Rapid recognition of methicillin-resistant Staphylococcus aureus by use of automated test systems

The ability to rapidly recognize methicillin-resistant Staphylococcus aureus by use of two automated instrument systems, the MS-2 system (Abbott Laboratories, Diagnostics Division, Irving, Tex.) and the AutoMicrobic system (Vitek Systems, Hazelwood, Mo.), was evaluated on a collection of 95 methicillin-resistant S. aureus isolates recovered from at least six geographical areas of the United States. Isolates were simultaneously tested with both systems, and the results were compared with MIC tests performed by the National Committee for Clinical Laboratory Standards agar dilution method. Methicillin-resistant S. aureus isolates were defined as those with a methicillin MIC greater than or equal to 8 micrograms/ml by the reference procedure. Overall, with the AutoMicrobic system, 94.7% of 95 methicillin-resistant S. aureus isolates were detected, and with the MS-2 system, 91.6% of the isolates were detected. Isolates with methicillin MICs greater than or equal to 32 micrograms/ml were readily detected with both systems (41 of 42 isolates). Of 53 isolates from three locales with methicillin MICs of 8 or 16 micrograms/ml, 90.6% (48) were detected by the AutoMicrobic system, whereas 86.8% (46) were detected by the MS-2 system. A program update which has been added to the MS-2 system prints a warning message indicating possible methicillin-resistant S. aureus with isolates which demonstrate multiple antibiotic resistance (greater than or equal to four drugs other than methicillin). This warning message would have provided presumptive recognition of six of eight isolates with discrepant results for methicillin by the MS-2 system.     

Validation of multiplex PCR strategy for simultaneous detection and identification of methicillin resistant Staphylococcus aureus

Multiplex polymerase chain reaction (PCR) strategy is described for rapid identification of clinically relevant methicillin resistant Staphylococcus aureus (MRSA) that targets mecA and coagulase genes. In this study, 150 staphylococcal clinical isolates were used that included 40 isolates of MRSA, 55 isolates of methicillin susceptible S. aureus (MSSA), 44 isolates of methicillin susceptible coagulase negative Staphylococcus spp. (MS-CoNS) and 11 isolates of methicillin resistant coagulase negative Staphylococcus spp. (MR-CoNS). Out of 55 S. aureus strains, three strains demonstrated mecA gene, which appeared to be oxacillin sensitive by disc diffusion. When (MS-CoNS) were evaluated, 10 isolates classified as oxacillin sensitive phenotypically, yielded positive results in PCR method. The results for mecA detection by PCR were more consistent with disk susceptibility tests in case of MRSA (100%) and MSSA (95%) isolates. In contrast to above results with MRSA and MSSA, mecA detection by PCR in MS-CoNS showed less correlation with disk susceptibility tests (77%). The results for coag detection by PCR were consistent with phenotypic tests in all isolates.     

Nosocomial infection caused by methicillin-resistant Staphylococcus aureus in Palestine

This report presents the prevalence of Palestinian isolates of methicillin-resistant Staphylococcus aureus (MRSA) in nosocomial infections and their antibiotic resistant pattern. A total of 321 clinical isolates of S. aureus were identified from different patients. The prevalence of methicillin resistance among S. aureus isolates was 8.7% (28 isolates). Resistance rates of MRSA to other antibiotics were as follows: 82.1% resistant to erythromycin, 67.9% to clindamycin, 64.3% to gentamicin, and 32.1% to ciprofloxacin. No co-trimoxazole- and vancomycin-resistant isolates were identified in this study. The proportion of methicillin resistance was highest among S. aureus isolates associated with upper respiratory specimens (42.8%); the proportion of methicillin resistance was 39.3% among skin ulcer isolates, 10.7% among urinary tract infection isolates, and lowest among isolates associated with blood and prostate discharge (3.6% each).     

