A rapid (20 h) solid screening medium for detecting methicillin-resistant Staphylococcus aureus

Color changing bioadhesive barrier for peripherally inserted central catheters

Bacteria migration at catheter insertion sites presents a serious complication (bacteraemia) with high mortality rates. One strategy to mediate bacteraemia is a physical barrier at the skin-catheter interface. Herein a colorimetric biosensor adhesive (CathoGlu) is designed and evaluated for both colorimetric detection of bacterial infection and application as a bacteria barrier. The design intent combines viscous, hydrophobic bioadhesive with an organic pH indicator (bromothymol blue). Visual observation can then distinguish healthy skin at pH = ∼5 from an infected catheter insertion site at pH = ∼8. The liquid-to-biorubber transition of CathoGlu formulation occurs via a brief exposure to UVA penlight, providing an elastic barrier to the skin flora. Leachates from CathoGlu demonstrate no genotoxic and skin sensitization effect, assessed by OECD-recommended in vitro and in chemico assays. The CathoGlu formulation was found non-inferior against clinically approved 2-octyl-cyanoacrylate (Dermabond™), and adhesive tape (Micropore™) within an in vivo porcine model. CathoGlu skin adhesive provides new opportunities to prevent sepsis in challenging clinical situations.

The Correlation between Airborne Methicillin-resistant Staphylococcus aureus with the Presence of MRSA Colonized Patients in a General Intensive Care Unit

Air sampling directly onto a methicillin-resistant Staphylococcus aureus (MRSA) selective agar was performed at six locations three times weekly over a period of 32 weeks in a new, initially MRSA-free Intensive Care Unit to examine if MRSA is present in air sample cultures and, if so, whether it is affected by the number of MRSA colonized patients present. A total of 480 air samples were collected on 80 days. A total of 39/480 (8.1%) samples were found to be MRSA positive of which 24/160 (15%) positive air samples were from the single rooms, where MRSA colonised patients were isolated, and 15/320 (4.7%) were from the open bed areas. A significant correlation was found between the daily number of MRSA colonized or infected patients in the Unit and the daily number of MRSA positive air samples cultures obtained (r2=0.128; P<0.005). The frequency of positive cultures was significantly higher in the single rooms than in the open bed areas (relative risk=3.2; P<0.001). The results from one of the single rooms showed a strong correlation between the presence of MRSA patients and MRSA positive air samples (relative risk=11.4; P<0.005). Our findings demonstrate that the presence of airborne MRSA in our unit is strongly related to the presence and number of MRSA colonized or infected patients in the Unit.

[Prevention of transmission of methicillin-resistant Staphylococcus aureus (MRSA) infection: standard precautions instead of isolation: a 6-year surveillance in a university hospital]

BACKGROUND

This study was carried out to evaluate whether the risk of acquiring methicillin-resistant Staphylococcus aureus (MRSA) is increased compared to reference data when standard precautions are practiced instead of strict or contact isolation.

METHODS

From 2000 through 2005, all patients with MRSA at a university hospital were prospectively surveyed. The isolates were investigated using molecular microbiology methods (PFGE, PCR) and compliance with hand hygiene was indirectly monitored using the consumption of disinfection solutions and glove use as surrogate markers.

RESULTS

The MRSA rates of the 797 patients were lower than for the reference data (PEG, KISS), the incidence of MRSA infections was reduced continuously, MRSA transmissions to contact patients could be demonstrated in about 30%, and the consumption of hand disinfection solutions as well as gloves was significantly higher in 2005 compared to 2000.

CONCLUSIONS

There is no evidence from our results that practicing standard precautions would increase the risk for the acquisition of MRSA compared to strict or contact isolation.

Population dynamics of methicillin-susceptible and -resistant Staphylococcus aureus in remote communities

Objectives

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) was first reported in remote regions of Western Australia (WA) in 1992 and is now the predominant MRSA isolated in the State. To gain insights into the emergence of CA-MRSA, 2146 people living in 11 remote WA communities were screened for colonization with S. aureus.

