Identification of methicillin-resistant strains of staphylococci by polymerase chain reaction

Distinct genotypic clusters of heterogeneously and homogeneously methicillin-resistant Staphylococcus aureus from a Greek hospital

Methicillin-resistant Staphylococcus aureus (MRSA) strains isolated over a one-year period from a Greek hospital were tested for their levels of resistance to methicillin by population analysis. Heterogeneously resistant strains belonged to classes I, II, and II/III, whereas homogeneously resistant ones belonged to class IV. Strains of all classes possessed the mecA gene. Pulsed-field gel electrophoresis (PFGE) of Smal-digested genomic DNA revealed that all heterogeneously resistant strains were also closely related, but in a cluster distinct from the heterogeneous one. The methicillin-sensitive strains displayed a greater variety of PFGE types compared to MRSA isolates.

Rapid diagnosis of methicillin-resistant Staphylococcus aureus bacteremia by nested polymerase chain reaction

OBJECTIVE

The purpose of this study was to establish a rapid and sensitive diagnostic method for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia in postoperative patients.

SUMMARY BACKGROUND DATA

As a result of diffusion and abuse of third-generation cephalosporin antibiotics in the 1980s in Japan, an outbreak of MRSA infection has been posed. In the field of surgery, severe postoperative infections with MRSA such as MRSA bacteremia, which may lead to multiple organ failure, have emerged with a high mortality.

METHODS

Thirty-five patients with high fever (above 38.5 C) or watery diarrhea or both within 2 weeks after gastrointestinal major surgery and 6 healthy volunteers were examined. Nested polymerase chain reaction was used to detect mecA and toxic shock syndrome toxin-1 (TSST-1) genes in blood specimens.

RESULTS

The mecA and TSST-1 genes were not detected in the blood samples of any of the six healthy volunteers. In all 12 samples from which MRSA colonies were isolated by blood culture, mecA and TSST-1 genes were detected. Although it took at least 48 hours to identify MRSA by the blood culture method, the presence of mecA and TSST-1 genes was determined by nested polymerase chain reaction method within only 3 to 4 hours after blood sampling.

CONCLUSIONS

This method, as a sensitive and rapid monitoring system for MRS bacteremia, would be clinically beneficial for prevention of cross infection and for early determination of appropriate treatment for infected patients.

Isolation of methicillin-resistant coagulase-negative staphylococci from chickens

Methicillin-resistant coagulase-negative staphylococci were isolated from the nares and skin of 1- to 8-week-old healthy chickens in three flocks from a farm. Isolation of methicillin-resistant coagulase-negative staphylococci was positive for 72 (25.7%) of the 280 chickens tested, with the frequency varying from 2.2 to 100% according to flock. A total of 45 appropriate isolates were selected and subjected to identification. Of the 45 methicillin-resistant coagulase-negative staphylococcal isolates selected, 37 were identified as Staphylococcus sciuri, 5 were identified as Staphylococcus epidermidis, and 3 were identified as Staphylococcus saprophyticus. The distribution of the species was different among the flocks. Comparative analysis of the SmaI-digested chromosomal DNA by pulsed-field gel electrophoresis revealed that the isolates could have originated from a single clone of each of S. sciuri and S. saprophyticus and three clones of S. epidermidis. By two methods based on the PCR technique, the mecA gene was detected in all five representative isolates of each methicillin-resistant coagulase-negative staphylococcal clone. The nucleotide sequence of a PCR fragment obtained from an isolate of S. sciuri was completely identical to the corresponding region of mecA genes reported in human methicillin-resistant Staphylococcus aureus isolates and Staphylococcus epidermidis isolates. The representative methicillin-resistant coagulase-negative staphylococcal isolates were resistant to many beta-lactam antibiotics, and some isolates were also resistant to macrolide and aminoglycoside antibiotics. This is the first evidence of the existence of methicillin-resistant coagulase-negative staphylococci from animals possessing the mecA gene.

