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

Comparison of rapid and conventional methods for investigating of mecA presence in Staphylococcus Species

Objectives:

Taking the determination of mecA gene by polymerized chain reaction (PCR) method as a reference in determining methicillin resistance in Staphylococcus species, we aimed at comparing the reliability levels of disk diffusion, latex agglutination test and chromogenic agar use methods.

Methods:

This prospective study was conducted on 228 Staphylococcus strains isolated between January 2020 and December 2020 in Samsun Training and Research Hospital. Disk diffusion, latex agglutination and chromogen agar medium methods were applied along with the polymerized chain reaction (PCR) method.

Results:

The mecA gene was detected in 47 of the isolates (20.6%) by the PCR method, and these isolates were accepted as methicillin-resistant. When the PCR result was taken as a reference, the sensitivity of the disk diffusion method became 100%, and specificity became 45.9%; sensitivity of latex agglutination was determined as 80.9%, and specificity as 70.2%; sensitivity of chromogenic agar as 85.1% and its specificity was found to be 95%. Only in S. aureus isolates, the highest sensitivity and specificity rate (100% and 88%, respectively) belonged to chromogenic agar.

Conclusion:

Chromogenic agar provides more reliable data for S. aureus isolates, and the combined use of all three methods does not significantly increase reliability.

Universal admission screening: a potential game-changer in hospitals with high prevalence of MRSA

INTRODUCTION

Despite the perception that meticillin-resistant Staphylococcus aureus (MRSA) is now under control in high-income countries, global prevalence remains high, even increasing in some regions. Universal admission screening and decolonization has been instituted in some hospitals to attempt control but the practice remains controversial.

METHODS

In 2014, Mater Dei Hospital in Malta introduced a universal admission screening policy, utilizing a novel, centralized and customized approach to achieve high compliance and low cost. Admissions are screened nasally by designated staff using chromogenic media, irrespective of risk factors. Carriers are decolonized without concurrent isolation or contact precautions. In this study, longitudinal, quasi-experimental evaluation was undertaken using time series analysis to analyse the impact of the intervention on the proportion of MRSA among clinical S. aureus isolates (%MRSA) and incidence per 1000 bed-days. A cost-utility analysis was also attempted to identify approximate quality-adjusted life years (QALYs) gained.

RESULTS

A transfer function model approach concluded that the intervention had a significant effect on both %MRSA and incidence. Six years following its introduction, the screening programme had led to an overall 43% long-term reduction in %MRSA from pre-screening levels [R²=0.687; Bayesian information criterion (BIC)=4.063], translating to a decrease in incidence of approximately 0.56 cases/1000 bed-days (R²=0.633, BIC=-3.063). No correlation was identified with consumption of antibiotics or alcohol hand rub. The annual cost-benefit of the programme was calculated at €1058 per QALY gain per year.

CONCLUSION

The universal admission screening and decolonization intervention was successful and cost-effective in this high-endemicity setting. It facilitated improvement in the prevalence of MRSA, achieving reduction levels rarely reported by Mediterranean hospitals.

Prevalence of Methicillin-resistant Staphylococcus aureus (MRSA) Carrier in Hemodialysis Patients at Dr. Soetomo Academic General Hospital

Chronic kidney disease (CKD) is now a global epidemic, and the prevalence is increasing worldwide. Hemodialysis    is one of the ways to treat by kidney function replacement. Infection is the number two cause of death in patients with hemodialysis (HD). Methicillin-resistant Staphylococcus aureus (MRSA) is a common cause of bacteriemia in patients with dialysis. The epidemiological data of MRSA carriers in CKD in Indonesia are still scarce. This study was to determine the prevalence of MRSA carriers in patients at The Kidney and Hypertension Outpatient-clinic and Hemodialysis Installation at Dr. Soetomo Academic General Hospital, Surabaya Indonesia. The study design was descriptive-analytic with a cross- sectional study design. Sampling was collected consecutively. Data on the general characteristics of the research subjects will be analyzed using a Chi-Squared test. There were 150 CKD stage five patients included in this study, the number  of patients has MRSA carrier were 6 (4%), among them, subjects underwent HD MRSA carrier were 2 subjects(2.7%), while for non-HD patients with MRSA were 4 subjects (5.3 %). There were no significant differences in MRSA carriers between HD and non HD groups (p=0.404). Comorbid factors that accompany MRSA carriers are diabetes mellitus, hypertension, kidney stones, gout, and systemic lupus erythematosus (SLE).  This study found, there were no significant differences in the incidence of MRSA carriers in stage five CKD non HD or HD groups. MRSA colonization exists in stage five CKD sufferers, so awareness of MRSA colonization

Occurrence and characteristics of methicillin-resistant Staphylococcus aureus (MRSA) in buffalo bulk tank milk and the farm workers in Italy

The aim of the present study was to assess the occurrence of MRSA in buffalo dairy farms and in buffalo tank milk from Italy, and to provide information about the antimicrobial resistance profile and molecular characteristics of the isolates. We collected 75 bulk tank milk (BTM) samples from 75 farms and 24 nasal swabs from 24 farm operators. Three (4%) of the 75 BTM samples and 1 (4%) of the 24 human nasal swabs tested positive for MRSA. The milk isolates belonged to the genotypes ST1/t127/Va and ST72/t3092/V, while the human isolate was characterized as ST1/t127/IVa. All isolates were multidrug resistant but vancomycin susceptible; they carried the icaA gene but tested negative for the pvl and ses genes. ST72 is a CA-MRSA commonly found in South Korea, and this is the first report of its detection in Europe. Although we found a low prevalence of MRSA in the farms we surveyed, this study clearly demonstrates, for the first time in Europe, that MRSA can be found in dairy buffalo farms and in raw buffalo milk. Therefore, the risk of human colonization/infection with MRSA linked to the handling of raw milk or consumption of contaminated dairy products should not be ruled out.