Expression of high-level methicillin resistance in Staphylococcus aureus from the Staphylococcus sciuri mec A homologue: role of mutation(s) in the genetic background and in the coding region of mec A

A close homologue of the mec A gene, the primary drug resistance determinant in methicillin resistant Staphylococcus aureus (MRSA), is ubiquitous in the animal commensal species Staphylococcus sciuri, yet most isolates of this staphylococcal species are susceptible to beta-lactam antibiotics including methicillin. Recently, we showed that in a methicillin-resistant mutant of S. sciuri prepared in the laboratory, the mec A homologue is converted to an antibiotic resistance gene by a point mutation introduced into the -10 consensus of the promoter and such promoter-up mutants of the S. sciuri mec A can express a significant degree of methicillin resistance when introduced into an antibiotic-susceptible strain of S. aureus. We now demonstrate that in this system further increase of the drug resistance phenotype may be achieved under antibiotic pressure by at least two different mechanisms. The first one of these involves the introduction of a point mutation at nucleotide Nt 1889 in the coding region of the S. sciuri-derived mec A determinant, resulting in the replacement of an asparagine with a threonine residue downstream of the conserved SXXK motif which causes extensive reduction in the beta-lactam antibiotic binding capacity (affinity) of the penicillin binding protein (PBP) encoded by the S. sciuri mec A homologue. A second, distinct, mechanism causing increased methicillin resistance is associated with mutation(s) of unknown nature in the genetic background of the S. aureus host.     

Evaluation of new Vitek 2 card and disk diffusion method for determining susceptibility of Staphylococcus aureus to oxacillin

Detection of methicillin resistance in Staphylococcus aureus is a challenge, especially low-level resistance, which is often misdiagnosed. The aim of this study was to compare the diagnostic accuracies of the automated Vitek 2 system and disk diffusion tests, using cefoxitin and moxalactam, for the detection of methicillin resistance in S. aureus strains. Four sets of genotypically diverse isolates were selected from a national reference collection, including mecA-negative S. aureus isolates (n = 56), hospital-acquired (n = 88) and community-acquired (n = 40) S. aureus isolates, and heterogeneous methicillin-resistant S. aureus isolates (n = 29). Oxacillin susceptibility was tested by the Vitek 2 system with the AST P549 card and by disk diffusion methods using 10, 30, and 60 microg cefoxitin and 30 microg moxalactam. Oxacillin resistance was confirmed by PCR for the mecA gene. The overall sensitivities for oxacillin resistance detection were 97.5% for the Vitek 2 automated system, 98.7% for 60-microg cefoxitin and moxalactam disk diffusion, and 99.6% for 10- and 30-microg cefoxitin disks, respectively. Methicillin-susceptible S. aureus isolates were correctly reported as susceptible by all methods. The median times for methicillin testing were 7 h for the Vitek 2 system versus 24 h for disk diffusion methods. In conclusion, the cefoxitin and moxalactam disk diffusion methods and the Vitek 2 automated system are highly accurate methods for methicillin resistance detection, including a range of representative Belgian methicillin-resistant S. aureus strains and unusual strains exhibiting cryptic or low-level oxacillin resistance.     

Interactions between Methicillin and Vancomycin in Methicillin-Resistant Staphylococcus aureus Strains Displaying Different Phenotypes of Vancomycin Susceptibility

Vancomycin-sensitive, -intermediate, and -heterointermediate methicillin-resistant Staphylococcus aureus isolates were tested by using E-tests to explore the interaction of methicillin and vancomycin. For the vancomycin-intermediate and -heterointermediate strains both drugs showed antagonism at concentrations below their MICs but synergy at methicillin concentrations near the MIC. This property could be used to screen for heterointermediate S. aureus strains.     

