Reductions in workload and reporting time by use of methicillin-resistant Staphylococcus aureus screening with MRSASelect medium compared to mannitol-salt medium supplemented with oxacillin

Methicillin-resistant Staphylococcus aureus (MRSA) is a significant pathogen in both nosocomial and community settings, and screening for carriers is an important infection control practice in many hospitals. In this retrospective study, we demonstrate that the implementation of an MRSA screening protocol using a selective chromogenic medium (MRSASelect) reduced the workload for this screening test by 63.7% overall and by 12.6% per specimen and reduced the turnaround time for reporting by an average of 1.33 days for all MRSA screening specimens, 1.97 days for MRSA-positive specimens, and 1.3 days for MRSA-negative specimens compared to standard mannitol-salt agar supplemented with 6 mg of oxacillin/liter.     

Performance of CHROMagar MRSA medium for detection of methicillin-resistant Staphylococcus aureus

CHROMagar MRSA was evaluated for its ability to identify methicillin-resistant Staphylococcus aureus (MRSA). A well-defined collection consisting of 216 MRSA strains and 241 methicillin-susceptible Staphylococcus aureus isolates was used. The sensitivity of CHROMagar MRSA after 24 h of incubation was 95.4%, increasing to 100% after 48 h. The specificity was already 100% after 24 h.     

Performance of MRSA ID, a new chromogenic medium for detection of methicillin-resistant Staphylococcus aureus

MRSA ID was evaluated for its ability to identify methicillin-resistant Staphylococcus aureus. A well-defined collection of staphylococci was used (n = 998). The sensitivity after 24 h was 96.4%, increasing to 98.8% after 48 h. The specificity was 98.2% after 24 h and decreased to 89.7% after 48 h.     

Evaluation of a new chromogenic medium, MRSA select, for detection of methicillin-resistant Staphylococcus aureus

We compared MRSA Select to mannitol-salt agar with 8 microg/ml cefoxitin for the detection of methicillin-resistant Staphylococcus aureus (MRSA) from 6,199 clinical samples submitted for MRSA screening. The sensitivities and specificities of MRSA Select and mannitol-salt agar with cefoxitin were 98% and 92% versus 90% and 78%, respectively (P<0.0001). Most (96%) MRSA were detected after overnight incubation using MRSA Select.     

Combined use of Pastorex Staph-Plus and either of two new chromogenic agars, MRSA ID and CHROMagar MRSA, for detection of methicillin-resistant Staphylococcus aureus

We describe the search toward a fast and reliable strategy to detect and confirm the presence of methicillin-resistant Staphylococcus aureus (MRSA) in screening samples. First, we evaluated the sensitivities and specificities of oxacillin resistance screening agar (ORSA) with enrichment (tryptic soy broth [TSB] and ORSA [TSB-ORSA]) and without enrichment (ORSA), MRSA ID (MRSA_ID) plates, and CHROMagar MRSA (C_MRSA) plates, all of which were inoculated with equal volumes of a suspension made by emulsifying screening swabs. Whereas the sensitivities after 48 h were similar for all media tested (77% for MRSA_ID and ORSA; 73% for C_MRSA and ORSA after enrichment [TSB-ORSA]), the specificities of MRSA_ID (98% after 24 h and 94% after 48 h) and C_MRSA (98% after 24 h and 90% after 48 h) were superior to the specificities of ORSAs (92% after 24 h and 83% after 48 h) and TSB-ORSA (86% after 24 h and 81% after 48 h). Subsequently, the performance of the Pastorex Staph-Plus agglutination test with presumptive MRSA isolates taken directly from chromogenic agars (direct_Pastorex agglutination) was compared to that of the Pastorex Staph-Plus agglutination test with isolates from blood agar subcultures (conventional_Pastorex agglutination). When the direct_Pastorex agglutination test on MRSA_ID plates was combined with Gram staining, the direct_Pastorex agglutination test with samples from MRSA_ID plates was as reliable as the conventional_Pastorex agglutination test with samples from blood agar subcultures from MRSA_ID plates. In contrast, the direct_Pastorex agglutination test with samples from C_MRSA plates gave false-negative results. Finally, we calculated the processing times of the four different strategies, namely, (i) enrichment in TSB supplemented with NaCl, subsequent culture on ORSA, and the conventional_Pastorex agglutination test; (ii) direct inoculation of ORSA combined with conventional_Pastorex agglutination test; (iii) direct inoculation of MRSA_ID plates combined with Gram staining and the direct_Pastorex agglutination test; and (iv) direct inoculation of C_MRSA plates combined with Gram staining and the direct_Pastorex agglutination test. We concluded that the use of MRSA_ID in combination with Gram staining and the direct_Pastorex agglutination test is faster and more specific than the other strategies tested.     