MRSA in swine, farmers and abattoir workers in Southern Italy

Methicillin-resistant Staphylococcus aureus (MRSA) is an important medical issue, since it causes serious and sometimes fatal infections in humans. Intensively reared swine may serve as reservoirs for MRSA that can infect swine workers, and also consumers (via contaminated meat). In this study, MRSA strains were isolated from 55 of the 85 (64.7%) intensive pig farms surveyed, and prevalence was greater on pig fattening farms than on breeding farms. In addition, we included in the study 63 foreign pigs imported for slaughter. Overall, the prevalence of MRSA in the 418 sampled swine was 59.1%; 12 genotypes were identified among the isolates; ST398 (96.4%) was most prevalent, followed by ST97 (2%), ST9 (0.8%) and ST1 (0.8%). MRSA isolates were also detected in 26 (17.3%) of the 150 operators included in the study; the genotypes detected were ST398 (85%), ST9 (7.6%), ST5 (3.8%) and ST1 (3.8%). All the strains were pvl negative and pia positive. Both swine and human strains displayed a multi-resistance pattern, and almost all were resistant to tetracycline. The results obtained in this study confirm the high prevalence of MRSA in swine reared and slaughtered in Italy, and underline the public health risk linked to the spread of antimicrobial-resistant Staphylococcus aureus among intensively reared pigs.

Skin asepsis protocols as a preventive measure of surgical site infections in dogs: chlorhexidine-alcohol versus povidone-iodine

Background

Most of surgical site infections (SSI) are caused by commensal and pathogenic agents from the patient’s microbiota, which may include antibiotic resistant strains. Pre-surgical asepsis of the skin is one of the preventive measures performed to reduce SSI incidence and also antibiotic resistance dissemination. However, in veterinary medicine there is no agreement on which biocide is the most effective.

The aim of this study was to evaluate the effectiveness of two pre-surgical skin asepsis protocols in dogs. A total of 46 animals were randomly assigned for an asepsis protocol with an aqueous solution of 7.5% povidone-iodine or with an alcoholic solution of 2% chlorhexidine. For each dog, two skin swab samples were collected at pre-asepsis and post-asepsis, for bacterial quantification by conventional techniques and isolation of methicillin-resistant species.

Results

Most samples collected at the post-asepsis did not present bacterial growth, both for the animals subjected to the povidone-iodine (74%) or to the chlorhexidine (70%) protocols. In only 9% of the cases a significant bacterial logarithmic reduction was not observed, indicating possible resistance to these agents. Also, the logarithmic reduction of the bacterial quantification from pre- and post-asepsis time, was not statistically different for povidone-iodine (6.51 ± 1.94 log10) and chlorhexidine (6.46 ± 2.62 log10) protocol.

From the 39% pre-asepsis swabs which showed bacterial growth in MRSA modified chromogenic agar medium, only one isolate was identified as Staphylococcus aureus and one as S. epidermidis. False positives were mainly other staphylococci species, as well as Enterobacteriaceae.

Conclusions

Pre-surgical skin asepsis protocols with povidone-iodine or chlorhexidine showed similar efficacy in the elimination of methicillin resistant bacteria and preventing surgical site infections in dogs undergoing surgery.

The Role of Negative Methicillin-Resistant Staphylococcus aureus Nasal Surveillance Swabs in Predicting the Need for Empiric Vancomycin Therapy in Intensive Care Unit Patients

OBJECTIVES The role of methicillin-resistant Staphylococcus aureus (MRSA) nasal surveillance swabs (nasal swabs) in guiding decisions about prescribing vancomycin is unclear. We aimed to determine the likelihood that patients with negative MRSA nasal swabs develop subsequent MRSA infections; to assess avoidable vancomycin days for patients with negative nasal swabs; and to identify risk factors for having a negative nasal swab and developing a MRSA infection during the intensive care unit (ICU) stay. METHODS This retrospective cohort study was conducted in 6 ICUs at a tertiary-care hospital from December 2013 through June 2015. The negative predictive value (NPV), defined as the ability of a negative nasal swab to predict no subsequent MRSA infection, was calculated. Days of vancomycin continued or restarted after 3 days from the collection time of the first negative nasal swab were determined. A matched case-control study identified risk factors for having a negative nasal swab and developing MRSA infection. RESULTS Of 11,441 patients with MRSA-negative nasal swabs, the rate of subsequent MRSA infection was 0.22%. A negative nasal swab had a NPV of 99.4% (95% confidence interval [CI], 99.1%-99.6%). Vancomycin was continued or started after nasal swab results were available in 1,431 patients, translating to 7,364 vancomycin days. No risk factors associated with MRSA infection were identified. CONCLUSIONS In our hospital with a low prevalence of MRSA transmission, a negative MRSA nasal swab was helpful in identifying patients with low risk of MRSA infection in whom empiric vancomycin therapy could be stopped and in whom the subsequent initiation of vancomycin therapy during an ICU admission could be avoided. Infect Control Hosp Epidemiol 2018;39:290-296.

High Occurrence of Methicillin-Resistant Staphylococcus aureus in Horses at Slaughterhouses Compared with Those for Recreational Activities: A Professional and Food Safety Concern?

The epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) in horses and its zoonotic potential is poorly understood. The objective of this study is to provide data on the prevalence and genetic characteristics of MRSA isolated from horses on farms, at racecourses, and at slaughterhouses in Italy, using standard and molecular methods. In addition, we report the prevalence of MRSA in horse handlers. Among 388 horses tested by nasal swabs, 27 (7%) were positive for MRSA ST398 (t011, t899, t1255) and ST1 (t127). The prevalence of MRSA in horses tested at slaughterhouses was significantly higher (p < 0.001) compared with those tested on farms and racecourses. Five (7%) out of 67 staff members working in close contact with horses (2 from slaughterhouse, 2 from riding stable, and 1 from racecourse) were carriers of MRSA ST398 (t011, t034) and ST1 (t127). The isolates from horses and humans carried SCCmec IVa or V and were pvl negative and pia positive. All the isolates from both horses and humans were resistant to at least two antimicrobial classes. The circulation of MRSA in horses and in humans working in close contact with them should be considered an emerging public health issue. In fact, it represents a potential risk for people who work in close contact with horses, and for horse meat consumers.