New chromogenic identification and detection of Staphylococcus aureus and methicillin-resistant S. aureus

This paper describes a new chromogenic plate medium, CHROMagar Staph aureus (CHROMagar, Paris, France), for the identification of Staphylococcus aureus on the basis of colony pigmentation. The abilities of CHROMagar Staph aureus, thermostable nuclease (DNase), and mannitol salt agar (MSA) to identify S. aureus isolates (n = 114) and discriminate between S. aureus and coagulase-negative staphylococci (CoNS; n = 22) were compared. CHROMagar Staph aureus proved to be more sensitive and specific than DNase and MSA, allowing a reliable, simple, and rapid method for the identification of S. aureus isolates. All CoNS encountered in this study with the exception of S. chromogenes could be easily differentiated from S. aureus on this medium. The supplementation with 4 microgram of oxacillin or methicillin per ml allowed simple identification of methicillin resistance in hospital-acquired S. aureus strains which show multiple-drug resistance profiles. Community-acquired methicillin-resistant S. aureus strains showing non-multi-drug resistance profiles require further evaluation on this new chromogenic medium. Methicillin or oxacillin resistance of all S. aureus isolates was confirmed by the detection of penicillin-binding protein 2a, encoded by the mecA gene, using the latex slide agglutination MRSA-Screen test (PBP 2' Test, DR900M; Oxoid).     

Nasal, axillary, and perineal carriage of Staphylococcus aureus among women: identification of strains producing epidermolytic toxin.

Following two outbreaks of staphylococcal scalded skin syndrome in a maternity unit, 500 pregnant women attending an antenatal clinic were screened for carriage of epidermolytic toxin producing Staphylococcus aureus. Nasal, axillary, and perineal swabs were collected from women whose gestational ages ranged from 12-40 weeks. Isolates of S aureus were purified, phage typed, and tested for methicillin sensitivity and production of epidermolytic toxin. The results showed that 164 (33%) women carried S aureus; of these, 100 (61%) were from the nose and three (2%) from axillae, but 41 (25%) strains were isolated from the perineum alone. Screening for nasal carriage alone will therefore miss 25% of carriers. More than one strain of S aureus was identified in seven of 20 women with multiple site carriage. Three (2%) methicillin resistant strains were isolated during the survey, and five (3%) isolates produced epidermolytic toxin. Phage typing identified 63 (34%) strains as non-typable, but 50% of isolates typed either groups I, II or III, and a further 10% represented varying combinations of these and other phage groups. These results provide baseline information on S aureus in the community, and identification of methicillin resistant and toxin producing strains shows a reservoir of outbreak potential which could become relevant on hospital admission of such a carrier.     

Antibiotic resistance of Staphylococcus aureus at north Lebanon: place of the methicillin resistance and comparison of detection methods

The aim of this study was to evaluate the susceptibility of 100 Staphylococcus aureus strains isolated from the laboratory of Microbiology of the Islami Hospital of Tripoli (Lebanon) to 19 antibiotics, and to determine the prevalence of methicillin resistant strains. 30% of strains studied were methicillin resistant, 96% were resistant to the penicillin G. Clavulanic acid restaurated the amoxicillin activity to 29%. The resistance level was 34% for amikacin, 3% for gentamycin and tobramycin, 10% for chloramphenicol, 44.33% for tetracyclin, 7% for erythromycin, 4.04% for clindamycin, 20% for trimethoprim-sulfametoxasol and 0% for vancomycin and teicoplanin. The methicillin-resistant Staphylococcus aureus possess more important resistant level in comparison with the methicillin sensitive strains. We compared the ability of latex agglutination test (Slidex(R) SARM, bioMérieux, France) to detect the production of penicillin-binding protein 2' (PBP 2') in 100 clinical isolates of S. aureus with two reference methods: the oxacillin disk diffusion test and the MIC determination by the E-test (AB BIODISK, Sweden). The two reference methods give the same results for the detection of methicillin resistant S. aureus. The Slidex test was positive for all 30 isolates determined to be methicillin resistant by the reference methods (sensitivity 100%). The latex test was negative for 42 of 70 isolates determined to be methicillin susceptible by the reference methods, and the latex test was positive for 28 isolates determined to be susceptible (specificity 60%).     