Methods

Antibiogram analysis, contour-clamped homogeneous electric field electrophoresis, multilocus sequence typing, Panton–Valentine leucocidin determinant detection and accessory genetic regulator typing were performed to characterize the isolates. MRSA was further characterized by staphylococcal cassette chromosome mec typing.

Results

The S. aureus population consisted of 13 clonal complexes and two Singleton lineages together with 56 sporadic isolates. Five lineages contained MRSA; however, these were not the predominant methicillin-susceptible S. aureus (MSSA) lineages. There was greater diversity amongst the MSSA while the MRSA appeared to have emerged clonally following acquisition of the staphylococcal cassette chromosome mec. Three MRSA lineages were considered to have been endemic in the communities and have subsequently become predominant lineages of CA-MRSA in the wider WA community. People colonized with MSSA tended to harbour clones of a different genetic lineage at each anatomical site while people colonized with MRSA tended to harbour clones of the same lineage at each site. Overall, the isolates were resistant to few antimicrobials.

Conclusions

Although the evidence suggests that in WA CA-MRSA strains arose in remote communities and have now disseminated into the wider community, there is no evidence that they arose from the predominant MSSA clones in these communities.

Molecular identification and characterization of mannitol-negative methicillin-resistant Staphylococcus aureus

We report on the isolation, molecular identification, and characterization of 5 mannitol-negative methicillin-resistant Staphylococcus aureus (MRSA) from clinical samples in KwaZulu-Natal (KZN) province, South Africa. Identification based on phenotypic testing and polymerase chain reaction detection of the S. aureus species-specific nuc gene and the coagulase gene indicated that the mannitol-negative isolates were S. aureus. Furthermore, they were mecA positive, and SCCmec typing demonstrated that all the isolates harbored type IV SCCmec. API STAPH (Biomerieux, Marcy-l'Etoile, France) misidentified 2 mannitol-negative MRSA that belonged to the major clone in KZN province, as Staphylococcus lugdunensis. Although the prevalence and mechanism of mannitol-negative MRSA is unknown, laboratories are encouraged to investigate S. aureus with atypical characteristics.

Development of a real-time Staphylococcus aureus and MRSA (SAM-) PCR for routine blood culture

The notification of "Gram-positive cocci, possibly staphylococcus" in a blood culture drawn from a seriously ill patient is responsible for a large amount of vancomycin prescribing in institutions where methicillin-resistant Staphylococcus aureus (MRSA) is an important cause of bacteraemia. A duplex real-time TaqMan polymerase chain reaction targeting the species-specific nuc gene, and the mecA gene encoding methicillin-resistance, was developed as a tool for rapid identification and detection of S. aureus and methicillin-resistance, and optimised for immediate as-needs testing. Three different DNA extraction methods achieved varying DNA quality, with PCR inhibition the main problem. Serial blood cultures (n=120) identified as possible staphylococci on Gram stain from our clinical laboratory were examined. There was one false negative result for a methicillin-resistant Staphylococcus epidermidis, which was positive on repeat testing, and one false negative result due to DNA extraction failure for MRSA from peritoneal dialysate inoculated into blood culture medium. Sensitivity and specificity of 97% and 100%, respectively, were obtained for mecA; and sensitivity and specificity of 98% and 100%, respectively, for nuc. Detection of slow-growing coagulase-negative staphylococci as co-infecting strains may be reduced. The assay quickly and reliably identified S. aureus in mixed infection, and identified methicillin resistance in both S. epidermidis and S. aureus strains.