Evaluation of two new chromogenic media, CHROMagar MRSA and S. aureus ID, for identifying Staphylococcus aureus and screening methicillin-resistant S. aureus

Thirty-nine methicillin-resistant Staphylococcus aureus (MRSA) isolates with diverse genetic backgrounds and two reference strains were correctly identified as S. aureus on CHROMagar MRSA and S. aureus ID media. Growth inhibition on CHROMagar MRSA was noted. A combination of cefoxitin disk and S. aureus ID was found suitable for rapid MRSA screening.     

Use of BBL CHROMagar MRSA medium for identification of methicillin-resistant Staphylococcus aureus directly from blood cultures

We evaluated the ability of BBL CHROMagar MRSA medium (Becton Dickinson, Sparks, MD) to identify methicillin-resistant Staphylococcus aureus (MRSA) directly upon subculture from positive blood culture bottles. There were 124 MRSA isolates recovered from blood cultures in the study. BBL CHROMagar MRSA medium was highly sensitive (97.6% [121/124] at 18 to 24 h of incubation and 100% [124/124] at 48 h) and 99.9% specific for identifying MRSA from positive blood cultures.     

Evaluation of three chromogenic media (MRSA-ID, MRSA-Select and CHROMagar MRSA) and ORSAB for surveillance cultures of methicillin-resistant Staphylococcus aureus

Screening specimens were homogenised in saline 0.9% w/v before either direct inoculation or following enrichment in broth on three chromogenic media (MRSA-ID, CHROMagar MRSA and MRSA Select) and ORSAB medium for the detection of methicillin-resistant Staphylococcus aureus (MRSA). In total, 102 of 466 specimens yielded MRSA on at least one medium. After incubation for 16-18 h, the sensitivity was 51%, 59%, 47% and 65% on MRSA-ID, CHROMagar MRSA, ORSAB and MRSA Select, respectively, compared with 82%, 75%, 67% and 80%, respectively, after 42 h, and 93%, 95%, 79% and not tested, respectively, following broth enrichment. There were significantly more MRSA colonies on MRSA-Select after 16-18 h than on ORSAB or MRSA ID (p 0.001 and 0.0022, respectively), whereas there were more MRSA colonies after 42 h on MRSA-ID and MRSA-Select than on ORSAB (p 0.0004 and 0.012, respectively). The specificity of the media for identifying MRSA based on the colour of colonies after incubation for 16-18 h was 100%, 99%, 99% and 100%, respectively, compared with 98%, 97%, 98% and 98%, respectively, after 42 h, and 100%, 99%, 100% and not tested, respectively, following broth enrichment. The speed of detection (mean time to report a positive result) was 1.65, 1.72, 2.31 and 1.35 days, respectively. For each of the three media tested following enrichment, the use of an enrichment broth increased the detection rate of MRSA by 16-24%.     

Multicenter evaluation of BBL CHROMagar MRSA medium for direct detection of methicillin-resistant Staphylococcus aureus from surveillance cultures of the anterior nares