A modified PCR-based method for rapid non-radioactive detection of clinically important pathogens

We have devised a sensitive and rapid method for the detection of several bacterial pathogens in clinical specimens using PCR. This method has been named Direct Labeling and Detection Procedure (DLDP) and is based on the direct incorporation of a nonradioactive digoxigenin label (DIG-11-dUTP) into a microbial species-specific gene fragment during amplification. Following amplification, the resulting PCR products are cleansed of nonincorporated DIG-11-dUTP, spotted onto a nylon membrane, fixed by UV-crosslinking and the labeled DNA is visualized by digoxigenin detection reagents. Using cultivated reference bacteria (Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa) we were able to demonstrate a rapid and sensitive detection of < 20 CFU of bacteria in human secretions (sputum, urine, mucous). The present study suggests that DLDP can be used as a reliable method for indication of bacteria in clinical or environmental specimens with the proviso that the selected corresponding oligonucleotide primers provide amplification of strong species-specific genes.

Rapid genotypic confirmation of methicillin resistance

Detection of phenotypic methicillin resistance in Staphylococcus aureus clinical strains by conventional disk diffusion testing is fraught with problems. We used gene amplification of the mecA locus by polymerase chain reaction (PCR), in conjunction with a capillary/air thermal cycler, to overcome both the inaccuracy of phenotypic methods and the lengthy processing times required for previous genotypic methods. The rapid PCR method correctly identified methicillin resistance in a consecutive series of 30 S. aureus isolates when compared with routine and reference phenotypic methods. The shorter processing time and smaller reagent volumes required for the air thermal cycler make same-day determination of methicillin resistance in clinical isolates feasible for diagnostic laboratories.

Anionic properties of beta-lactam-enhancing factor on methicillin-resistant Staphylococcus aureus

We found a Factor (factor T) in aged mixtures of tungstate and phosphate which greatly enhanced the antibacterial effects of beta-lactams upon methicillin-resistant Staphylococcus aureus (MRSA). Factor T suppressed penicillinase production and the amount of penicillin-binding protein-2' in the membrane fraction, thus sensitizing MRSA strains to beta-lactams. In addition, Factor T caused a metachromatic reaction and prolonged the blood coagulation time, indicating that it is a heparin-like polyanion. Since Factor T becomes ineffective in the presence of a polycation, a charge interaction may play an important role in the enhancing effect. One possibility is that Factor T non-specifically inhibits several anion-sensitive enzymes. Factor T inhibited several nucleotide-interacting enzymes, but not most serum enzymes.

Comparison of PCR detection of mecA with standard susceptibility testing methods to determine methicillin resistance in coagulase-negative staphylococci

Approximately 75% of coagulase-negative staphylococci are resistant to methicillin, but it is suspected that even more resistance exists that is not detected by standard susceptibility assays. To determine the most accurate assay for measuring resistance, we compared the detection of mecA by PCR with detection by National Committee for Clinical Laboratory Standards methods using oxacillin as the class drug. Strains from 11 species of coagulase-negative staphylococci were selected such that 84% were susceptible by the broth microdilution method. Of 45 mecA-positive strains, 1 strain was unable to express the mecA gene product after induction and was not included in further analyses. For microdilution with 2% NaCl, the disk test without salt, and agar screen containing 4% NaCl plus-6 micrograms of oxacillin per ml, the sensitivities in detecting the 44 mecA-positive strains were 50, 84, and 70%, respectively, at 24 h and 77, 82, and 100%, respectively, at 48 h. The specificities of microdilution, disk, and agar screen in detecting the 97 strains lacking mecA were 100, 89, and 100%, respectively, at 24 h. Only the disk test proved to be less specific at 48 h (81%). Furthermore, for 10 of the mecA-positive strains plus an additional 8 strains subsequently added to the analyses, the MICs were 2 micrograms/ml at 24 h by the broth microdilution method; all 18 strains were positive for mecA by PCR. Thus, an oxacillin MIC of > or = 2 micrograms/ml indicated resistance and is probably a more appropriate breakpoint than the current National Committee for Clinical Laboratory Standards breakpoint of 4 micrograms/ml for coagulase-negative staphylococci. Strains for which MICs are < 2 micrograms/ml may be methicillin resistant and should be verified as susceptible by oxacillin agar screening with incubation for 48 h.