Identification and molecular characterization of mannitol salt positive, coagulase-negative staphylococci from nasal samples of medical personnel and students

The identification of mannitol salt positive, coagulase-negative staphylococci (CNS) is often disregarded when Staphylococcus aureus is screened in clinical samples using mannitol salt agar. However, the emergence of CNS as important human pathogens has indicated that reliable methods for the identification of clinically significant CNS are of great importance in understanding the epidemiology of infections caused by them. The identification and molecular characterization of mannitol salt positive CNS from nasal samples of medical personnel and students is reported here. A total of 84 mannitol salt positive staphylococcal isolates were obtained from 240 nasal samples, of which 15 were CNS. The API STAPH system classified the CNS isolates into six species, and one-third of the isolates were identified with confidence levels of <80 %. 16S-23S rRNA intergenic spacer length polymorphism analysis (ITS-PCR) identified only two species (Staphylococcus haemolyticus and Staphylococcus saprophyticus). This identification was confirmed by antibiotyping, species-specific PCR and PFGE. The results from this study indicate that ITS-PCR is a potentially useful and reliable tool, enabling hospital laboratories to obtain rapid, full and accurate identification of CNS at the species level.

Use of broth enrichment and real-time PCR to exclude the presence of methicillin-resistant Staphylococcus aureus in clinical samples: a sensitive screening approach

A rapid and sensitive method for excluding the presence of methicillin-resistant Staphylococcus aureus (MRSA) in clinical samples was developed and evaluated. The method utilised an MRSA-selective enrichment broth for 16 h, followed by PCR quantification of the nuc gene. Samples below a quantitative PCR threshold were reported as MRSA-negative. Broths from PCR-positive samples were subcultured for MRSA isolation. Clinical samples (n = 334) in a constructed high prevalence population were analysed in parallel with a selective plating method. The new broth-PCR assay increased the number of positive samples by 35% (49 vs. 66), and 94% of negative samples were reported within 24 h. To reduce costs and workload, 665 clinical samples were grown separately in enrichment broth and then pooled in the PCR step. The broth-PCR assay increased the number of MRSA positive samples from 11 to 15 compared with selective plating. Most (89%) of the culture-negative samples were also PCR-negative and could be reported within 24 h. The growth of 25 European EMRSA strains was tested in the selective enrichment broth. On average, the MRSA strains showed a 300 000-fold increase in CFU, compared with 30-fold for the eight methicillin-sensitive Staphylococcus aureus strains tested.

Evaluation of separate vs pooled swab cultures, different media, broth enrichment and anatomical sites of screening for the detection of methicillin-resistant Staphylococcus aureus from clinical specimens

Early identification of methicillin-resistant Staphylococcus aureus (MRSA) carriers is a major component of an MRSA control programme. The cost and laboratory workload could be markedly reduced by processing multiple swabs from one person in one culture broth (specimen pooling). We evaluated the sensitivity for MRSA detection and the growth rate of pooled swabs compared with individual processing. In total, 1254 swabs from 423 subjects (two to five swabs per subject) were submitted for detection of MRSA. Swabs were suspended in 2-mL volumes of sterile Todd-Hewitt Broth and divided into two 1-mL aliquots. One aliquot of the suspension was processed as a single specimen, and the other aliquot was mixed (pooled) with other suspensions in which swabs from the same patient were suspended. Forty-four (10%) pooled samples were positive for MRSA. Specimens from seven additional patients that were negative when pooled were positive when processed separately. There was no case where the pooled specimen was positive but the separate specimens were negative. The diagnostic sensitivity of pooled surveillance cultures compared with single cultures, when only subjects colonized by MRSA were considered, was 86% and the false-negative rate was 14%. Eighty percent of the pooled positive cultures were detected by the third day and all were detected by the fourth day. Fifty-four percent of the specimens processed separately were detected by the second day and all were detected by the fourth day. Pooling of specimens decreases the sensitivity of MRSA detection compared with processing each swab separately, particularly in swabs with a low number of colony-forming units. In all subjects whose pooled samples were negative but whose swabs examined separately were positive, the swabs examined separately were negative on primary plates and positive only after culturing in enrichment broth.