Active surveillance for methicillin-resistant Staphylococcus aureus (MRSA) is among the strategies recommended by the Society for Healthcare Epidemiology of America for control of nosocomial MRSA infections. Infection control and laboratory personnel desire rapid, sensitive, and inexpensive methods to enhance surveillance activities. A multicenter study was performed to evaluate a new selective and differential chromogenic medium, BBL CHROMagar MRSA (C-MRSA) medium (BD Diagnostics, Sparks, MD), which enables recovery and concomitant identification of MRSA strains directly from nasal swab specimens taken from the anterior nares. Specimens were inoculated to C-MRSA and Trypticase soy agar with 5% sheep blood agar (TSA II, BD Diagnostics). Mauve colonies on C-MRSA at 24 h and 48 h and suspicious colonies on TSA II were confirmed as Staphylococcus aureus by Gram stain morphology and a coagulase test. In addition, the results of C-MRSA were compared to results of susceptibility testing (five different methods) of S. aureus strains isolated on TSA II. A total of 2,015 specimens were inoculated to C-MRSA and TSA II. Three hundred fifty-four S. aureus isolates were recovered; 208 (59%) were oxacillin (methicillin) susceptible and 146 (41%) were oxacillin resistant (MRSA). On C-MRSA, 139/146 or 95.2% of MRSA isolates were recovered, whereas recovery on TSA II was 86.9% (127/146) (P = 0.0027). The overall specificity of C-MRSA was 99.7%. When C-MRSA was compared to each susceptibility testing method, the sensitivity and specificity, respectively, were as follows: oxacillin MIC by broth microdilution, 94.4% and 96.7%; oxacillin screen agar, 94.3% and 96.7%; PBP2' latex agglutination, 93.7% and 98.5%; cefoxitin disk diffusion, 95.0% and 98.1%; and mecA PCR, 95.1% and 98.1%. In this study, C-MRSA was superior to TSA II for recovery of MRSA from surveillance specimens obtained from the anterior nares and was comparable to conventional, rapid, and molecular susceptibility methods for the identification of MRSA isolates.     

Development and evaluation of a chromogenic agar medium for methicillin-resistant Staphylococcus aureus

We describe here the development and evaluation of MRSA ID, a new chromogenic agar medium for the specific isolation and identification of methicillin-resistant Staphylococcus aureus (MRSA). We used S. aureus ID (bioMérieux, La Balme Les Grottes, France) and supplemented it with various antimicrobials, including cefoxitin, ciprofloxacin, oxacillin, and methicillin. Cefoxitin proved to be superior to the other antimicrobials for the selection of MRSA from other strains of S. aureus. MRSA ID (consisting of S. aureus ID supplemented with 4 mg of cefoxitin/liter) was evaluated by the use of 747 swabs from various clinical sites. All specimens were also cultured on CHROMagar MRSA and oxacillin resistance screening agar base (ORSAB) and in selective mannitol broth (SMB). A total of 85 MRSA strains were isolated by a combination of all methods. After 22 to 24 h of incubation, 80% of the MRSA strains were isolated as green colonies on MRSA ID, compared with 59 and 62% of the strains that were isolated as colored colonies on CHROMagar MRSA and ORSAB, respectively. After 48 h of incubation, 89, 72, and 78% of the MRSA strains were isolated on MRSA ID, CHROMagar MRSA, and ORSAB, respectively. Sixty-five percent of the strains were isolated by growth in SMB. The specificities of MRSA ID, CHROMagar MRSA, ORSAB, and SMB were 99.5, 99.3, 97.9, and 92.8%, respectively, after 22 to 24 h of incubation. We conclude that MRSA ID is a sensitive and specific medium for the isolation and identification of MRSA.     

Three-Way Comparison of BBL CHROMagar MRSA II,MRSASelect, and Spectra MRSA for Detection of Methicillin-Resistant Staphylococcus aureus Isolates in Nasal Surveillance Cultures

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of hospital-acquired and life-threatening infections. Active surveillance programs for MRSA utilize either molecular or culture-based methods. A prospective study was performed to compare the performance of selective and differential chromogenic media, BBL CHROMagar MRSA II (CMRSA II; BD Diagnostics, Sparks, MD), MRSASelect (Bio-Rad Laboratories, Redmond, WA), and Spectra MRSA (Remel, Lenexa, KS), for the detection of MRSA in nasal swab specimens. A total of 515 compliant remnant nasal swab specimens were sequentially used to inoculate BBL Trypticase soy agar with 5% sheep blood (TSA II) and each chromogenic medium. After 24 h of incubation, colony color reactions and morphology on chromogenic media were compared to suspicious colonies on nonselective TSA II. MRSA on TSA II was confirmed by Gram staining, a coagulase test, and a cefoxitin disk test. The overall prevalence of MRSA and methicillin-susceptible S. aureus (MSSA) on TSA II was 12.4% (64/515) and 9.7% (50/515), respectively. When each chromogenic medium was compared to the standard culture method, the sensitivity and specificity, respectively, were as follows: CMRSA II, 87.7% and 98.6%; MRSASelect, 89.0% and 93.4%; and Spectra MRSA, 83.6% and 92.1%. The positive predictive values were highest for CMRSA II (91.4%), followed by MRSASelect (69.1%) and Spectra MRSA (63.5%). False-positive results on chromogenic media were mainly due to color interpretation. The negative predictive values for all three media were greater than 97%. In conclusion, CMRSA II gave the best overall results for detecting MRSA from nasal specimens.     