Detection of oxacillin-resistance in Staphylococcus aureus by MicroScan MIC panels in comparison to four other methods

Two hundred fifty-two isolates of Staphylococcus aureus were tested for oxacillin susceptibility by MicroScan Gram positive overnight and rapid MIC panels. Results were compared with nonautomated methods including disk diffusion, MRSA Crystal ID, and Etests using MRSA Screen Agar as reference. One hundred sixty-nine isolates (67.1%) were oxacillin-susceptible and 83 (32.9%) were resistant. All methods agreed for 234 (92.9%) isolates. Very major error rates were 1.2% for disk diffusion, 3.6% for Etest, and 0 for all other methods. Major error rates were 5.3% for MicroScan overnight panels, 3% for rapid panels, 2.4% for disk diffusion, 1.2% for Etest, and 0.6% for MRSA Crystal ID. Nine oxacillin-susceptible isolates with borderline MICs and discrepant results for 1 or more methods were tested for the mec A gene and all were negative. Each was susceptible to beta lactam/beta lactamase inhibitor combinations, suggesting that false resistance may have been due to excessive beta lactamase production. Oxacillin-resistant S. aureus with borderline MICs determined by MicroScan should be confirmed by an alternate method. The most practical and cost-effective means among those we tested is the MRSA Screen Agar.

Specific detection of methicillin-resistant Staphylococcus species by multiplex PCR

In Staphylococcus aureus, mecA and femA are the genetic determinants of methicillin resistance. By using a multiplex PCR strategy, 310- and 686-bp regions of the mecA and femA genes, respectively, were coamplified to identify susceptible (lacking mecA) and resistant (mecA+) staphylococci and to differentiate S. aureus (femA+) from coagulase-negative staphylococci (lacking femA). A third staphylococcal genomic sequence, corresponding to IS431 and spanning 444 bp, was used as a PCR control. One hundred sixty-five staphylococcal strains were tested. All 72 methicillin-resistant strains were found to be mecA+, and 92 of the 93 susceptible isolates lacked mecA. Only one coagulase-negative Staphylococcus isolate carrying the mecA gene was highly susceptible to oxacillin. The femA determinant was a unique feature of S. aureus; it was found in 100% of the S. aureus strains tested but was undetectable in all of the coagulase-negative staphylococci tested. The possibility of directly detecting the mecA and femA genes in blood samples was also investigated. After two amplification steps, a sensitivity of 50 microorganisms per ml of freshly collected spiked blood was achieved. In conclusion, coamplification of mecA and femA determinants proved to be very reliable both for rapid detection of methicillin resistance and differential diagnosis between S. aureus and other staphylococci. This technique, which can be successfully performed with blood samples, could be a useful tool in the diagnosis and treatment monitoring of staphylococcal infections.

Evaluation of the BBLreg CrystalTM MRSA ID System for Rapid Detection of Methicillin Resistance in Staphylococcus aureus

Twenty-four clinical isolates of Staphylococcus aureus collected from various geographic areas and four reference strains were studied by (i) agar diffusion with disks impregnated with 5 microg oxacillin and reading after incubation at 30C for 24 hours, (ii) Southern hybridization with a probe specific for the mecA gene, and (iii) the BBLreg CrystalTM MRSA ID system. There was perfect correlation between the three methods: the BBLreg CrystalTM MRSA ID system detected methicillin resistance in the fifteen strains hybridizing with the mecA probe and classified as resistant by the oxacillin disk diffusion test; the thirteen remaining strains were susceptible by agar diffusion and by the BBLreg test and did not hybridize with the mecA probe. The BBLreg CrystalTM MRSA ID System, therefore, appears to be an accurate method for rapid detection of Staphylococcus aureus exhibiting homogeneous resistance to methicillin.

Immunological detection of penicillin-binding protein 2' of methicillin-resistant staphylococci by using monoclonal antibodies prepared from synthetic peptides

Two synthetic peptides 31 and 32 amino acids in length were prepared as deduced from a known amino acid sequence of penicillin-binding protein 2' (PBP2') of methicillin-resistant Staphylococcus aureus. Two monoclonal antibodies were generated from fused cells of myeloma cells and splenic cells of mice immunized with the synthetic peptides. Western blot (immunoblot) analysis demonstrated specific binding of the antibodies to PBP2' of a methicillin-resistant S. aureus strain. An immunoradiometric assay was developed by using these antibodies for simple detection of PBP2'.