Mannitol salt agar-cefoxitin combination as a screening medium for methicillin-resistant Staphylococcus aureus

In disk diffusion tests, cefoxitin is now considered a better indicator than oxacillin for the presence of the mecA gene in Staphylococcus aureus. A logical extension of this work is the incorporation of cefoxitin into media selective for methicillin-resistant Staphylococcus aureus (MRSA). This paper describes the development and subsequent testing of mannitol salt agar containing 4 mg/liter cefoxitin with a unique collection of well-characterized MRSA strains, including low-level methicillin-resistant strains and an equal number of known mecA-negative S. aureus strains. The agar supported the growth of 96.6% of the mecA-positive strains in the collection and inhibited the growth of 100% of the mecA-negative strains. These results suggest that selective media based on cefoxitin are superior to those based on oxacillin for the detection of MRSA.

Outbreak of methicillin-resistant Staphylococcus aureus in two nursing homes in Central Norway

Until recently, infections with methicillin-resistant Staphylococcus aureus (MRSA) have mainly been associated with hospital outbreaks in Norway. However, increasingly cases are contracted outside hospitals. This paper reports the first two outbreaks of MRSA in two nursing homes in central Norway, affecting 23 residents and five staff members. Pulsed-field gel electrophoresis analysis showed that all strains from nursing home A were identical and that the strains from nursing home B were genotypically similar with one or two band differences. Multi-locus sequence typing (MLST) showed that the strains from the two nursing homes belong to clonal complex 45, with each strain being a single-locus variant of sequence type 45 (ST 45), a well-known European epidemic strain. No evident source of the two outbreaks was found, and there was no obvious connection between the two outbreaks. The latter is also supported by the minor differences observed by MLST, suggesting a connection at some time in the past. The outbreaks led to a heavier workload and economic strain on both nursing homes. The outbreak in nursing home A was brought to an end, whereas two residents remained colonized in nursing home B despite several eradication attempts. These outbreaks show the potential for MRSA spread in a nursing home. If the prevalence of MRSA in Norway continues to increase, nursing home staff and residents may have to be included in the groups to be screened for MRSA upon hospital admission.

Rapid detection of mecA and nuc genes in staphylococci by real-time multiplex polymerase chain reaction

A multiplex real-time polymerase chain reaction (RT-PCR) targeting the mecA and nuc genes was developed for the detection of methicillin resistance and identification of Staphylococcus aureus. Novel mecA and nuc primers and fluorescence resonance energy transfer hybridization probes specific for the mecA and nuc genes were evaluated. The assay was performed using the LightCycler system (Roche Molecular Biochemicals, Mannheim, Germany) and evaluated against the traditional gel-based multiplex PCR (PCR-gel) method currently used at Royal Perth Hospital. Clinical isolates (n = 222) and isolates from a culture collection library (n = 206) were tested by both assays in parallel. The RT-PCR assay was 100% sensitive and specific for the detection of methicillin resistance and for the identification of S. aureus when compared with the PCR-gel assay. Results from the RT-PCR assay showed 5 isolates with lower efficiency fluorescence curves for the nuc gene PCR fragment. DNA sequencing showed mutations within the region of the probe-binding sites compared with the reference strain. The results of the RT-PCR assay were available within 2 h. This rapid mecA/nuc RT-PCR assay is a suitable and practical tool for the routine detection of methicillin resistance and identification of S. aureus, which can be easily incorporated into the diagnostic molecular microbiology laboratory work flow.