Carriage, Clinical Microbiology and Transmission of Staphylococcus aureus

Staphylococcus aureus is one of the most important bacterial pathogens in clinical practice and a major diagnostic focus for the routine microbiology laboratory. It is carried as a harmless commensal in up to two-thirds of the population at any one time predominantly not only in the anterior nares, but also in multiple other sites such as the groin, axilla, throat, perineum, vagina and rectum. It colonizes skin breach sites, such as ulcers and wounds, and causes superficial and deep skin and soft tissue infections and life-threatening deep seated infections particularly endocarditis and osteomyelitis. S. aureus is constantly evolving through mutation and uptake of mobile genetic elements that confer increasing resistance and virulence. Since the 1960s, hospitals have had to contend with emergence of methicillin-resistant S. aureus (MRSA) strains that spread better in hospitals than methicillin-susceptible S. aureus (MSSA) and are harder to treat. Since the 1980s, distinct community MRSA strains have also emerged that cause severe skin and respiratory infections. Conventional identification of MSSA and MRSA in the microbiology laboratory involves microscopy, culture and biochemical analysis that for most samples is straightforward but slow, taking at least 48 h. This delay has significant consequences for individual patient care and public health, through inadequate or excessive empiric antibiotic use, and failure to implement appropriate infection control measures for MRSA-colonized patients during those first 48 h. This unmet need has driven development of rapid molecular diagnostics that either complement or replace conventional culture techniques in the laboratory, or can be placed in the clinical environment as point-of-care (POC) devices. These new technologies provide results to clinicians anything from within an hour to 24 h, depending on sample and clinical setting, and should transform management of patients with S. aureus and other bacterial diseases; however, uptake is often slow due to the disruptive effect of new technologies, costs of transition and uncertainty of the optimal solution given successive advances. More evidence of the health economic, clinical and antimicrobial resistance benefit will help support introduction of these new technologies. Finally, preventing MRSA transmission has been a priority for healthcare organizations for many years. There have been significant recent reductions in transmission following local and national campaigns to re-enforce basic and heightened infection control interventions such as universal hand hygiene, barrier nursing, decolonization and isolation of MRSA-colonized patients detected through routine culture or screening policies. Developments in whole genome sequencing are providing greater insight into reservoirs and routes of transmission that should help better target interventions to ensure sustainable control of endemic strains and to identify and prevent emergence of new strains.

Prospective Two-Center Comparison of Three Chromogenic Agars for Methicillin-Resistant Staphylococcus aureus Screening in Hospitalized Patients

Three chromogenic media, chromID MRSA SMART (SMART), chromID MRSA first generation (chromID), and Brilliance MRSA (OX2), were evaluated for methicillin-resistant Staphylococcus aureus (MRSA) screening using 1,220 samples. The sensitivity at 24 h was significantly better with the SMART agar (66.4%) than that with chromID agar (50.5%). Enrichment and incubation until 48 h are still needed for an optimal yield.

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.

Improved microbiological diagnostic due to utilization of a high-throughput homogenizer for routine tissue processing

Tissue specimens are valuable materials for microbiological diagnostics and require swift and accurate processing. Established processing methods are complex, labor intensive, hardly if at all standardizable, and prone to incorporate contaminants. To improve analyses from tissue samples in routine microbiological diagnostics, by facilitating, fastening, and standardizing processing as well as increasing the microbial yield, performance of Precellys 24 high-throughput tissue homogenizer was evaluated. Therefore, tissue samples were artificially inoculated with Staphylococcus aureus, Escherichia coli, and Candida albicans in 3 different ways on the surface and within the material. Microbial yield from homogenized samples was compared to direct plating method. Further, as proof of principle, routine tissue samples from knee and hip endoprosthesis infections were analyzed. The process of tissue homogenization with Precellys 24 homogenizer is easy and fast to perform and allows for a high degree of standardization. Microbial yield after homogenization was significantly higher as compared to conventional plating technique.

Assessment of the Influence of Test Characteristics on the Clinical and Cost Impacts of Methicillin-ResistantStaphylococcus aureusScreening Programs in US Hospitals

Objective.

To compare the impacts of different methicillin-resistantStaphylococcus aureus(MRSA) screening test options (eg, polymerase chain reaction [PCR], rapid culture) and program characteristics on the clinical outcomes and budget of a typical US hospital.

Methods.

We developed an Excel-based decision-analytic model, using published literature to calculate and compare hospital costs and MRSA infection rates for PCR- or culture-based MRSA screening and then used multivariate sensitivity analysis to evaluate key variables. Same-day PCR testing for a representative 370-bed teaching hospital in the United States was assessed in different populations (high-risk patients, intensive care unit [ICU] patients, or all patients) and compared with other test options.

Results.

Different screening program populations (all patients, high-risk patients, ICU patients, or patients with previous MRSA colonization or infection only) represented a potential savings of $12,158-$76,624 per month over no program ($188,618). Analysis of multiple test options in high-risk population screening indicated that same-day PCR testing of high-risk patients resulted in fewer infections over 1,720 patient-days (2.9, compared with 3.5 for culture on selective media and 3.8 for culture on nonselective media) and the lowest total cost ($112,012). The costs of other testing approaches ranged from $113,742 to $123,065. Sensitivity analysis revealed that variations in transmission rate, conversion to infection, prevalence increases, and hospital size are important to determine program impact. Among test characteristics, turnaround time is highly influential.

Conclusion.

