Alternative use for spectra MRSA chromogenic agar in detection of methicillin-resistant Staphylococcus aureus from positive blood cultures

Evaluation of agar culture plates to efficiently identify small colony variants of methicillin-resistant Staphylococcus aureus

Background: Small-colony variants of methicillin-resistant Staphylococcus aureus (SCV-MRSA) recently were described as slow-growing, thymidine-dependent strains; typically, SCV-MRSA were isolated from patients receiving sulfamethoxazole-trimethoprim, but detection of these strains frequently was delayed because of their small colony size and slow growth. Bacteremia cases due to SCV-MRSA sometimes become lethal when the initiation of treatment with intravenous anti-methicillin-resistant Staphylococcus aureus (MRSA) drugs starts too late. Methods: Here, we evaluated the use of general MRSA-specific agar plates in Japan, including MS-CFX, X-MRSA, and CHROMagar, for the efficient detection of SCV-MRSA, including the comparative detection efficiencies of these media for stock strains and clinical isolates. Results: Among the three MRSA-specific agar plates that were tested, X-MRSA and CHROMagar showed similar detection efficiencies for both 24 and 48 hrs culturing; in contrast, MS-CFS did not permit the detection of SCV-MRSA in stock strains. For clinical isolates of SCV-MRSA, X-MRSA plates permitted detection of the smallest and slowest-growing colonies of SCV-MRSA at 48 hrs of culturing; in contrast, CHROMagar and MS-CFX sometimes did not identify SCV-MRSA at 24 and 48 hrs. Conclusion: Optimization of media and incubation times will be necessary for efficient identification for SCV-MRSA, which would prevent delays in diagnosis and treatment with anti-MRSA drugs.

Screening agars for MRSA: evaluation of a stepwise diagnostic approach with two different selective agars for the screening for methicillin-resistant Staphylococcus aureus (MRSA)

BACKGROUND

Colonization with methicillin-resistant Staphylococcus aureus (MRSA) poses a hygiene risk that does not spare field hospitals or military medical field camps during military deployments. Diagnostic options for unambiguously identifying MRSA isolates are usually scarce in military environments. In this study, we assessed the stepwise application of two different selective agars for the specific identification of MRSA in screening analyses.

METHODS

Nasal swabs from 1541 volunteers were subjected to thioglycollate broth enrichment and subsequently screened on CHROMagar MRSA selective agar for the identification of MRSA. The MRSA identity of suspicious-looking colonies was confirmed afterwards or excluded by another selective agar, chromID MRSA. All isolates from the selective agars with MRSA-specific colony morphology were identified by biochemical methods and mass spectrometry.

RESULTS

The initial CHROMagar MRSA screening identified suspicious colonies in 36 out of 1541 samples. A total of 25 of these 36 isolates showed MRSA-like growth on chromID agar. Out of these 25 isolates, 24 were confirmed as MRSA, while one isolate was identified as Staphylococcus kloosii. From the 11 strains that did not show suspicious growth on chromID agar, 3 were methicillin-sensitive Staphylococcus aureus (MSSA, with one instance of co-colonization with Corynebacterium spp.), 2 were confirmed as MRSA (with 1 instance of co-colonization with MSSA), 2 were lost during passaging and could not be re-cultured, one could not be identified by the applied approaches, and the remaining 3 strains were identified as Staphylococcus saprophyticus, Staphylococcus hominis (co-colonized with Macrococcus caseolyticus) and Staphylococcus cohnii, respectively.

CONCLUSIONS

The application of the selective agar CHROMagar MRSA alone proved to be too non-specific to allow for a reliable diagnosis of the presence of MRSA. The combined use of two selective agars in a stepwise approach reduced this non-specificity with an acceptably low loss of sensitivity. Accordingly, such a stepwise screening approach might be an option for resource-restricted military medical field camps.

Separation of methicillin-resistant from methicillin-susceptible Staphylococcus aureus by electrophoretic methods in fused silica capillaries etched with supercritical water

Identification and prevention of Staphylococcus aureus-caused infections may benefit from a fast and dependable method to distinguish between the methicillin-resistant (MRSA) and methicillin-susceptible (MSSA) S. aureus strains. The current methods involving polymerase chain reaction and/or other molecular tests are usually laborious and time-consuming. We describe here a fast and low-cost method employing capillary zone electrophoresis (CZE) to distinguish between MRSA and MSSA. The method makes use of a supercritical water-treated fused silica capillary, the inner surface of which has subsequently been modified with (3-glycidyloxypropyl)trimethoxysilane. With optimized proportions of suitable additives to the background electrolyte, a CZE separation of MRSA from MSSA may be completed within 12 min. The cells were baseline-resolved, and resolution was determined to be 3.61. The isoelectric points of MSSA and MRSA were found to be the same for both groups of these strains, pI = 3.4.

Evaluation of MRSASelect (™) chromogenic medium for the early detection of methicillin-resistant Staphylococcus aureus from blood cultures

INTRODUCTION

Staphylococcus aureus bacteremia is associated with considerable morbidity and mortality. In theory, reducing the turnaround time in reporting of methicillin-resistant S aureus (MRSA) among patients with bactermia could assist with the rapid optimization of antimicrobial therapy.

OBJECTIVE

To evaluate the sensitivity and specificity of MRSASelect (Bio-Rad Laboratories, USA), a chromogenic medium, in the early detection of MRSA from blood cultures growing Gram-positive cocci in clusters, and to confirm that routine use of this medium would, in fact, reduce turnaround time for MRSA identification.

METHODS

The present study was conducted at three microbiology laboratories in Manitoba. Between April 2010 and May 2011, positive blood cultures with Gram-positive cocci in clusters visualized on Gram stain were subcultured to both MRSASelect and routine media. MRSA isolates were identified using conventional microbiological methods from routine media and using growth with the typical colony morphology (pink colony) on MRSASelect medium.

RESULTS

A total of 490 blood cultures demonstrating Gram-positive cocci in clusters on Gram stain were evaluated. S aureus was recovered from 274 blood cultures, with 51 S aureus isolates (51 of 274 [18.6%]) identified as MRSA. MRSASelect medium had a sensitivity of 98%, a specificity of 100%, a positive predictive value of 100% and a negative predictive value of 99.8% for the recovery and identification of MRSA directly from positive blood culture bottles. In addition, use of MRSASelect medium was found to improve turnaround time in the detection of MRSA by almost 24 h relative to conventional methods.

DISCUSSION

These data support the utility of MRSASelect medium for the rapid identification of MRSA from positive blood cultures. Further clinical studies are warranted to determine whether the improvement in turnaround time will result in a measurable reduction in suboptimal antimicrobial therapy and/or improvement in patient outcome.