Comparison of culture screening methods for detection of nasal carriage of methicillin-resistant Staphylococcus aureus: a prospective study comparing 32 methods

Screening for carriage of methicillin-resistant Staphylococcus aureus (MRSA) is fundamental to modern-day nosocomial infection control, both for epidemiologic investigation and day-to-day decisions on barrier isolation. Numerous microbiologic techniques have been advocated for screening for nasal carriage of MRSA, including the use of charcoal rather than rayon swabs, preincubation of swabs in Stuart's medium, preincubation of swabs in salt-containing trypticase soy broth (TSB), use of mannitol-salt agar (MSA), use of MSA containing oxacillin (MSA(Ox)), use of Mueller-Hinton agar containing oxacillin (MHA(Ox)), and the use of MSA containing lipovitellin with an oxacillin disk (MSAL(Ox)). We report a prospective clinical trial undertaken to test all of these methods concurrently. Patients at high risk for MRSA carriage were screened with eight consecutive nasal swabs (four standard rayon, four charcoal-coated rayon), which were processed by primary plating on MSA, MSA(Ox), MHA(Ox), and MSAL(Ox); Stuart's preincubation for 72 h followed by plating on the solid media; overnight enrichment in salt-containing TSB followed by plating; and Stuart's preincubation for 72 h followed by overnight enrichment in TSB and plating. All of the above methods were repeated with charcoal swabs. Each patient was screened by 32 culture methods. Forty-three (42%) of 102 patients studied were positive for MRSA by one or more methods. Among the four media evaluated with direct plating, MSAL(Ox) was 11 to 25% more sensitive for detecting MRSA (MSAL(Ox) versus MSA(Ox) or MHA(Ox) or MSA, each P < 0.01). Preincubation in Stuart's medium for 72 h did not enhance recovery of MRSA. Enrichment in salt-containing TSB further increased yield 9%. MSAL(Ox) also showed the best specificity, 93%. Charcoal swabs showed no advantage over standard rayon swabs. Our results suggest that the highest yield will be achieved by using standard rayon swabs that are enriched overnight in TSB with inoculation onto MSAL(Ox) medium. Direct inoculation of swabs onto MSAL(Ox) allows detection of 90% of MRSA carriers.

A preliminary evaluation of a new selective agar supplemented with desferrioxamine for detection of methicillin-resistant Staphylococcus aureus

The aim of the present study was to evaluate the performance of two new selective screening agars, Colombia agar supplemented with 1000 mg/L desferrioxamine, 5 mg/L amphotericin B, 16 mg/L polymyxin B, and 2 mg/L methicillin (CMDAP agar) or 0.5 mg/L oxacillin (CODAP agar), for detection of methicillin-resistant Staphylococcus aureus (MRSA). Both the CMDAP and the CODAP agar effectively inhibited growth of 151 isolates of coagulase-negative staphylococci (CoNS), 45 of Enterobacteriaceae and six Candida spp. examined. The sensitivity and specificity of the CMDAP and CODAP agars for detection of MRSA was calculated by comparing the growth of 52 MRSA with the inhibition of 74 mecA negative S. aureus and of 151 CoNS. The performance of the new agars was compared with four previously described MRSA screening agars. The sensitivity and specificity for detection of MRSA after incubation at 35 degrees C for 24 h was 0.94 and 0.91, respectively, for the CMDAP agar, 0.60 and 0.90 for the CODAP agar, 0.98 and 0.57 for methicillin aztreonam mannitol salt agar (MAMSA), 0.23 and 0.84 for oxacillin mannitol salt agar (OMSA), 0.48 and 0.76 for oxacillin Mueller-Hinton agar (OMHA) and 0.75 and 0.77 for lithium oxacillin mannitol salt agar (LOMSA). Agars supplemented with desferrioxamine, CMDAP and CODAP, were more specific for detecting MRSA compared with agars not supplemented with desferrioxamine. The detection rate was higher for agars supplemented with methicillin than for agars supplemented with oxacillin.