All screening options showed reductions in infection rates and cost impact improvement over no screening program. Among the options, same-day PCR testing for high-risk patients slightly edges out the others in terms of fewest infections and greatest potential cost savings.

Analysis of reporting time for identification of methicillin-resistant Staphylococcus aureus carriers using ChromID MRSA

We assessed the reporting times for identification of nasal methicillin-resistant Staphylococcus aureus (MRSA) carriers in 2011 in a university-affiliated hospital using surveillance cultures incubated for 1 and 2 days with ChromID MRSA (bioMérieux, France). Of 2,732 nasal swabs tested, MRSA was detected in 829 (85.6%) and 140 (14.4%) swabs after 1 and 2 days of incubation, respectively, and the median reporting times for positive specimens were 33.7 hr (range, 18.2-156.9 hr) and 108.1 hr (range, 69.8-181.0 hr), respectively. Detection rate after 1-day incubation was 85%. Additional 1-day incubation improved detection rate; however, it prolonged the reporting times of positive specimens approximately up to 4 days because of the need for confirmatory tests such as species identification and susceptibility tests. Following a 2-day culture with ChromID MRSA, rapid confirmatory tests are warranted to reduce delay in identifying MRSA carriers.

Evaluation of a chromogenic biplate medium (ChromID MRSA/ChromID S. aureus) for the simultaneous detection of methicillin-resistant and methicillin-susceptible Staphylococcus aureus in preoperative screening samples from the anterior nares

We evaluated the performance of the ChromID MRSA/ChromID S. aureus biplate for the simultaneous detection of Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) in preoperative screening samples. The sensitivity and specificity were 94.2% and 93.6%, respectively, for the S. aureus compartment and 92.9% and 99.7% for the MRSA compartment after 48 h incubation.

[Antibiotic ointments and methicillin-resistant Staphylococcus aureus with a reservoir in a healthcare worker in a tertiary hospital]

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) has become an important hospital-acquired pathogen, with transfer of the organism from a carrier or infected patient to uninfected patients by the hands or clothing of staff as the main mode of transmission.

METHODS

Investigation of a cluster of new cases of MRSA resistant to mupirocin and fusidic acid, using epidemiological and microbiological resources.

RESULTS

From September 2010 to February 2012, sixteen patients had at least one culture positive for MRSA resistant to mupirocin and fusidic acid. Some not apparently related cases and outbreaks appeared. By analysing cultures taken from patients and staff using pulsed-field gel electrophoresis, it was demonstrated that most likely this situation was started by an auxiliary nurse who was a carrier of the MRSA. Healthcare worker decontamination using oral antibiotic therapy was unsuccessful. Eventually, the situation was controlled by placing the carrier in a different job, with no further cases to date (September, 2012).

CONCLUSION

This report illustrates the risk of nosocomial transmission linked to care delivered by healthcare workers.

MRSA screening by the Xpert MRSA PCR assay: pooling samples of the nose, throat, and groin increases the sensitivity of detection without increasing the laboratory costs

The performance of the Xpert MRSA polymerase chain reaction (PCR) assay on pooled nose, groin, and throat swabs (three nylon flocked eSwabs into one tube) was compared to culture by analyzing 5,546 samples. The sensitivity [0.78, 95 % confidence interval (CI) 0.73-0.82] and specificity (0.99, 95 % CI 0.98-0.99) were similar to the results from published studies on separated nose or other specimens. Thus, the performance of the Xpert MRSA assay was not affected by pooling the three specimens into one assay, allowing a higher detection rate without increasing laboratory costs, as compared to nose samples alone.

Performance of culture media for the isolation and identification of Staphylococcus aureus from bovine mastitis

Rapid isolation and identification of pathogens is a major goal of diagnostic microbiology. In order to isolate and identify Staphylococcus aureus, a number of authors have used a variety of selective and/or differential culture media. However, to date, there are no reports comparing the efficacy of selective and differential culture media for S. aureus isolation from bovine mastitis cases using the 16S rRNA (rrs) gene sequence as a gold standard test. In the present study, we evaluated the efficacy of four selective and/or differential culture media for the isolation of S. aureus from milk samples collected from cows suffering from bovine mastitis. Four hundred and forty isolates were obtained using salt-mannitol agar (SMA, Bioxon), Staphylococcus-110 agar (S110, Bioxon), CHROMAgar Staph aureus (CSA, BD-BBL) and sheep's blood agar (SBA, BD-BBL). All bacterial isolates were identified by their typical colony morphology in the respective media, by secondary tests (for coagulase and β-haemolysis) and by partial 16S rRNA (rrs) gene sequencing as a gold standard test. Sensitivity, positive predictive and negative predictive values were higher for SMA (86.96, 52.63 and 95.95%, respectively) compared with S110 (70.00, 23.73 and 90.91%, respectively), CSA (69.23, 28.13 and 95.74%, respectively) and SBA (68.75, 37.93 and 89.58%, respectively) while specificity values were similar for all media. Data indicated that the use of culture media for S. aureus isolation combined with determination of coagulase activity and haemolysis as secondary tests improved accuracy of the identification and was in accordance with rrs gene sequence-analysis compared with the use of the culture media alone.

Methicillin-resistant Staphylococcus aureus in nasal surveillance swabs at an intensive care unit: an evaluation of the LightCycler MRSA advanced test

Background

We compared the LightCycler MRSA advanced test (Roche Diagnostics, Germany) with enrichment culture methods to evaluate the relative diagnostic performance of the LightCycler MRSA advanced test for active surveillance in a high-prevalence setting.

Methods

A total of 342 nasal swab specimens were obtained from patients in the intensive care unit at admission and on the seventh day for follow-up. The results of LightCycler MRSA advanced test were compared to those of the enrichment culture. For discrepant results, mecA gene PCR was performed.

