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.     

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.     

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.     

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.     

Prospective comparison of a new chromogenic medium, MRSASelect, to CHROMagar MRSA and mannitol-salt medium supplemented with oxacillin or cefoxitin for detection of methicillin-resistant Staphylococcus aureus

MRSASelect agar was compared to CHROMagar, mannitol-salt agar with oxacillin, and mannitol-salt agar with cefoxitin (MSA-CFOX) for the isolation of methicillin-resistant Staphylococcus aureus (MRSA). The sensitivities and specificities were 97.3% and 99.8%, 82.9% and 99.1%, 80.2% and 79%, and 99.1% and 84.8%, respectively. MSA-CFOX and MRSASelect had a high sensitivity. MRSASelect, however, was more specific and proved to be a more reliable and rapid medium for the detection of MRSA.     

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.     

Rapid detection of methicillin-resistant Staphylococcus aureus by Crystal MRSA ID System.

A commercially available method for the rapid detection of methicillin-resistant Staphylococcus aureus (BBL Crystal MRSA ID System) was evaluated and compared with conventional methods. All 52 isolates of methicillin-susceptible and 142 isolates of intrinsic methicillin-resistant S. aureus were correctly identified in 4 h by the test method, whereas correct identification took 11 to 24 h by conventional methods. The test is simple, rapid, and easy to perform and the results are easy to interpret.     

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%.     

Rapid confirmation of suspected methicillin-resistant Staphylococcus aureus colonies on chromogenic agars by a new commercial PCR assay, the GenomEra MRSA/SA Diagnose

A new automated closed tube PCR assay, the GenomEra(?) MRSA/SA Diagnose (Abacus Diagnostica Oy, Finland) was evaluated for rapid confirmation of methicillin-resistant Staphylococcus aureus (MRSA) from cultured screening specimens. The ability of the assay to detect genotypically different MRSA strains was studied with a collection of 304 MRSA isolates covering 68 spa types. The specificity was investigated with a collection of 146 non-MRSA staphylococcus isolates. The usefulness of the assay for clinical purposes was assessed by a sequential combination of MRSA screening culture and confirmation of the colonies with the GenomEra MRSA/SA Diagnose assay. A total of 145 suspected MRSA colonies on chromogenic plates were analyzed this way. All MRSA isolates from the culture collection and from the clinical screening specimens were confirmed as MRSA with the GenomEra MRSA/SA Diagnose assay and none of the non-MRSA staphylococci caused false-positive results, which indicates both sensitivity and specificity of 100%. The combination of GenomEra MRSA/SA Diagnose with preceding culture on selective MRSA agar permitted MRSA confirmation within 24 h. This practice offers a reliable and quick detection of MRSA that is also suitable in areas where several strain types cause epidemics.     

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.     

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.     

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).     

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.     

Evaluation of four selective agars and two enrichment broths in screening for methicillin-resistant Staphylococcus aureus

To evaluate methicillin-resistant Staphylococcus aureus detection, we tested in vitro four selective agars and two enrichment broths apart and in combination. Tryptone soya broth with salt, aztreonam, and cefoxitin appeared to be the most sensitive medium. This broth was superior to a phenol red mannitol broth with aztreonam and ceftizoxime.     

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.     

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 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.     

Detection of methicillin-resistant Staphylococcus aureus (MRSA) in blood with the EVIGENE MRSA detection kit

A total of 200 blood cultures containing putative staphylococci were analyzed by a commercial gene probe hybridization assay (EVIGENE; Statens Serum Institut, Copenhagen, Denmark), and 18 were identified as methicillin-resistant Staphylococcus aureus (MRSA) positive. Of these, 17 were positive by PCR and 16 were positive by culture. Detailed analysis of the discrepant results showed that the EVIGENE kit allowed specific identification of MRSA in blood cultures without any of the drawbacks associated with PCR.     

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.