COMPARISON OF RAPID METHOD OF DNA EXTRACTION USING MICROWAVE IRRADIATION WITH CONVENTIONAL PHENOL CHLOROFORM TECHNIQUE FOR USE IN MULTIPLEX PCR FOR mec A AND fem B GENES TO IDENTIFY GENOTYPES OF MRSA FROM CULTURES

Methicillin Resistant Staphylococcus aureus (MRSA) infection is a major cause of morbidity and mortality in hospitalised patients and requires vancomycin for effective therapy. Rapid identification of MRSA is vital to control MRSA outbreaks in hospitals. Identification of MRSA is a time consuming process requiring more than 48 hours and is labour intensive involving culture, biochemical tests and antimicrobial susceptibility testing. In this study we have used microwave irradiation of the bacterium obtained from cultures which was then directly subjected to a multiplex PCR technique to accurately and rapidly identify the presence of mec A and fem B genes which characterise MRSA. This has been compared with the standard method of lysing the bacterium and DNA extraction using phenol chloroform method followed by multiplex PCR. The microwave lysis method followed by direct PCR has been found to be less time consuming, 5 hours, as compared to 9 hours by conventional technique. Use of this strategy would enable early identification and early implementation of control measures.

Methods for identifying methicillin resistancein Staphylococcus aureus

Five methods for the detection of methicillin-resistant Staphylococcus aureus were used to examine a collection of 100 clinical isolates comprising both susceptible and resistant strains. The disc diffusion test with oxacillin had a sensitivity of 93.3% and a specificity of 92.0% whereas mannitol salt agar containing oxacillin had a sensitivity of 100% and specificity 80.6% with high inoculum. With a low inoculum the sensitivity was 90.7% and specificity 96.0%. The MRSA screen test (Denka Seiken Co. Ltd., Japan) and Evigene MRSA Detection Kit (State Serum Institute, Denmark) tests were in complete agreement with results obtained with polymerase chain reaction assays amplifying mecA and nuc gene sequences.

A purpose built MRSA cohort unit

The control of hospital-acquired infection, in particular methicillin-resistant Staphylococcus aureus (MRSA) remains a challenge. Our hospital has established a purpose built 11-bed cohort unit with on-site rehabilitation for care of patients colonized with MRSA, in an attempt to improve their quality of care. Prior to the opening of this unit a number of concerns were voiced and the aim of this study was to address these. First, to establish if patient cohorting reduces the likelihood of successful decolonization, second, to evaluate the risk of staff colonization, and finally to see if successful environmental control of MRSA is possible.A patient database was established detailing patient demographics, infection rates, eradication and reacquisition rates. Staff screening was performed weekly, at the start of a period of duty. Sixty environmental sites were screened before unit opening, at 48h, six weeks and at six months. There were 88 admissions in the first six months; 62 patients were colonized with MRSA, and 26 patients (10 surgical, 16 medical) had MRSA infections. Twenty-three of 88 patients (26%) were successfully decolonized, which compares favourably with an eradication rate of 20% for the rest of the hospital. Twenty staff members participated in weekly screening. Five staff members colonized with MRSA were detected and all were successfully decolonized. Environmental control was achieved with a combination of a daily detergent clean and a once weekly clean with phenolic disinfectant. Our preliminary data suggest that, despite cohorting patients colonized with MRSA, with proper education and supervised cleaning protocols, it is possible to control environmental MRSA load, successfully decolonize patients and limit the risk of staff colonization.

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.

Community strain of methicillin-resistant Staphylococcus aureus involved in a hospital outbreak

Western Australia (WA) has been able to prevent methicillin-resistant Staphylococcus aureus (MRSA) strains from outside of the state from becoming established in its hospitals. Recently, a single-strain outbreak of MRSA occurred in a WA metropolitan teaching hospital following admission of an infected patient from a remote community. The strain responsible for the outbreak was unrelated to any imported strains and spread rapidly in the hospital. Screening of two remote communities in the region from which the index case came revealed that 42% of the people in one community and 24% in the other carried MRSA. Isolates were typed by resistance pattern, plasmid analysis, contour-clamped homogeneous electric field electrophoresis, bacteriophage pattern, and coagulase gene restriction fragment length polymorphism. It was found that of the people carrying MRSA, 39% in the former community and 17% in the latter community were carrying an MRSA strain which was indistinguishable from the strain that caused the hospital outbreak.