Results

For the detection of methicillin-resistant Staphylococcus aureus (MRSA), the LightCycler MRSA advanced test showed 98.5% sensitivity and 78.6% specificity and had positive and negative predictive values of 75.0% and 98.8%, respectively. A total of 46 samples had discrepant results between the LightCycler MRSA advanced test and enrichment culture. Of the 44 specimens that were positive in the LightCycler MRSA advanced test but negative by enrichment culture, mecA genes were detected in 37 specimens. In addition, of the original 44 cases, 21 patients had a history of MRSA colonization or infection within the last month; of those 21 specimens, 20 were positive for mecA gene as shown by PCR. Seven mecA-negative discrepant specimens comprised 3 methicillin-sensitive S. aureus-culture positive and only 2 patients had MRSA infections.

Conclusions

Despite its low specificity and positive predictive value, the LightCycler MRSA advanced test could serve as a rapid test for patients colonized with MRSA.

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.

Evaluation of the BD GeneOhm MRSA and VanR Assays as a Rapid Screening Tool for Detection of Methicillin-Resistant Staphylococcus aureus and Vancomycin-Resistant Enterococci in a Tertiary Hospital in Saudi Arabia

Objectives. The aim of this study was to evaluate the diagnostic performance of BD GeneOhm VanR Assay, a rapid PCR test that detects the presence of vanA and/or vanB genes and the performance of BDGeneOhm MRSA Assay which detects the staphylococcal cassette chromosomemec (SCCmec cassette) carrying the mecA gene and Staphylococcus aureus specific sequence located within the orfX gene. Methods. 300 duplicate rectal swabs collected consecutively were analyzed for the presence of VRE by culture and BD PCR. 2267 duplicate swabs were collected (728 nasal and 1539 groin swabs) and analyzed for the presence of MRSA by culture method and BD PCR. Results. Compared to culture, the BD GeneOhm VanR Assay showed a sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of 100%, 91.1%, 23.5%, and 100%, respectively. The BD GeneOhm MRSA Assay revealed sensitivity, specificity, PPV, and NPV of 97.2%, 99.4%, 89.7%, and 99.9%, respectively, for nasal swabs. For groin swabs, it was 100%, 98.7%, 61.5% and 100%, respectively. Conclusion. The BD GeneOhm vanR Assay is a good screening test for rapid exclusion of VRE carriers in hospitals. The BD GeneOhm MRSA Assay represents a reliable screening test. The true strength of the BD GeneOhm Assay for MRSA and VRE is its exceptionally high NPV making the test an ideal tool for rapid exclusion of MRSA and VRE carriers in hospitals. As a consequence, this would dramatically shorten the patient isolation time.

Evaluation of the impact of direct plating, broth enrichment, and specimen source on recovery and diversity of methicillin-resistant Staphylococcus aureus isolates among HIV-infected outpatients

We compared recovery of Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) from nasal and groin swab specimens of 600 HIV-infected outpatients by selective and nonselective direct plating and broth enrichment. Swabs were collected at baseline, 6-month, and 12-month visits and cultured by direct plating to mannitol salt agar (MSA) and CHROMagar MRSA (CM) and overnight broth enrichment with subculture to MSA (broth). MRSA isolates were characterized by pulsed-field gel electrophoresis (PFGE), staphylococcal cassette chromosome mec (SCCmec) typing, and PCR for the Panton-Valentine leukocidin. At each visit, 13 to 15% of patients were colonized with MRSA and 30 to 33% were colonized with methicillin-susceptible S. aureus (MSSA). Broth, CM, and MSA detected 95%, 82%, and 76% of MRSA-positive specimens, respectively. MRSA recovery was significantly higher from broth than CM (P ≤ 0.001) or MSA (P ≤ 0.001); there was no significant difference in recovery between MSA and CM. MSSA recovery also increased significantly when using broth than when using MSA (P ≤ 0.001). Among specimens collected from the groin, broth, CM, and MSA detected 88%, 54%, and 49% of the MRSA-positive isolates, respectively. Broth enrichment had a greater impact on recovery of MRSA from the groin than from the nose compared to both CM (P ≤ 0.001) and MSA (P ≤ 0.001). Overall, 19% of MRSA-colonized patients would have been missed with nasal swab specimen culture only. USA500/Iberian and USA300 were the most common MRSA strains recovered, and USA300 was more likely than other strain types to be recovered from the groin than from the nose (P = 0.05).

Screening, isolation, and decolonisation strategies in the control of meticillin resistant Staphylococcus aureus in intensive care units: cost effectiveness evaluation

OBJECTIVE

To assess the cost effectiveness of screening, isolation, and decolonisation strategies in the control of meticillin resistant Staphylococcus aureus (MRSA) in intensive care units.

DESIGN

Economic evaluation based on a dynamic transmission model.

SETTING

England and Wales. Population Theoretical population of patients on an intensive care unit.

MAIN OUTCOME MEASURES

Infections, deaths, costs, quality adjusted life years (QALYs), incremental cost effectiveness ratios for alternative strategies, and net monetary benefits.

RESULTS

All decolonisation strategies improved health outcomes and reduced costs. Although universal decolonisation (regardless of MRSA status) was the most cost effective in the short term, strategies using screening to target MRSA carriers may be preferred owing to the reduced risk of selecting for resistance. Among such targeted strategies, universal admission and weekly screening with polymerase chain reaction coupled with decolonisation using nasal mupirocin was the most cost effective. This finding was robust to the size of intensive care units, prevalence of MRSA on admission, proportion of patients classified as high risk, and precise value of willingness to pay for health benefits. All strategies using isolation but not decolonisation improved health outcomes but costs were increased. When the prevalence of MRSA on admission to the intensive care unit was 5% and the willingness to pay per QALY gained was between £20,000 (€23,000; $32,000) and £30,000, the best such strategy was to isolate only those patients at high risk of carrying MRSA (either pre-emptively or after identification by admission and weekly screening for MRSA using chromogenic agar). Universal admission and weekly screening using polymerase chain reaction based detection of MRSA coupled with isolation was unlikely to be cost effective unless prevalence was high (10% of patients colonised with MRSA on admission).

