Performance of a new chromogenic medium, BBL CHROMagar MRSA II (BD), for detection of methicillin-resistant Staphylococcus aureus in screening samples

Sensitivity and Specificity of a Novel Colony Characteristic for Determination of Methicillin-Resistant Staphylococcus aureus

Purpose: To assess colony morphology of Staphylococcus aureus isolates for target shape (T1) and its utility in the identification of methicillin-resistant S. aureus (MRSA).

Methods: Staphylococcus species isolated from blood cultures were studied for colony morphology characteristics. A polymerase chain reaction (PCR) test was performed on positive blood culture bottles for the detection of S. aureus and methicillin resistance. Colony morphology was read at 24 and 48 hours and defined as follows: target shaped (T1) - an elevated colony center encircled by a pale zone, which is surrounded by a single ring of peripheral enhancement giving a ‘target’ appearance; dome-shaped (T2) with an elevated center lacking the ‘target’ appearance.

Results: At 48 hours, 73.7% of MRSA and 59.5% of coagulase-negative staphylococci (CoNS) showed T1 morphology. T1 morphology has a sensitivity of 73.68% and specificity of 93.55% amongst S. aureus for identification of methicillin resistance and a high positive predictive value (95.45%) at 48 hours.

Conclusion: T1 morphology has a modest sensitivity with specificity and positive predictive value amongst S. aureus for identification of methicillin resistance at 48 hours. It can be potentially used for the identification of MRSA, especially in resource-limited settings and wherein a molecular test is not repeated if PCR testing has already identified methicillin-sensitive S. aureus (MSSA) on a recent specimen on the same patient.

Genetic and phenotypic study of methicillin-resistant Staphylococcus aureus among patients and health care workers in Mansoura University Hospital, Egypt

BACKGROUND AND OBJECTIVES

Staphylococcus aureus is common pathogen that is associated with many hospital acquired infections. The virulence of S. aureus is identified with resistance to antibiotics especially to methicillin. Therefore the aims of the present study were to detect the carrier rates of methicillin-resistant S. aureus (MRSA) among health care Workers (HCWs) and patients and to compare use of specific chromogenic agar for MRSA culture with PCR for detection of MRSA genes.

MATERIALS AND METHODS

Samples obtained were subjected to full microbiological laboratory studies involving culture on specific chromogenic medium and antibiotics susceptibility testing for detection of MRSA and their resistance rates to other commonly used antibiotics. Furthermore multiplex PCR was carried out to detect SCCmecA genes.

RESULTS

Staphylococcus aureus was isolated from 70 (29.9%) of the studied subjects. MRSA isolates (n=28) had high resistance rates for the used antibiotics and the most common resistance was for ciprofloxacin and chloramphenicol (57.1% for each). MRSA was isolated mainly from health care workers (17.02%). The frequency of SCCmecA was 60.7% for type I, 25% for type III and 14.3% for type II. Chromogenic agar identified correctly MRSA isolates in 92.9%. PCR was positive in all isolates with resistance to cefoxitin disc.

CONCLUSIONS

The present study highlights that MRSA carriage is common among health care workers in one Egyptian tertiary care hospital. The major genotype of MRSA is belonging to SCCmecA type I followed by type III and type II. ChromID medium is an accurrate culture method for detection of MRSA compared to molecular method.

Screening for methicillin-resistant Staphylococcus aureus colonization using sponges

OBJECTIVE Nasal swab culture is the standard method for identifying methicillin-resistant Staphylococcus aureus (MRSA) carriers. However, this method is known to miss a substantial portion of those carrying MRSA elsewhere. We hypothesized that the additional use of a sponge to collect skin culture samples would significantly improve the sensitivity of MRSA detection. DESIGN Hospitalized patients with recent MRSA infection were enrolled and underwent MRSA screening of the forehead, nostrils, pharynx, axilla, and groin with separate swabs and the forehead, axilla, and groin with separate sponges. Staphylococcal cassette chromosome mec (SCCmec) typing was conducted by polymerase chain reaction (PCR). PATIENTS A total of 105 MRSA patients were included in the study. RESULTS At least 1 specimen from 56.2% of the patients grew MRSA. Among patients with at least 1 positive specimen, the detection sensitivities were 79.7% for the swabs and 64.4% for the sponges. Notably, 86.4% were detected by a combination of sponges and nasal swab, and 72.9% were detected by a combination of pharyngeal and nasal swabs, whereas only 50.9% were detected by nasal swab alone (P<0.0001 and P=0.0003, respectively). Most isolates had SCCmec type II (59.9%) and IV (35.7%). No correlation was observed between the SCCmec types and collection sites. CONCLUSION Screening using a sponge significantly improves MRSA detection when used in addition to screening with the standard nasal swab.