Bactericidal activity of skin disinfectants on methicillin-resistant Staphylococcus aureus

We studied bactericidal activity of 10% povidone-iodine, 0.5% chlorhexidine gluconate, and 0.5% chlorhexidine in 80% ethanol on four strains of methicillin-resistant and two strains of methicillin-susceptible Staphylococcus aureus. The pathogen was exposed to each of the disinfectants for 15, 30, 60, 120, and 240 s at room temperature. The inocula from these suspensions were cultured 72 h at 37 degrees C after the antimicrobial activity of the disinfectants in the suspensions was inactivated by 1:1000 dilution with neutralizer. No organism grew in any of the strains after exposure to 0.5% chlorhexidine in 80% ethanol. The 15-, 30-, and 60-s exposure to 10% povidone-iodine reduced the mean colony count by 55.2%, 91.2%, and 96.7%, respectively, and the exposures to 0.5% chlorhexidine gluconate reduced the mean colony count by 37.2%, 77.1%, and 93.3%, respectively. The difference in colony count between disinfectants was significant at 15- and 30-s exposures (P < 0.01 and 0.05, respectively). The results suggest that bactericidal activity of 0.5% chlorhexidine in 80% ethanol is more potent and more rapid against methicillin-susceptible and methicillin-resistant strains of S. aureus.

Detection of methicillin-resistant Staphylococcus aureus (MRSA) with antibodies against synthetic peptides derived from penicillin-binding protein 2'

Ten kinds of peptides (21 to 32 amino acids in length) were synthesized based on the reported amino acid sequences of the penicillin-binding protein 2' (PBP2') of methicillin-resistant Staphylococcus aureus (MRSA). Antibodies against these synthetic peptides (SPs) were generated by immunizing rabbits. The antibodies raised against all the peptides except for one reacted to PBP2' of MRSA and to SPs used for immunization but not to any other protein of MRSA or methicillin-susceptible S. aureus (MSSA) tested by ELISA and Western blotting. A sandwich immunoradiometric assay (IRMA) for the detection of PBP2' was developed using these antibodies. The method could detect PBP2' extracted from as few as 3 x 10(4) cells of a clinical MRSA isolate, and a good correlation between cell number and signal radio-count was observed. IRMA was positive for all 51 methicillin-resistant staphylococci isolated from patients, and was negative for all the 28 methicillin-susceptible ones and 19 strains of other bacterial species. IRMA could be a simple and reliable method for MRSA detection in the clinical bacterial laboratory.

Identification and ribotypes of Staphylococcus caprae isolates isolated as human pathogens and from goat milk

We report five cases of human infection with Staphylococcus caprae. Two were community acquired (one case each of endocarditis and urinary tract infection); the other three were acquired in a hospital (two cases of bacteremia associated with intravenous access and one case of urinary tract infection). Analysis of human isolates and goat isolates from eight herds showed that they could be misidentified by some commercial identification systems but were clearly identified as S. caprae by ribotyping, according to their species-specific ribotype. Phylogenetic methods applied to the ribotypes did not reveal two distinct lineages corresponding to the goat and human origins of the isolates, although human ribotypes were clearly distinguishable by the presence of a core of four specific bands. The latter observation may reflect some degree of evolutionary change within the species between human and goat isolates.

Revised interpretation of oxacillin MICs for Staphylococcus epidermidis based on mecA detection

In 1992 and 1993, at The Ohio State University Medical Center, a larger proportion of Staphylococcus epidermidis strains required oxacillin MICs of 1 to 2 micrograms/ml than did Staphylococcus aureus strains. mecA genotype was correlated with antimicrobial susceptibility for selected clinical S. epidermidis strains. All 14 strains that required oxacillin MICs of < or = 0.25 microgram/ml and 2 of 5 strains that required oxacillin MICs of 0.5 microgram/ml were susceptible by 1-microgram oxacillin disk test and were mecA negative. Three of 5 strains that required oxacillin MICs of 0.5 microgram/ml and all 18 strains that required oxacillin MICs of > or = 1.0 microgram/ml were resistant by oxacillin disk test and were mecA positive. Current National Committee for Clinical Laboratory Standards MIC interpretive criteria may underestimate methicillin resistance among S. epidermidis strains.