CONCLUSIONS

MRSA control strategies that use decolonisation are likely to be cost saving in an intensive care unit setting provided resistance is lacking, and combining universal screening using polymerase chain reaction with decolonisation is likely to represent good value for money if untargeted decolonisation is considered unacceptable. In intensive care units where decolonisation is not implemented, evidence is insufficient to support universal screening for MRSA outside high prevalence settings.

Evaluation of salt concentrations, chromogenic media and anatomical sampling sites for detection of methicillin-resistant Staphylococcus aureus in pigs

The performance of chromogenic media for the detection of methicillin-resistant Staphylococcus aureus (MRSA) in humans, has been evaluated in numerous studies. However, few comparative studies have been performed for the detection of MRSA in pigs. In this study two different salt concentrations (2.5% or 7.5% NaCl) were evaluated in the enrichment nutrient broth and three selective chromogenic media (chromID MRSA, BrillianceMRSA and MRSASelect) for their ability to detect MRSA in swabs from 29 pigs obtained from three different anatomical sampling sites (anterior nares, skin behind both ears and perineum). ChromID MRSA showed the highest relative sensitivity and specificity after enrichment in 7.5% NaCl, followed by MRSASelect and BrilianceMRSA. For all chromogenic media more MRSA-positive results were obtained for specimens collected from skin behind the ears than for specimens taken from both nares and perineum. The results with regard to the anatomical sampling sites were confirmed in a larger study on three different pig farms involving 60 pigs per farm. Skin behind the ears was the anatomical site with the highest relative sensitivity (91.4%) for MRSA detection compared to perineum and anterior nares, with a relative sensitivity of 76.5% and 75.3%, respectively. An increased relative sensitivity could be achieved when combining two anatomical sites. Sampling of anterior nares and skin behind the ears appeared to be the most sensitive combination with a relative sensitivity of 98.2%. These results show that sampling of only the anterior nares underestimates the real pig MRSA prevalence.

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.

Quantification of methicillin-resistant Staphylococcus aureus strains in marine and freshwater samples by the most-probable-number method

Recreational beach environments have been recently identified as a potential reservoir for methicillin-resistant Staphylococcus aureus (MRSA); however, accurate quantification methods are needed for the development of risk assessments. This novel most-probable-number approach for MRSA quantification offers improved sensitivity and specificity by combining broth enrichment with MRSA-specific chromogenic agar.

The value of multiple surveillance cultures for methicillin-resistant Staphylococcus aureus

BACKGROUND

We evaluated our experience in a low prevalence setting to determine the extent to which multiple swabs increased detection rates and the incremental costs of doing so.

METHODS

Nasal and groin swabs submitted in pairs were cultured onto a single plate (Oxoid MRSA Denim Blue Agar; Oxoid Company, Napean, ON, Canada). We determined whether MRSA was detected when swabs submitted in the preceding 3 days were negative. We explored the costs associated with screening and of each additional colonized patient detected.

RESULTS

In all, 60,049 paired nose and perineal swabs were submitted from 21,599 patients. In all, there were 12,750 duplicate, 1437 triplicate, and 112 instances when >4 swabs were processed within 3 days. The first culture was positive in 106 of 12,750 (0.83%%), 42 of 12,750 (0.33%) on the second when the first was negative, 7 of 1642 (0.43%) on the third or subsequent swab pair when the preceding 2 were negative.

CONCLUSION

Overall, the sensitivity of the first of multiple cultures of a set was 74.3%. Had the 14,392 multiple samples not been submitted, 49 colonized patients would not have been identified. Additional laboratory costs associated with multiple samples equaled $2088 per patient identified.

Evaluation of a new selective medium, BD BBL CHROMagar MRSA II, for detection of methicillin-resistant Staphylococcus aureus in different specimens

The sensitivity of screening for methicillin-resistant Staphylococcus aureus (MRSA) can be improved by adding other specimen sites to nares. We describe an evaluation of a new selective medium, BBL CHROMagar MRSA II (CMRSAII), for its ability to detect MRSA from different specimen types. CMRSAII is a chromogenic medium which incorporates cefoxitin for the detection of MRSA. A study was performed at four clinical laboratories with the following specimens: 1,446 respiratory, 694 stool, 1,275 skin, and 948 wound specimens and 688 blood culture bottles containing Gram-positive cocci. The recovery of MRSA on traditional culture media was compared to results with CMRSAII. S. aureus was tested by cefoxitin disk diffusion. CMRSAII was interpreted as positive for MRSA at 24 h (range, 18 to 28 h) based solely on the visualization of mauve-colored colonies and at 48 h (range, 36 to 52 h) based on detection of mauve colonies with subsequent confirmation as S. aureus (by coagulase or latex agglutination testing). MRSA was recovered more frequently on CMRSAII (89.8% at 24 h and 95.6% at 48 h) than on traditional culture plates (83.1% at 24 h and 79.8% at 48 h) for all specimen types combined (P < 0.001). The percent sensitivities of CMRSAII at 24- and 48-h reads, respectively, were 85.5 and 92.4% for respiratory specimens, 87.9% and 98.3% for stool specimens, 88.4% and 96.1% for skin specimens, 92.1% and 94.6% for wound specimens, and 100% and 100% for positive blood cultures. The specificity was 99.8% for respiratory specimens and 100% for all others. In conclusion, CMRSAII is a reliable screening medium for multiple specimen types.

[Performance evaluation of BD GeneOhm MRSA PCR assay for detection of nasal colonization of methicillin-resistant Staphylococcus aureus at endemic intensive care units]

BACKGROUND

The BD GeneOhm MRSA PCR assay (Becton Dickinson, USA) is a qualitative real-time PCR test for rapid detection of nasal colonization of methicillin-resistant Staphylococcus aureus (MRSA). We evaluated the performance of BD GeneOhm MRSA PCR assay versus MRSASelect (Bio-Rad, France) and broth enrichment cultures for detection of MRSA from nasal swabs.