Evaluation of CHROMagar Staph. aureus, a new chromogenic medium, for isolation and presumptive identification of Staphylococcus aureus from human clinical specimens

CHROMagar Staph. aureus (CSA) is a new chromogenic medium for presumptive identification of Staphylococcus aureus as mauve colonies after 24 h of incubation. We conducted a preliminary study with 100 S. aureus and 45 coagulase-negative Staphylococcus (CoNS) stock isolates plated on CSA. All S. aureus isolates yielded mauve colonies after 24 h of incubation at 37 degrees C, while CoNS isolates grew as blue, white, or beige colonies. Culture on CSA was then prospectively compared to a conventional laboratory method, i.e. , culture on 5% horse blood agar (HBA), catalase test, and latex agglutination test (HBA-catalase-latex), for isolation and presumptive identification of S. aureus from 2,000 consecutive clinical samples. Among the 310 S. aureus isolates recovered by at least one of the two methods, 296 grew as typical mauve colonies on CSA, while only 254 yielded catalase-positive, latex-positive colonies on HBA. The sensitivity of CSA was significantly higher than that of the conventional method (95.5 and 81.9%, respectively; P < 0.001) and allowed the recovery of important clinical isolates that were undetected on blood agar. The specificities of the two methods were not significantly different, although that of CSA was slightly higher (99.4% versus 98.9% for HBA-catalase-latex; P = 0. 08). On the basis of its excellent sensitivity and specificity, ease of identification of positive colonies, and absence of complementary testing, CSA can be recommended as a routine plating medium for presumptive identification of S. aureus in clinical specimens.     

Evaluation of S. aureus ID, a new chromogenic agar medium for detection of Staphylococcus aureus

S. aureus ID (bioMérieux, La Balme Les Grottes, France) is a new chromogenic agar medium designed to enable the isolation of staphylococci and the specific identification of Staphylococcus aureus. S. aureus produces green colonies on this medium due to production of α-glucosidase. To evaluate this medium, a total of 350 wound swabs were cultured onto S. aureus ID, CHROMagar Staph. aureus, and conventional media routinely used in our laboratory. After 18 to 20 h of incubation, 96.8% of strains formed green colonies on S. aureus ID compared with 91.1% of strains forming mauve colonies on CHROMagar Staph. aureus. A total of 94.3% of strains were recovered within 18 to 20 h with conventional media. The sensitivity was increased after 48 h of incubation to 98.7, 96.2, and 95.6% with S. aureus ID, CHROMagar Staph. aureus, and conventional media, respectively. A total of 97.4% of green colonies on S. aureus ID were confirmed as S. aureus compared with 94.4% of mauve colonies on CHROMagar Staph. aureus. We conclude that S. aureus ID is a highly sensitive and specific medium for the isolation and identification of S. aureus from wound swabs.     

Methicillin-resistant Staphylococcus aureus (MRSA) detection: comparison of two molecular methods (IDI-MRSA PCR assay and GenoType MRSA Direct PCR assay) with three selective MRSA agars (MRSA ID, MRSASelect, and CHROMagar MRSA) for use with infection-control swabs