Multiplex PCR for Simultaneous Identification of Staphylococcus aureus and Detection of Methicillin and Mupirocin Resistance

In this work, we describe a multiplex PCR assay for the detection of clinically relevant antibiotic resistance genes harbored by some Staphylococcus aureus isolates and for the simultaneous identification of such isolates at the species level. Conditions were optimized for the simultaneous detection of the 310-, 456-, and 651-bp regions of the mecA (encoding high-level methicillin resistance), ileS-2 (encoding high-level mupirocin resistance), and femB (encoding a factor essential for methicillin resistance) genes, respectively, from a single colony in a single reaction tube. The femB PCR fragment allows the specific identification of S. aureus. Validation of the method was performed using 50 human isolates of methicillin-resistant S. aureus (MRSA) and the appropriate control strains. This assay offers a rapid, simple, feasible, specific, sensitive, and accurate identification of mupirocin-resistant MRSA clinical isolates and could be systematically applied as a diagnostic test in clinical microbiology laboratories, facilitating the design and use of antibiotic therapy.     

Effect of the source of Mueller-Hinton agar and resistance frequency on the detection of methicillin-resistant Staphylococcus aureus.

Inconsistencies in the results of disk diffusion tests of oxacillin against Staphylococcus aureus that occurred when using commercially prepared Mueller-Hinton agar from different sources led us to evaluate the ability of media from different sources to detect resistance to oxacillin, methicillin, and nafcillin in S. aureus. Mueller-Hinton agar from five manufacturers was prepared in our laboratory and used for standard disk diffusion and agar dilution tests. Ten oxacillin-resistant S. aureus isolates, of which three were definitive-resistant and seven were occult-resistant, were examined. All definitive-resistant strains were resistant to all three antimicrobial agents on four out of five agars. The occult-resistant strains were consistently detected as resistant on only one of the agars. With only slight differences, oxacillin, methicillin, and nafcillin resistance was more readily detected by disk diffusion and agar dilution when initially incubated at 30 degrees C, and extended incubation improved the detection. The frequency of resistance within a population of occult-resistant cells was low compared with the frequency within a population of definitively resistant cells. The heterogeneity of colony morphology and apparent growth rates within a population of occult-resistant cells contributed to the problem of detecting some resistant isolates. Definitive-resistant isolates were characterized by a very high and stable frequency of resistance. Occult-resistant strains were characterized by a lower frequency of resistance, although the true frequency of resistance may be difficult to ascertain because of heterogeneity in growth rates.     

Staphylococcus aureus susceptibility to innate antimicrobial peptides,beta-defensins and CAP18, expressed by human keratinocytes

The antimicrobial peptides human beta-defensin-1 (hBD1), hBD2, hBD3, and CAP18 expressed by keratinocytes have been implicated in mediation of the innate defense against bacterial infection. To gain insight into Staphylococcus aureus infection, the susceptibility of S. aureus, including methicillin-resistant S. aureus (MRSA), to these antimicrobial peptides was examined. Based on quantitative PCR, expression of hBD2 mRNA by human keratinocytes was significantly induced by contact with S. aureus, and expression of hBD3 and CAP18 mRNA was slightly induced, while hBD1 mRNA was constitutively expressed irrespective of the presence of S. aureus. Ten clinical S. aureus isolates, including five MRSA isolates, induced various levels of expression of hBD2, hBD3, and CAP18 mRNA by human kertinocytes. The activities of hBD3 and CAP18 against S. aureus were found to be greater than those of hBD1 and hBD2. A total of 44 S. aureus clinical isolates, including 22 MRSA strains, were tested for susceptibility to hBD3 and CAP18. Twelve (55%) and 13 (59%) of the MRSA strains exhibited more than 20% survival in the presence of hBD3 (1 microg/ml) and CAP18 (0.5 microg/ml), respectively. However, only three (13%) and two (9%) of the methicillin-sensitive S. aureus isolates exhibited more than 20% survival with hBD3 and CAP18, respectively, suggesting that MRSA is more resistant to these peptides. A synergistic antimicrobial effect between suboptimal doses of methicillin and either hBD3 or CAP18 was observed with 10 MRSA strains. Furthermore, of several genes associated with methicillin resistance, inactivation of the fmtC gene in MRSA strain COL increased susceptibility to the antimicrobial effect mediated by hBD3 or CAP18.     