Carriage and Genetic Diversity of Methicillin-Resistant Staphylococcus aureus among Patients and Healthcare Workers in a Serbian University Hospital

OBJECTIVES

There is a paucity of data on methicillin-resistant Staphylococcus aureus (MRSA) epidemiology among Balkan countries. The aim of our study was to determine the prevalence of nasal and pharyngeal carriages and diversity of MRSA among patients and healthcare workers (HCWs) in the major referral centre in Serbia, and to evaluate performance of three different media for MRSA screening.

METHODS

Nasal and pharyngeal swabs were obtained from 195 patients and 105 HCWs in Emergency Department (ED), Surgical Department (SD) and Medical Department (MD). After broth enrichment, samples were inoculated onto MRSA-ID, ORSA and oxacillin-MSA and incubated for 24/48 hours. Characterisation of isolated MRSA strains was determined by MLVA, spa, SCCmec and agr typing, PVL genes detection and antimicrobial susceptibility.

RESULTS

MRSA carriage prevalence was 11.8% in patients and 7.6% in HCWs. Introduction of pharyngeal swabs in screening procedure increased MRSA carriage rate by over 30%. Variable found to be independently associated with an increased risk for MRSA carriage was ED (odd ratio (OR) = 4.45, 95% confidence interval (CI) 1.78-11.14). A higher risk of multidrug-resistant MRSA carriage was observed among patients (OR = 22; 95% CI 1.92-251.54). CC5-MRSA-SCCmecI was the dominant clone among patients and HCWs in ED and MD, while high genetic diversity of community-associated MRSA (CA-MRSA) was shown in SD especially among HCWs. MRSA-ID was superior to the other tested media with a sensitivity/specificity of 95.2% and 99.6% after 48 hours of incubation.

CONCLUSIONS

These results indicate high MRSA carriage rate in the hospital and emergence of CA-MRSA through HCWs in these settings. MRSA-ID was the optimal available choice for MRSA screening.

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.

Detection of oxacillin-susceptible mecA-positive Staphylococcus aureus isolates by use of chromogenic medium MRSA ID

Reports of oxacillin-susceptible mecA-positive Staphylococcus aureus strains are on the rise. Because of their susceptibility to oxacillin and cefoxitin, it is very difficult to detect them by using routine phenotypic methods. We describe two such isolates that were detected by chromogenic medium and confirmed by characterization of the mecA gene element.

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.

Transmission rates, screening methods and costs of MRSA--a systematic literature review related to the prevalence in Germany

Methicillin-resistant Staphylococcus aureus (MRSA) infections represent a serious challenge for health care institutions, which is inherent in the combination of prevalence, transmission rates and costs. Furthermore, performing an MRSA screening requires information on the complex system of effectiveness, accuracy and costs of different screening methods. The purpose of this study was to give an overview of parameters with decisive significance for the burden of MRSA and the selection of a specific MRSA screening strategy. A systematic literature search for peer-reviewed health economic studies associated with MRSA was performed (from 1995 to the present). Eighty-seven different studies met all inclusion and exclusion criteria. Primary outcomes included the prevalence of MRSA, MRSA transmission rates, performance characteristics of MRSA screening methods, costs for pre-emptive isolation precautions and costs per MRSA case. The prevalence rates reported for all inpatients (1.2-5.3 %) as well as for inpatients with risk factors or patients in risk areas (3.85-20.6 %) vary greatly. The range of cross-transmission rates per day reported for patients with MRSA in isolation is 0.00081-0.009 and for carriers not in isolation is 0.00137-0.140, respectively. For polymerase chain reaction (PCR) methods, the mean sensitivity and specificity were 91.09 and 95.79 %, respectively. Culture methods show an average sensitivity of 89.01 % and an average specificity of 93.21 %. The turn-around time for PCR methods averages 15 h, while for the culture method, it can only be estimated as 48-72 h. This review filtered important parameters and cost drivers, and covered them with literature-based averages. These findings serve as an ideal evidence base for further health economic considerations of the cost-effectiveness of different MRSA screening methods.

Performance evaluation of a modified chromogenic medium, ChromID MRSA New, for the detection of methicillin-resistant Staphylococcus aureus from clinical specimens

A novel chromogenic medium for the detection of methicillin-resistant Staphylococcus aureus (MRSA), ChromID MRSA New, was evaluated and compared with the original ChromID MRSA agar, using 355 consecutive screening specimens from nose (120), throat (121) and perineum (114). The specimens were collected with an E-swab and inoculated within 24 hours onto both ChromID MRSA New and on ChromID MRSA. ChromID MRSA New was more sensitive than ChromID MRSA in detecting MRSA after 24 hours of incubation (94.3% versus 81.4%; p < 0.05). With the ChromID MRSA New, processing time is reduced from 48 h to 24 h and confirmation of the resistance to methicillin is redundant.