Methicillin-resistant Staphylococcus aureus (MRSA) is an increasing problem. Rapid detection of MRSA-colonized patients has the potential to limit spread of the organism. We evaluated the sensitivities and specificities of MRSA detection by two molecular methods (IDI-MRSA PCR assay and GenoType MRSA Direct PCR assay) and three selective MRSA agars (MRSA ID, MRSASelect, and CHROMagar MRSA), using 205 (101 nasal, 52 groin, and 52 axillary samples) samples from consecutive known MRSA-infected and/or -colonized patients. All detection methods had higher MRSA detection rates for nasal swabs than for axillary and groin swabs. Detection of MRSA by IDI-MRSA was the most sensitive method, independent of the site (94% for nasal samples, 80% for nonnasal samples, and 90% overall). The sensitivities of the GenoType MRSA Direct assay and the MRSA ID, MRSASelect, and CHROMagar MRSA agars with nasal swabs were 70%, 72%, 68%, and 75%, respectively. All detection methods had high specificities (95 to 99%), independent of the swab site. Extended incubation for a further 24 h with selective MRSA agars increased the detection of MRSA, with a corresponding decline in specificity secondary to a significant increase in false-positive results. There was a noticeable difference in test performance of the GenoType MRSA Direct assay in detection of MRSA (28/38 samples [74%]) compared with detection of nonmultiresistant MRSA (17/31 samples [55%]) (susceptible to two or more non-beta-lactam antibiotics). This was not observed with selective MRSA agar plates or IDI-MRSA. Although it is more expensive, in addition to rapid turnaround times of 2 to 4 h, IDI-MRSA offers greater detection of MRSA colonization, independent of the swab site, than do conventional selective agars and GenoType MRSA Direct.     

New chromogenic identification and detection of Staphylococcus aureus and methicillin-resistant S. aureus

This paper describes a new chromogenic plate medium, CHROMagar Staph aureus (CHROMagar, Paris, France), for the identification of Staphylococcus aureus on the basis of colony pigmentation. The abilities of CHROMagar Staph aureus, thermostable nuclease (DNase), and mannitol salt agar (MSA) to identify S. aureus isolates (n = 114) and discriminate between S. aureus and coagulase-negative staphylococci (CoNS; n = 22) were compared. CHROMagar Staph aureus proved to be more sensitive and specific than DNase and MSA, allowing a reliable, simple, and rapid method for the identification of S. aureus isolates. All CoNS encountered in this study with the exception of S. chromogenes could be easily differentiated from S. aureus on this medium. The supplementation with 4 microgram of oxacillin or methicillin per ml allowed simple identification of methicillin resistance in hospital-acquired S. aureus strains which show multiple-drug resistance profiles. Community-acquired methicillin-resistant S. aureus strains showing non-multi-drug resistance profiles require further evaluation on this new chromogenic medium. Methicillin or oxacillin resistance of all S. aureus isolates was confirmed by the detection of penicillin-binding protein 2a, encoded by the mecA gene, using the latex slide agglutination MRSA-Screen test (PBP 2' Test, DR900M; Oxoid).     

Comparison of MRSASelect Agar,CHROMagar Methicillin-Resistant Staphylococcus aureus (MRSA) Medium,and Xpert MRSA PCR for Detection of MRSA in Nares: Diagnostic Accuracy for Surveillance Samples with Various Bacterial Densities