Reevaluation of the ability of the standardized disk diffusion test to detect methicillin-resistant strains of Staphylococcus aureus.

To reevaluate the ability of the disk diffusion method to detect methicillin-resistant Staphylococcus aureus, 73 such isolates from 13 cities were tested for antimicrobial susceptibility with the standardized disk diffusion test. Duplicate plates were incubated at 30 and 35 degrees C and read after 18, 24, and 48 h. After incubation at 35 degrees C for 24 h, 97% of isolates appeared resistant to methicillin, and 99% appeared resistant to oxacillin. A significantly smaller proportion of isolates appeared resistant to cephalothin (P less than 0.001) and cefamandole (P less than 0.001). Isolates from some cities had no zones of inhibition around methicillin and oxacillin disks, whereas those from other cities had measurable zones of inhibition, with light growth inside the zones. Patterns of growth around cephalothin and cefamandole disks also varied among isolates from different cities. Incubation at 30 degrees C for 24 h did not result in better detection of methicillin or oxacillin resistance. All study isolates appeared resistant to methicillin and oxacillin after 48 h of incubation at 35 degrees C. The results suggest that methicillin-resistant S. aureus strains from many areas will be detected if standardized disk diffusion tests are incubated at 35 degrees C for 24 h.     

Reliability of the MS-2 system in detecting methicillin-resistant Staphylococcus aureus.

The MS-2 system (Abbott Diagnostics, Division of Abbott Laboratories, Dallas, Tex.) is an automated system capable of rapid antimicrobial susceptibility testing. However, the short incubation periods used by the device may adversely affect its ability to detect slowly growing resistant organisms. Shortly after the introduction of the MS-2 system into the University of Mississippi Medical Center clinical microbiology laboratory, we noted discrepancies between the MS-2 and the disk diffusion susceptibility reports when methicillin-resistant Staphylococcus aureus isolates were tested. Subsequently, we determined the susceptibilities of 75 such isolates by the MS-2 and Kirby-Bauer disk diffusion methods and measured the minimum inhibitory concentrations of methicillin, oxacillin, and cephalothin for 33 of the 75 isolates by standardized agar dilution techniques. There was only 47% overall agreement between the MS-2 and disk diffusion methods when methicillin was tested and 15% agreement when cephalothin was the test drug. There was 93% or more overall agreement between the two methods when other antimicrobial agents were tested. The minimum inhibitory concentration of methicillin was greater than or equal to 16 micrograms/ml for all 33 isolates evaluated by the agar dilution method. A comparison of the MS-2 and agar dilution results revealed an overall agreement of 49% when the susceptibilities to methicillin were determined. The MS-2 system reported that multiple methicillin-resistant S. aureus isolates obtained from a single patient were either resistant, intermediate, or sensitive to methicillin. Inconsistent results were also obtained when a single isolate was tested simultaneously in 10 cuvette cartridges. We conclude that the MS-2 system does not reliably detect methicillin and cephalothin resistance among S. aureus.     

Incidence of glycopeptide hetero-intermediate Staphylococcus aureus strains in Maltese hospitals

The incidence of hetero-intermediate glycopeptide susceptibility among Staphylococcus aureus isolates in Malta, a country with a high incidence of methicillin resistance, was studied by screening 454 non-repetitive S. aureus isolates on teicoplanin-supplemented agar plates, followed by Etests and genotypic studies. All strains were susceptible to vancomycin, but four (0.88%) exhibited teicoplanin MICs of > 12 mg/L. High methicillin-resistant S.aureus endemicity was not an accurate predictor of the emergence of non-susceptibility to glycopeptides.     