Direct identification of Staphylococcus aureus in blood cultures by detection of the gene encoding the thermostable nuclease or the gene product

This study compares methods for direct identification of S. aureus in blood cultures by detection of the thermonuclease (TNase) of this bacterium or the nuc gene encoding it. The protein was detected by an enzyme diffusion test in o-toluidine blue DNA agar with a test time of at least 4 h, by a monoclonal antibody (MAb)-based sandwich enzyme-linked immunosorbent assay (sELISA) with a test time of approximately 4 h, and by a MAb-based sandwich enzyme-linked immunofiltration assay (sELIFA) with a test time of 25-30 min (sample preparation included). The nuc gene was amplified by a polymerase chain reaction (PCR) with a test time (amplification plus detection) of approximately 3.5 h. The tests were optimized for direct examination of blood-containing cultures. All tests were positive with 67/67 blood cultures which grew S. aureus, negative with 35/35 cultures which grew coagulase-negative staphylococci, and negative with 37/37 cultures with various other bacteria. These results showed positive agreement with those of the commercial AccuProbe test but not with the StaphAurex agglutination kit. With an artificially seeded blood culture, minimum total times required (incubation plus testing) were as follows: nuc-PCR, 9.5 h; sELIFA, 12.5 h; enzymatic test, 16-36 h; AccuProbe, 14 h. Direct examination of both the nuc gene and the mecA gene encoding methicillin resistance demonstrated the mecA gene in all the coagulase-negative staphylococci (48.6%) which showed oxacillin resistance. The sELIFA had the particular advantage of its short test time, the PCR its high sensitivity and the possibility of simultaneous detection of the species-specific nuc gene and genes encoding other clinically important characters of the bacteria. These tests offer the prospect of direct application to a variety of clinical specimens for rapid diagnosis of S. aureus infection.

Species-specific identification of methicillin resistance in staphylococci

The ability to identify methicillin-resistant staphylococci by the disc diffusion method was evaluated using discs containing oxacillin (1, 5 and 10 micrograms), methicillin (10 micrograms) and cephalexin (30 micrograms). Strains of Staphylococcus aureus (67 strains) and coagulase-negative staphylococci (72 novobiocin-sensitive and 27 novobiocin-resistant strains) were studied using two inoculum densities (10(6) cfu/ml and 10(8) cfu/ml). Inhibitory zones were recorded after 18, 24 and 42 hours of incubation. A mecA-specific application of the polymerase chain reaction was used as a reference method. The inoculum of 10(8) cfu/ml and incubation for 24 hours were optimal for the identification of methicillin-resistant strains. However, one single disc was not sufficient for the identification of methicillin resistance in the different staphylococcal species. The mecA-positive strains of Staphylococcus aureus and novobiocin-resistant coagulase-negative species were clearly separated from the mecA-negative strains when the 5 micrograms oxacillin disc was used, whereas the 1 microgram oxacillin disc was optimal for the identification of the mecA-positive novobiocin-sensitive coagulase-negative strains.

Diversity of methicillin-resistant Staphylococcus aureus isolated in a Canadian hospital

Three neonates and three other patients located elsewhere in the hospital became infected with Staphylococcus aureus. Initial automated microdilution susceptibility testing with oxacillin and disk diffusion testing with amoxicillin-clavulanic acid indicated the isolates had borderline oxacillin resistance (MICs 4 micrograms/ml), presumably due to hyperproduction of beta-lactamase. Chromosomal DNA restriction fingerprinting and phage typing revealed the neonatal isolates to be identical; whereas, the other patients were infected with three different strains. Further analysis of the four strains by Southern hybridization with a mecA specific oligoprobe and a quantitative beta-lactamase assay demonstrated that two strains carried the mecA gene (coding for low affinity penicillin-binding protein 2a), and two strains were hyperproducers of beta-lactamase, including one which was mecA gene positive. One strain neither carried the mecA gene nor hyperproduced beta-lactamase. The two mecA gene positive strains displayed oxacillin MICs of 16 micrograms/ml on dilution susceptibility testing in 4% NaCl supplemented Mueller-Hinton agar. Hence, they were considered intrinsically methicillin-resistant Staphylococcus aureus. Both oxacillin and amoxicillin-clavulanic acid MICs were increased on NaCl supplementation. Results of amoxicillin-clavulanic acid disk diffusion susceptibility testing did not correlate with quantitative beta-lactamase production. It is recommended that clinical laboratories do not use amoxicillin-clavulanic acid disk diffusion assays to differentiate suspected borderline resistance due to beta-lactamase hyperproduction from mecA gene expression of PBP-2a since additional mechanisms may account for resistance.