Rapid laboratory methods provide optimal support for active surveillance efforts to screen for methicillin-resistant Staphylococcus aureus (MRSA). Most laboratories struggle to determine the optimal use of resources, considering options to balance cost, speed, and diagnostic accuracy. To assess the performance of common methods, the first comparison of MRSASelect agar (MS) and CHROMagar MRSA (CA), with and without broth enrichment followed by a 24-h subculture to MS, was performed. Results were compared to those of the Xpert MRSA assay. For direct culture methods, the agreement between MS and CA was 98.8%. At 18 h, direct MS identified 93% of all positive samples from direct culture and 84% of those identified by the Xpert MRSA. For Trypticase soy broth-enriched MS culture, incubated overnight and then subcultured for an additional 24 h, the agreement with Xpert MRSA was 96%. The agreement between direct MS and Xpert MRSA was 100% when semiquantitative culture revealed a bacterial density of 2+ or greater; however, discrepancies between culture and Xpert MRSA arose for MRSA bacterial densities of 1+ or less, indicating low density as a common cause of false-negative culture results. Since 1+ or less was established as the most common MRSA carrier state, broth enrichment or PCR may be critical for the identification of all MRSA carriers who may be reservoirs for transmission. In this active-surveillance convenience sample, the use of broth enrichment followed by subculture to MS offered a low-cost but sensitive method for MRSA screening, with performance similar to that of Xpert MRSA PCR.The spread of methicillin-resistant Staphylococcus aureus (MRSA) is a major concern in healthcare settings because human “carriers” can spread MRSA to others, resulting in increased morbidity, mortality, and costs (13, 14, 17). One strategy to help prevent and control MRSA infections is the use of active surveillance cultures to screen patients for nasal carriage of MRSA, a practice coupled with appropriate barrier precautions for colonized or infected patients. Active surveillance programs are growing in number in the United States (10, 20, 23, 24), despite differences in opinions about the practice and reported gaps in the literature (19). Therefore, clinical microbiology laboratories must respond to provide support for active surveillance screening methods.There is continued debate about the practicality and cost benefit of different MRSA detection methods. Typically, agar culture or PCR methods are used (4); however, additional workload and costs cause laboratories to struggle with resource allocation issues, which often depend on the interplay between assay accuracy, turnaround time, cost per test, and workforce availability. While PCR results can be available in as little as 2 h and are known to provide excellent sensitivity and specificity for MRSA screening (4, 25, 28), PCR methods are more costly than culture methods (6). Alternately, selective and differential chromogenic agars have gained popularity because of lower cost and mid-range speed. Because of their resistance to antimicrobials in the agar, MRSA colonies grow and subsequently produce distinct color changes caused by the cleavage of a chromogenic substrate by a specific enzyme of S. aureus. Results can be achieved in as little as 18 to 24 h of incubation, depending on the agar used (1, 4, 5, 15, 27; BBL CHROMagar MRSA package insert no. 8012632, 2006 [Becton Dickinson and Company]; MRSASelect package insert nos. 63747 and 63749, 2007 [Bio-Rad Laboratories]). Although less costly, direct agar cultures are relatively insensitive in comparison to PCR and broth enrichment cultures. Broth enrichment cultures can improve sensitivity in comparison to that of nonenriched (direct) cultures (2, 9, 16, 21); however, enrichment delays results and adds to workload and costs. Thus, a discussion of the advantages and limitations of each methodological approach become critical. Decisions about the type of screening method used must be made based on the performance of the methods and resources available for each health care system. Clearly, the most sensitive screening system is desirable; however, at times, financial realities may affect our clinical practices.Because of the critical balance between speed, accuracy, cost, and hands-on time associated with each screening method, we designed and evaluated a broth enrichment algorithm that relies on the use of chromogenic agar as the subculture medium for broth enrichment in order to maximize both the speed and sensitivity of agar-based approaches. We compared the method''s performance to that of a PCR reference method, the Cepheid Xpert MRSA assay.To our knowledge, the study is the first clinical assessment of Bio-Rad''s MRSASelect agar (MS) and Becton Dickinson''s BBL CHROMagar MRSA (CA) combined with broth enrichment and compared to the Xpert MRSA assay. It is also the first study to describe the performance of the methods relative to the various bacterial densities of MRSA which are found in active surveillance samples from hospitalized patients. The study goal was to validate an off-label use of MS as a subculture medium, in combination with tryptic soy broth (TSB) enrichment, to improve the recovery of MRSA bacteria with minimum impact to workload and cost.(This information was presented, in part, at the 108th General Meeting of the American Society for Microbiology, Boston, MA, 1 to 5 June 2008 [18].)     

Evaluation of a cefoxitin disk diffusion test for the routine detection of methicillin-resistant Staphylococcus aureus

Two oxacillin disk methods were compared with a cefoxitin disk diffusion test for detection of methicillin-resistant Staphylococcus aureus (MRSA), with PCR for mecA as the reference method. When tested with 115 MRSA and 350 methicillin-susceptible S. aureus isolates, the cefoxitin disk test (specificity 100%, sensitivity 96.5%) was superior to the oxacillin disk methods (specificity 99.1%, sensitivity 90.4%). Testing with both oxacillin and cefoxitin disks would give better sensitivity (100%) than the cefoxitin test alone, but at the expense of specificity (99.1%). The cefoxitin disk test required no special test conditions and would improve the reliability of routine tests for detection of MRSA.