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%.     

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.     

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.     

Comparison of three selective chromogenic media for Methicillin-Resistant Staphylococcus aureus detection

PURPOSE: Rapid detection of Methicillin-Resistant Staphylococcus aureus (MRSA) is of major importance in hospital hygiene. In order to reduce the response time from screening laboratories, new selective chromogenic media have been developed and marketed by major microbiology companies. In this context, an evaluation of their performances was needed. MATERIALS AND METHODS: Media produced by Bio-Rad Laboratories (MRSASelect), Becton Dickinson (CHROMagar MRSA) and bioMérieux (chromID MRSA) were studied by 203screening samples, 110 of which were MRSA positive. Each Stahylococcus aureus was identified by catalase detection, Staphytect Plus Dryspot latex agglutination test and free coagulase detection, in addition to mannitol fermentation and Voges-Proskauer tests in the case of doubtful identification. Resistance was verified by checking the inhibition zone diameter of under 20 mm on 30 microg cefoxitin disks. RESULTS: Bio-Rad Laboratories, Becton Dickinson and bioMérieux media read at 24 or 24/48 hours have a respective sensitivity of 91, 71 and 85/92%. The specificity of these media is 99, 100 and 99/93%. CONCLUSION: These media proved to be new powerful and rapid tools used in screening for MRSA detection. MRSASelect is one of the most effective media showing high sensitivity and specificity and the easiest interpretation. chromID MRSA exhibits similar performance but needs more time to be as effective as Bio-Rad media while CHROMagar MRSA isn't enough efficient with its slightly lack of sensitivity to perform a reliable screening.     

Performance of CHROMagar selective medium and oxacillin resistance screening agar base for identifying Staphylococcus aureus and detecting methicillin resistance

Two new selective media, oxacillin resistance screening agar base (ORSAB) and CHROMagar Staph aureus (CSA), were evaluated for identification of Staphylococcus aureus and for screening of methicillin resistance by addition of antimicrobial agents to these media. A well-defined collection consisting of 1,140 staphylococci was used. A total of 624 were S. aureus, of which 358 were methicillin susceptible and 266 were methicillin resistant, and 516 were coagulase-negative staphylococci. The methicillin-resistant S. aureus (MRSA) strains were selected based on the results of phage typing; 247 different types were included in the analysis. For identification of S. aureus, both media performed better after 24 h than after 48 h. The sensitivities at 24 h were comparable (CSA, 98.6%; ORSAB, 97.1%), but the specificity of CSA was significantly higher (CSA, 97.1%; ORSAB, 92.1%). For screening of methicillin resistance, antibiotic supplements were added to both media. The sensitivity was lower after 24 h (CSA, 58.6%; ORSAB, 84.2%) and increased significantly after 48 h (CSA, 77.5%; ORSAB, 91.4%). At both time intervals ORSAB was significantly more sensitive than CSA. However, the specificities of both media were high after 24 h (CSA, 99.1%; ORSAB, 98.3%) and decreased significantly after 48 h of incubation (CSA, 94.7%; ORSAB, 95.5%). In conclusion, for identification of S. aureus, CSA is more accurate than ORSAB because of a significantly higher specificity. For screening of MRSA, ORSAB performs better than CSA, but the usefulness in clinical practice is limited because a significant number of strains are not detected.     

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.     

Methicillin-resistant Staphyloccocus aureus: evaluation of five selective media

This study evaluates the performance in isolating methicillin-resistant Staphylococcus aureus (MRSA) of three media: the reduced salt formulation of mannitol salt agar plus oxacillin (MMSAO); CHROMagar Staph aureus plus ciprofloxacin (CHRAC); and Halifax MRSA medium (HMO), against the previously recommended mannitol salt agar (7% salt) plus oxacillin (OMSAO) and Baird-Parker medium plus ciprofloxacin (BPC). MRSA screening swabs were plated out onto the five selective media and the plates examined at 24 and 48 h. Suspected colonies were confirmed as MRSA by detection of heat-labile DNase, coagulase and/or protein A, and by confirming resistance to methicillin. Of 719 specimens examined, 191 grew MRSA on at least one medium. The relative sensitivities of the five media at 48 h were as follows: BPC, 94%; CHRAC, 70%; OMSAO, 61%; HMO, 56%; and MMSAO, 46%. In addition, BPC gave the least number of unnecessary investigations for non-MRSA isolates. The current advantage of BPC when performing direct culture for MRSA was confirmed. The other ciprofloxacin-containing medium also produced reasonable results. Of the two mannitol salt agar media, the formulation with 7% salt gave better results. HMO proved unreliable at isolating MRSA.     