Testing of methicillin resistance by in vitro susceptibility and the presence of the mecA gene in clinical Staphylococcus aureus isolates in Finland

A total of 140 epidemiologically unrelated Staphylococcus aureus strains collected in Finland between 1983 and 1994 were sent to the reference laboratory with verified or suspected methicillin resistance. These strains and 37 S. aureus strains previously identified as methicillin-susceptible were retested using 5 different susceptibility test methods including agar screening, disc diffusion, growth around methicillin (25 micrograms) test strips and minimal inhibitory concentration (MIC) determinations by an agar dilution method and E-test. The isolates were also analyzed for the presence of the mecA gene by the polymerase chain reaction (PCR). Based on in vitro susceptibility, 69 strains were identified as methicillin-resistant and were positive for the mecA gene in PCR, while 84 strains were methicillin-susceptible and negative for this gene. Susceptibility testing gave conflicting results for 24 (14%) strains. When the tests were repeated in triplicate for each isolate, discrepant results were still achieved with 18 of the 24 strains in at least 2 different tests. Thus, based on in vitro susceptibility, these strains could not be definitely classified as resistant or susceptible to methicillin. Yet 7 of them were positive for the mecA gene as an indication of genetic resistance to methicillin. Corroborating earlier studies, these results illustrate the difficulty of detecting methicillin resistance/susceptibility based only on susceptibility testing and underscore the importance of confirming methicillin resistance in S. aureus in specialized laboratories.

Detection of mecA, femA, and femB genes in clinical strains of staphylococci using polymerase chain reaction

MecA, a structural gene located on the chromosome of Staphylococcus aureus, characterizes methicillin-resistant S. aureus (MRSA), and femA and femB(fem) genes encode proteins which influence the level of methicillin resistance of S. aureus. In order to examine effectiveness of detecting mecA and fem genes in identification of MRSA, the presence of these genes in 237 clinically isolated strains of staphylococci was investigated by polymerase chain reaction (PCR). An amplified mecA DNA fragment of 533 base pairs (bp) was detected in 100% of oxacillin-resistant S. aureus, in 16.7% of oxacillin-sensitive S. aureus, in 81.5% of S. epidermidis, and in 58.3% of other coagulase-negative staphylococci (CNS). While the PCR product of femA (509 bp) or femB (651 bp) was obtained from almost all the S. aureus strains except for five oxacillin-resistant strains (2.5%), neither of these genes were detected in CNS. Therefore, the detection of femA and femB together with mecA by PCR was considered to be a more reliable indicator to identify MRSA by differentiating it from mecA-positive CNS than single detection of mecA.

Possible mechanism of action of beta-lactam-enhancing factor on methicillin-resistant Staphylococcus aureus

We have recently found a factor (Factor T) in aged mixtures of tungstate and phosphate which greatly enhances the antibacterial effects of beta-lactams on methicillin-resistant strains of staphylococcal species such as methicillin-resistant Staphylococcus aureus (MRSA), but shows only weak effects on methicillin-susceptible S. aureus and bacterial strains other than staphylococci. Factor T alone did not strongly inhibit cell metabolism and bacterial growth unless an excess amount was added. When Factor T was added to the culture medium beforehand, the growth of MRSA cells was rapidly suppressed just after addition of oxacillin (MPIPC). However, the growth of the cells was inhibited gradually when these two reagents were added in reverse order. For full expression of the enhancing effect, it seemed necessary for cells of MRSA strains to be incubated with Factor T for at least 2-3 hr. When the cells were washed after being sensitized by incubating them for 5 hr with Factor T, it took approximately 1 hr for the cells to recover their resistance to MPIPC. Factor T reduced the amount of penicillin-binding protein-2' (PBP 2'), and thus sensitized the MRSA strains to beta-lactams.