METHODS

From August 2008 to January 2009, 295 nasal swabs were taken from patients in intensive care units and transported to the laboratory with BD CultureSwab Liquid Stuart Single Swab (Becton Dickinson, USA). The swabs were inoculated onto MRSASelect first and then suspended into GeneOhm sample buffer: 100 microL of the suspension was inoculated into 6.5% NaCl-tryptic soy broth (Becton Dickinson, USA), which was subcultured on MRSASelect after overnight incubation (TSBS). Performances of GeneOhm MRSA and MRSASelect were compared to TSBS.

RESULTS

With GeneOhm MRSA, 125 swabs (44.6%) were positive for MRSA, 13 (4.4%) were unresolved, and 2 were not determined. With MRSASelect and TSBS 86 (29.4%) and 106 swabs (36.2%), respectively, were positive. The sensitivity, specificity, and positive and negative predictive value of GeneOhm MRSA were 85.8%, 77.5%, and 72.8% and 93.5%, respectively, and corresponding values for MRSASelect were 78.3%, 94.8%, and 96.5% and 88.9%. Of the 33 patients whose 34 specimens were found false positive in GeneOhm MRSA, 23 patients were MRSA-positive either previously or subsequently to this study. All of the 10 patients with false-negative specimens in GeneOhm MRSA PCR assay were previously MRSA or methicilln-resistant coagulase negative staphylococci (MRCNS)-positive and were treated for MRSA, but they became MRSA-positive after 1 to 4 negative surveillance cultures.

CONCLUSIONS

GeneOhm MRSA PCR assay showed a relatively high negative predictive value. However, its low specificity and frequent occurrence of unresolved results would be problematic in the endemic areas with a high prevalence of MRSA.

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

Two chromogenic media for the detection of MRSA were compared: BBL CHROMagar MRSA II (BD) and MRSA ID agar (bioMérieux). Following overnight nonselective enrichment, 1,919 screening samples were inoculated on both chromogenic agars. After 24 h, the sensitivities of both media were high and comparable. Both media showed an important decrease in specificity after 48 h of incubation (decreases of 8% for MRSA II and 10% for MRSA ID), but MRSA II was significantly more specific at both time points.

Evaluation of chromogenic media for detection of methicillin-resistant Staphylococcus aureus

Rapid laboratory diagnosis is critical for treating, managing, and preventing methicillin-resistant Staphylococcus aureus (MRSA) infections. We evaluated and compared the potential for MRSA detection of five chromogenic media, Brilliance MRSA agar (Oxoid), ChromID (bioMérieux), MRSASelect (Bio-Rad), CHROMagar (CHROMagar Microbiology), and BBL-CHROMagar (BD Diagnostics). Media were tested with log serial dilutions (10(0) to 10(6) CFU) of pure isolates of MRSA (n = 60), non-MRSA (n = 27), and defined mixtures thereof simulating clinical samples (n = 84). Further evaluations were done on pre-enriched nasal and groin screening swabs (n = 213) from 165 hospitalized patients. Randomized samples were spiral plated on each medium and independently scored by five investigators for characteristic colonies at 24 and 48 h of incubation. Confirmatory testing of up to five putative MRSA colonies recovered from each medium was done. The cumulative average sensitivity with isolates, mixtures, and clinical samples was the highest for Brilliance MRSA agar (97%) and similar for the other four media (>or=92%). The cumulative average specificity was the highest for BBL-CHROMagar (99%), followed by MRSASelect (98%), CHROMagar (97%), ChromID (89%), and Brilliance MRSA agar (86%). All of the media detected MRSA at 10 and 1 CFU, although at these low loads, few MRSA samples harboring SCCmec type III or IV were misinterpreted as non-MRSA by investigators. False-positive results were mainly due to methicillin-resistant S. epidermidis. For an arbitrary MRSA prevalence of 5% and based on patient sample evaluations, the positive predictive values for BBL-CHROMagar and CHROMagar ( approximately 84%) were the highest. The negative predictive values of all of the media were >or=92% for MRSA prevalences ranging from 5% to 30%. In conclusion, BBL-CHROMagar and CHROMagar gave the best overall results for detection of MRSA, irrespective of the sample concentration, investigator, or incubation period.

Staphylococcus aureus carriage in care homes: identification of risk factors, including the role of dementia

The aim of this study was to investigate the prevalence and associated risk factors of methicillin-susceptible and methicillin-resistant Staphylocccus aureus (MSSA and MRSA) carriage in care homes, with particular focus on dementia. A point-prevalence survey of 748 residents in 51 care homes in Gloucestershire and Greater Bristol was undertaken. Dementia was assessed by the clock test or abbreviated mini-mental test. Nasal swabs were cultured for S. aureus on selective agar media. Multivariable analysis indicated that dementia was not a significant risk factor for MSSA (16.2%) or MRSA (7.8%); and that residents able to move around the home unassisted were at a lower risk of MRSA (P=0.04). MSSA carriage increased with increasing age (P=0.03) but MRSA carriage decreased with increasing age (P=0.05). Hospitalization in the last 6 months increased the risk of MSSA (P=0.04) and MRSA (P=0.10). We concluded that cross-infection through staff caring for more dependent residents may spread MRSA within care homes and from the recently hospitalized. Control of MSSA and MRSA in care homes requires focused infection control interventions.

Rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) in diverse clinical specimens by the BD GeneOhm MRSA assay and comparison with culture

The efficacy of the BD GeneOhm methicillin-resistant Staphylococcus aureus (MRSA) assay was assessed by analyzing nasal swabs and swabs from other body sites for the presence of MRSA in a low-prevalence area. From 681 patients with a high risk for MRSA carriage, 1,601 specimens were collected and transported in Amies agar. After discordant analysis, the sensitivity, specificity, positive predictive value, and negative predictive value of the BD GeneOhm MRSA assay were 84.3%, 99.2%, 88.4%, and 98.9%, respectively, compared to culture.

