PCR-based assays, such as the Cepheid Xpert MRSA/SA Blood Culture assay 24 and the BD GeneOhm StaphSR assay, 25,26 are commonly used molecular tests for direct detection of S. aureus from blood cultures.…”

“…The use of rapid molecular screening tests on nasal swabs may reduce the turnaround time of conventional screening to only a few hours, allowing for faster isolation of patients. A number of molecular assays are available for detection of MRSA (þ/À MSSA) in nasal swabs, 12 which, similar to the assays described above, rely on detection of S. aureus species markers, and elements of the SCCmec complex. As such, these screening assays also are subject to the same technical considerations around false positive and negative results, in addition to logistical issues around laboratory workflow and timely reporting of results.…”