Cloning and characterization of a gene affecting the methicillin resistance level and the autolysis rate in Staphylococcus aureus

Tn918 mutagenesis of a high-level methicillin-resistant Staphylococcus aureus (methicillin MIC, 800 micrograms/ml) led to the isolation of a low-resistance mutant. The Tn918 insert was transferred back to the parent to produce strain SRM563 (methicillin MIC, 12.5 micrograms/ml), which showed heterogeneous resistance. Twenty-two clinical isolates of methicillin-resistant S. aureus were transformed with DNA of SRM563. In the transformants of most strains, instances of reduced resistance were observed with concomitant increases of autolysis rate induced by Triton X-100 and were generally more profound in high-resistance strains. Two transformants exhibited a decrease of the autolysis rate and little reduction of resistance. In the transformant of methicillin-susceptible strain RN2677, an increase of the autolysis rate and little reduction of resistance were observed. The production of low-affinity penicillin-binding protein (PBP2') did not significantly decrease in the mutants. Insertion of Tn918 occurred within the 3'-terminal region of a novel gene designated llm, which was cloned and sequenced. RNA blot analysis demonstrated that the gene was transcribed. The encoded protein was composed of 351 amino acid residues with a molecular weight of 38,512 and was hydrophobic, suggesting its location on the membrane. The gene was detected by PCR in all S. aureus strains tested but not in the other 26 staphylococcal species. Comparison of the 3'-terminal sequences of the gene among several S. aureus strains showed that, whereas nucleotide substitutions occurred at the third position in seven of eight 3'-terminal codons, only C-terminal amino acid variation of glutamate or aspartate was observed.

Development of enzyme-labeled oligonucleotide probe for detection of mecA gene in methicillin-resistant Staphylococcus aureus

A DNA hybridization method with an enzyme-labeled oligonucleotide probe (mecA-ELONP) was developed to detect the methicillin-resistant gene (mecA) in methicillin-resistant Staphylococcus aureus. For rapid identification, bacterial colonies were transferred from agar plates directly onto nylon membranes. Lysis of cells, denaturation of DNA, and hybridization were performed on the membranes. These procedures required only 3 h for completion. The results obtained by this test closely corresponded with those obtained by determining the MICs of oxacillin against S. aureus. The results of the mecA-ELONP also correlated well with those of a commercially available PCR test. Thus, mecA-ELONP proved to be a reliable and convenient method for the rapid identification of methicillin-resistant S. aureus, which could be useful in clinical microbiology laboratories.

Multiplex PCR for identification of methicillin-resistant staphylococci in the clinical laboratory

A multiplex PCR assay for detection of the staphylococcal mecA gene (the structural gene for penicillin-binding protein 2a) was compared with agar dilution and disk diffusion susceptibility test methods for identifying methicillin resistance. The multiplex PCR assay combined two primer sets (mecA and 16S rRNA) in a single reaction. A total of 500 staphylococcal isolates (228 isolates of Staphylococcus aureus and 272 isolates of coagulase-negative staphylococci) from clinical specimens were studied. For S. aureus, 40 of 40 mecA-positive isolates and 4 of 188 mecA-negative isolates were oxacillin resistant (positive and negative predictive values of 100 and 98%, respectively). In 3 of 4 discordant isolates, resistance was due to hyperproduction of beta-lactamase. For coagulase-negative staphylococci, 148 of 159 mecA-positive isolates and 0 of 113 mecA-negative isolates were oxacillin resistant (positive and negative predictive values of 93 and 100%, respectively). Twenty-six isolates were categorized as indeterminate because of the absence of a detectable 16S rRNA product. Four of these 26 isolates contained mecA when retested. The assay is designed to be incorporated into the work flow of the clinical microbiology laboratory and allows for the identification of intrinsic resistance in a timely and reliable manner.