A comparison of PCR detection of mecA with oxacillin disk susceptibility testing in different media and sceptor automated system for both Staphylococcus aureus and coagulase-negative staphylococci isolates

PURPOSE: To evaluate three methods for 406 isolates of Staphylococcus aureus and coagulase-negative staphylococci (CNS) for the detection of methicillin resistance (MR) using National Committee for Clinical Laboratory Standards (NCCLS) new interpretive criteria. METHODS: We used polymerase chain reaction (PCR) as a gold standard method to evaluate three methods [disk diffusion with Mueller-Hinton agar (MHA) and mannitol salt agar (MSA) and Sceptor system (Becton Dickinson, USA)] for the detection of mecA gene. The isolates that were methicillin-resistant with any of the three tests were evaluated further for MR by E-test. RESULTS: MHA, MSA and Sceptor showed sensitivities of 100, 100 and 99% for S. aureus and 100, 82.6 and 72.1% for CNS, respectively. The specificities of the same methods were found as 100, 90.1 and 99.3% for S. aureus and 79.2, 95.8 and 97.2% for CNS, respectively. E-test showed 100% sensitivity for both S. aureus and CNS. Forty-eight CNS and 16 S. aureus isolates, which presented discrepancies with the three phenotypic methods (MHA disk diffusion method, MSA disk diffusion method and Sceptor), were correctly classified as resistant/susceptible with the E-test when compared with PCR. Only five CNS isolates, which were mecA-negative with PCR were resistant with E-test. Analysis of 248 S. aureus revealed that MHA is superior to other phenotype-based susceptibility testing methods in detecting MR. When we examined the results of 158 CNS, none of the three methods proved efficient in detecting MR. CONCLUSIONS: We conclude that although the accuracy of the MHA disk diffusion test for the detection of MR approaches the accuracy of PCR for S. aureus isolates, the need for easy and reliable methods of detecting MR in CNS still remains.     

New quadriplex PCR assay for detection of methicillin and mupirocin resistance and simultaneous discrimination of Staphylococcus aureus from coagulase-negative staphylococci

Major challenges in diagnostic molecular microbiology are to develop a simple assay to distinguish Staphylococcus aureus from the less virulent but clinically important coagulase-negative staphylococci (CoNS) and to simultaneously determine their antibiotic resistance profiles. Multiplex PCR assays have been developed for the detection of methicillin- and mupirocin-resistant S. aureus and CoNS but not for the simultaneous discrimination of S. aureus from CoNS. We designed a new set of Staphylococcus genus-specific primers and developed a novel quadriplex PCR assay targeting the 16S rRNA (Staphylococcus genus specific), nuc (S. aureus species specific), mecA (a determinant of methicillin resistance), and mupA (a determinant of mupirocin resistance) genes to identify most staphylococci, to discriminate S. aureus from CoNS and other bacteria, and to simultaneously detect methicillin and mupirocin resistance. Validation of the assay with 96 ATCC control strains and 323 previously characterized clinical isolates, including methicillin- and mupirocin-sensitive and -resistant S. aureus and CoNS isolates and other bacteria, demonstrated 100% sensitivity, specificity, and accuracy. This assay represents a simple, rapid, accurate, and reliable approach for the detection of methicillin- and mupirocin-resistant staphylococci and offers the hope of preventing their widespread dissemination through early and reliable detection.     

Detection of methicillin-resistant staphylococci by using the polymerase chain reaction.