Comparative evaluation of five culture media with triplex PCR assay for detection of methicillin-resistant Staphylococcus aureus

The emergence of methicillin-resistant Staphylococcus aureus (MRSA) is responsible for nosocomial and community-acquired infections. Hence, rapid and accurate laboratory diagnosis of MRSA is a vital constituent of control measures. The present study evaluated five different methods for the identification of MRSA. A total of 207 S. aureus clinical isolates that consisted of 89 MRSA and 118 methicillin-susceptible S. aureus (MSSA) strains confirmed by PCR were tested. MRSA strains were evaluated by five different methods: chromogenic MRSA agar (CMRSA), oxacillin resistance screening agar base (ORSAB), mannitol salt oxacillin agar (MSO), mannitol salt cefoxitin agar with two different concentrations of cefoxitin [4 microg/ml (MSC-4) and 6 microg/ml (MSC-6)]. The results of the different methods were compared to mecA PCR as the gold standard. MSC-6 showed only six false-positive MRSA in comparison with PCR. The sensitivities and specificities of MSC-6, MSC-4, MSO-4, ORSAB, and CMRSA were as follows: 98.9/94.9%, 100/83.1%, 89.9/87.3%, 97.8/96.6%, and 95.5/94.9%, respectively. In comparison with PCR, it was found that both MSC-6 and ORSAB were relatively the least expensive screening tests ($0.70 and $1.00, respectively). In conclusion, all methods were comparable, but MSC-6 was the least expensive medium for MRSA screening.

Recent advances in diagnostic microbiology

The past decade has seen a surge in the development of a variety of molecular diagnostics designed to rapidly identify or characterize medically important microorganisms. We briefly review important advances in molecular microbiology, and then discuss specific assays that have been implemented in clinical microbiology laboratories throughout the country. We also discuss emerging methods and technologies that will soon be more widely used for the prompt and accurate detection of the agents of infectious diseases.

Impact of a short period of pre-enrichment on detection and bacterial loads of methicillin-resistant Staphylococcus aureus from screening specimens

We compared the impacts of direct plating on a chromogenic medium and of plating after enrichment (4 h and overnight) on the detection of methicillin-resistant Staphylococcus aureus (MRSA) from 52 patient screening samples. MRSA colony counts for approximately 70% of samples after overnight pre-enrichment were >20-fold higher than the counts after the other two treatments. The qualitative differences (sample was MRSA positive/negative) between the results of the direct plating and 4-h pre-enrichment treatments were not significant, indicating no advantage of the latter; however, the number of samples positive for MRSA increased significantly after an overnight sample pre-enrichment (P < 0.005).

Prevalence and distribution of meticillin-resistant Staphylococcus aureus within the environment and staff of a university veterinary clinic

OBJECTIVES

To characterise the distribution of meticillin-resistant Staphylococcus aureus within the environment of a university small animal hospital and compare this with the distribution among staff.

METHODS

Samples were collected from 140 environmental sites and the anterior nares of 64 staff members at the University of Glasgow Small Animal Hospital on a single day (d1). Sixty of the environmental sites were resampled 14 days later (d14).

RESULTS

Meticillin-resistant S aureus was isolated from two of 140 (1.4 per cent; 95 per cent confidence interval: 1.7 to 5.1) environmental sites on d1 and one of 60 (1.7 per cent; 95 per cent confidence interval: 0.4 to 8.9) on d14. Two of the 64 staff sampled were positive for meticillin-resistant S aureus (3.1 per cent; 95 per cent confidence interval: 0.4 to 8.4).

CLINICAL SIGNIFICANCE

A lower prevalence of meticillin-resistant S aureus was observed in the environment than previously reported. The location, relatedness between isolates and the presence of Panton-Valentine leucocidin indicates that the source of the environmental meticillin-resistant S aureus was most likely to have been human rather than animal in these cases. This study presents important information regarding the potential source and distribution of meticillin-resistant S aureus within veterinary hospital environments and highlights potential variability of prevalence of meticillin-resistant S aureus within and between veterinary institutions.

Evaluation of isolation procedures and chromogenic agar media for the detection of MRSA in nasal swabs from pigs and veal calves

Since the emergence of MRSA in livestock, screening of animals for the detection of MRSA is widely practised. Different procedures are published for animal samples but a systematic comparison of methods has not been performed. The objective of this study was to compare three available commonly used procedures and three chromogenic agars for detecting MRSA in nasal swabs from pigs (n=70) and veal calves (n=100). Procedures 1 and 2 used a pre-enrichment comprising Mueller Hinton broth with 6.5% NaCl followed by selective enrichment with 4 microg/ml oxacillin+75 microg/ml aztreonam (procedure 1) and 5 microg/ml ceftizoxime+75 microg/ml aztreonam (procedure 2) respectively. Procedure 3 used a selective enrichment broth only, containing 4% NaCl, 5 microg/ml ceftizoxime+50 microg/ml aztreonam. After selective enrichment, media were streaked on to three different chromogenic agars. Significantly more MRSA were found for pig as well as for veal calf samples with procedures 1 and 2. No significant differences were found between procedures 1 and 2. For nasal swabs from pigs significantly more MRSA-positive samples were found when MRSA Screen (Oxoid) or MRSASelect (Bio-Rad) agars were used compared to MSRA ID (bioMérieux). For calf samples no significant differences between the different agars were found. In conclusion, the results of this study show that procedures 1 and 2, both using additional high salt pre-enrichment are superior and should be recommended for MRSA detection in nasal swabs from pigs and veal calves. The preferred choice of chromogenic agar depends on the sample matrix.