Detection of nasal colonization methicillin-resistant Staphylococcus aureus: a prospective study comparing real-time genic amplification assay vs selective chromogenic media

In contrast to "classical" genic amplification, real-time genic amplification can be performed in every laboratory without the need of sophisticated isolation procedures. Moreover, real-time genic amplification allows an early detection of meticillin resistant Staphylococcus aureus colonization, 2 hours compared to 1 or 2 days for culture. OBJECTIVE: In order to assess the feasibility on Smartcycler of the IDI-MRSA real-time genic amplification assay in comparison with chromogenic media. METHODS: A prospective study has been initiated in July 2004: nasal swabs were taken from patients entering the ICU, vascular surgery, diabetology and geriatry wards. During a 4 months period, 682 specimens have been obtained from 508 patients. RESULTS: Sixty-four (9.3%) patients were positive by genic amplification and selective agar culture (CHROMagar MRSA, MRSASelect and/or ORSAB), 19 (2.9%) were positive by genic amplification only (3 of these patients were under antibiotic treatment); 572 specimens remained negative by both methods. The sensitivity and specificity of this assay were 100% and 96% respectively with a positive predictive value of 70% and negative predictive value of 100%. Initially 82 nasal specimens were unresolved (12%). 38 were resolved following a freeze-thaw cycle. Thus, 44 (6.4%) were unresolved specimens. Comparison between CHROMagar MRSA and MRSASelect showed a good correlation for the detection at 24 hours (5.5% and 5.6% respectively). These two chromogenic media allowed a much better detection of MRSA than ORSAB medium within 24H. CONCLUSION: The results obtained by the early real-time genic amplification for the detection of meticillin resistant Staphylococcus aureus are promising. Despite 6.4% amplification failure, we consider that IDI-MRSA real-time genic amplification assay represents a significant breakthrough in the detection of colonization.     

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.     

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.     

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.     

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.     

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.     

Direct detection of Staphylococcus osteoarticular infections by use of Xpert MRSA/SA SSTI real-time PCR

We evaluated the Xpert MRSA/SA SSTI real-time PCR assay (Cepheid, Sunnyvale, CA) directly on perioperative bone and joint samples. The sensitivity and specificity for detection of methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant Staphylococcus aureus (MRSA), and methicillin-resistant coagulase-negative Staphylococcus were, respectively, 100% and 98.3%, 100% and 100%, and 100% and 95.3%. The median total test turnaround time was 72 min for PCR versus 79 h for culture. Using these rapid results, appropriate antibiotic treatment could be rapidly initiated.     

Methicillin resistance among isolates of Staphylococcus aureus: antibiotic sensitivity pattern & phage typing

Out of 3988 clinical specimens from hospital admitted patients 230 strains of Staphylococcus aureus were isolated, 45 strains (19.56%) were Methicillin resistance Staphylococcus aureus (MRSA). All MRSA strains were beta lactamase producers. Multidrug resistance was observed among MRSA strains more commonly than in methicillin sensitive strains of Staphylococcus aureus (MSSA). Maximum strains were resistant to penicillin (100%), co-trimoxa zole (97%) & chloramphenicol (93.33%). As least resistant to gentamicin & ciprofloxacin shown by MRSA, these drugs can be used in few situations after susceptibility test. All strains of MRSA were sensitive to vancomicin (100%). Majority of strains (34 out of 45) showed MIC values of 4 ug/ml. Twenty eight out of 44 strains were non typable using routine phages. Study revealed that MRSA with associated multidrug resistance is common in this region. There is need to develop local set of MRSA phages for improvement of typability.     

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.     

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 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.     

Rapid differentiation of methicillin-resistant and methicillin-susceptible Staphylococcus aureus by flow cytometry after brief antibiotic exposure

We noticed that methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) isolates yielded side-scatter (SSC) and fluorescence intensity (FI) differences on flow cytometry (FCM) following incubation in oxacillin broth. The purpose of this study was to determine whether MRSA and MSSA could be reliably differentiated by FCM. S. aureus isolates were incubated in oxacillin-containing Mueller-Hinton broth, stained using the FASTEST total viable organisms kit, and analyzed by FCM in the MicroPRO instrument. SSC versus FI were examined, and gates 1 and 2 were defined to encompass the majority of MSSA and MRSA signal events, respectively. A count ratio (CR) was defined as the ratio of counts in gate 2 to those in gate 1. Initially, 33 isolates were tested after 4 h of incubation for proof-of-concept. Twenty others were then tested after incubation intervals ranging from 30 min to 4 h to determine the earliest possible time for differentiation. Next, 100 separate isolates were tested to determine the best CR cutoff value. Finally, the CR was validated by using an independent cohort of 121 isolates. We noted that MRSA isolates had higher SSC and FI readings than did MSSA isolates after 2 h of incubation. The receiver-operator characteristics curve showed that a CR cutoff of 0.0445 reliably differentiated MRSA from MSSA. In the validation cohort, this cutoff had a sensitivity of 100% and a specificity of 98.7% for identifying MRSA from among S. aureus isolates, following 2 h of incubation. This study demonstrates that MRSA and MSSA can be accurately differentiated by FCM after 2 h of incubation in an oxacillin-containing liquid culture medium.