A polymerase chain reaction (PCR)-based test was developed for the detection of mecA in staphylococci. To facilitate this process, a rapid cell lysis procedure was established for the release of DNA from staphylococcal strains. Primers based on the DNA sequence of the mecA gene from Staphylococcus aureus were used in PCRs to screen for the presence of this gene in a total of 98 staphylococcal isolates. Fifty-one isolates were mecA positive (17 S. aureus strains and 34 coagulase-negative staphylococci including S. epidermidis, S. haemolyticus, and S. simulans). Results obtained with PCRs were generally consistent with those of standard microbiological assays. PCRs designed to detect the femA gene (factor essential for methicillin resistance) revealed the presence of the gene in all S. aureus strains examined regardless of the susceptibility profiles of the strains to methicillin. In contrast, femA could not be detected in coagulase-negative staphylococci by PCR with the same primers. Low-stringency hybridization suggested the presence of a gene structurally related to femA in S. epidermidis and other coagulase-negative staphylococci examined.     

Detection of Staphylococcus aureus enterotoxins A to D by real-time fluorescence PCR assay

Staphylococcus aureus is one of the most significant pathogens causing nosocomial and community-acquired infections. Among the secreted staphylococcal virulence factors, there is a growing list of enterotoxins which can induce gastroenteric syndrome and toxic shock syndrome. Here, we developed a real-time fluorescence PCR assay (TaqMan PCR) for the detection of genes encoding staphylococcal enterotoxins A, B, C1, and D (SEA, SEB, SEC1, and SED) of S. aureus as well as the mecA gene encoding methicillin resistance and the femB gene as a specific genomic marker for S. aureus. SEA to SED were selected because they are the four classically described enterotoxins of S. aureus and because they were detected by latex agglutination. In order to evaluate the reliability of TaqMan PCR, we investigated 93 isolates of S. aureus derived from patients at our hospital over 5 months and compared the results with data obtained by a commercially available reversed passive latex agglutination assay (SET-RPLA) for these isolates. Thirteen enterotoxin genes were detected by TaqMan PCR; however, no proteins expressed by these genes were detected by SET-RPLA. As a result, more isolates of S. aureus (n = 44) were found positive by TaqMan PCR for one or more enterotoxin genes than by SET-RPLA for the respective proteins expressed by these genes (n = 40). We conclude that TaqMan PCR is more sensitive because it offers the possibility for determining enterotoxins on a genotypic basis. Additionally, the assay allows the parallel detection of genes for SEA to SED and methicillin resistance in S. aureus. Furthermore, real-time PCR is well suited for screening large numbers of samples at the same time, allowing rapid, reliable, efficient, and cost-saving routine laboratory diagnosis.     

Rapid Slide Latex Agglutination Test for Detection of Methicillin Resistance in Staphylococcus aureus

The MRSA screen test (Denka Seiken Co., Ltd.), a commercially available, rapid (20-min) slide latex agglutination test for the determination of methicillin resistance by detection of PBP 2a in Staphylococcus aureus, was compared with the oxacillin agar screen test and PCR detection of the mecA gene. A total of 563 S. aureus isolates were tested. Two hundred ninety-six of the isolates were methicillin-susceptible isolates from cultures of blood from consecutive patients. Also, 267 methicillin-resistant isolates that comprised 248 different phage types were tested. Methicillin resistance was defined as the presence of the mecA gene. Of the 267 mecA gene-positive isolates, 263 were positive by the MRSA screen test (sensitivity, 98.5%), and all the mecA-gene negative strains were negative by the MRSA screen test (specificity, 100%). The oxacillin agar screen test detected methicillin resistance in 250 of the mecA gene-positive isolates (sensitivity, 93.6%). The sensitivity of the MRSA screen test was statistically significantly higher than the sensitivity of the oxacillin agar screen test (P < 0. 05). The MRSA screen test is a highly sensitive and specific test for the detection of methicillin resistance. Also, it offers results within half an hour and is easy to perform, which makes this test a valuable tool in the ongoing battle against methicillin-resistant S. aureus.