Laboratory diagnosis of oxacillin resistance in Staphylococcus aureus by a multiplex-polymerase chain reaction assay

A polymerase chain reaction (PCR) test was developed in which the mecA gene responsible for the intrinsic resistance to oxacillin in Staphylococcus aureus and the gyrA gene, always present in this species, were amplified in one operation. Among the 468 clinical isolates tested, the results obtained for 454 of the isolates (97%) were consistent with those of MIC determination. Discrepant results were noted for strains with low-level oxacillin resistance (MICs, 4-8 micrograms/ml) and mecA gene negative. For these strains, susceptibility to oxacillin was restored in the presence of a beta-lactamase inhibitor, which suggested a resistance by penicillinase hyperproduction. In contrast, all of the high-level resistant strains (MICs, > 8 micrograms/ml) carried the mecA gene. The presence of this gene has frequently been associated with resistance to gentamicin, tetracycline, erythromycin, lincomycin, and pefloxacin. The PCR assay described in this study can be accomplished with ease and total confidence in the clinical microbiologic laboratory for a rapid and effective establishment of antistaphylococcal chemotherapy.

Nucleotide sequence of the structural gene for the penicillin-binding protein 2 of Staphylococcus aureus and the presence of a homologous gene in other staphylococci

The structural gene for penicillin-binding protein 2 (PBP2) of Staphylococcus aureus was cloned and sequenced. The nucleotide sequence of the 2,458-bp chromosomal fragment was determined, and the 2,148-bp coding region for PBP2 was identified. Determination of ten N-terminal amino acids of the PBP2 protein indicated that N-terminal methionine had been removed from the primary translational product. Thus, PBP2 is comprised of 715 amino acids with a molecular mass of 79,147. Nucleotide sequences having some homology with the PBP2 gene and proteins cross-reactive with anti-PBP2 antibody were detected in some other species of staphylococci by polymerase chain reaction and Western blot analysis, respectively.

Comparison of phenotypic methods and DNA hybridization for detection of methicillin-resistant Staphylococcus aureus

One hundred thirty-eight Staphylococcus aureus isolates from patients with severe staphylococcal infections were collected in 15 French hospitals. Detection of the mec gene was performed by dot blot hybridization with a specific DNA probe. Dot blot results were used to characterize the isolates as methicillin susceptible (77 isolates) or resistant (61 isolates). The isolates were screened for methicillin resistance by an agar spread method on Mueller-Hinton plates containing oxacillin (2 and 10 micrograms/ml) and were incubated at 37 degrees C, with 10(8) CFU as the inoculum. MICs of oxacillin and methicillin were determined by the agar dilution method on Mueller-Hinton plates without NaCl, by using 10(5) CFU per spot, after 24 and 48h of incubation at 30 or 37 degrees C. Moderately elevated MICs were found for 20 isolates (14.5%). The mec gene was detected in six (30%) of the isolates expressing a low level of resistance to methicillin and/or oxacillin. As determined by comparison with probe hybridization results, the spread plate method with oxacillin at 2 micrograms/ml was more sensitive (sensitivity, 100%) and specific (specificity, 100%) than agar dilution with either methicillin or oxacillin in identifying methicillin resistance or susceptibility. Determinations of methicillin and oxacillin MICs by the agar dilution method had a specificity of 99 to 100% depending on the conditions of incubation, but the sensitivity was below 85% whatever the duration or temperature of incubation.

Comparison of tests for detection of methicillin-resistant Staphylococcus aureus in a clinical microbiology laboratory

By microdilution testing, 186 of 1,450 clinical isolates of Staphylococcus aureus were preliminarily classified as oxacillin resistant (MIC > or = 4 micrograms/ml); 15 of these isolates gave conflicting results by alternative methods and were studied further. Only 2 of these (MIC > 4 micrograms/ml) were mecA positive; 13 were inhibited by oxacillin at 4 micrograms/ml. Significant numbers of S. aureus strains classified as resistant with an oxacillin MIC of 4 micrograms/ml may prove susceptible by other methods.

National survey of methicillin-resistant Staphylococcus aureus in Belgian hospitals: detection methods, prevalence trends and infection control measures. The Groupement pour le Dépistage, l'Etude et la Prévention des Infections Hospitalières

A questionnaire survey of Belgian acute care hospitals was conducted to determine the methods used for detection of methicillin-resistant Staphylococcus aureus (MRSA), to estimate the prevalence of this organism during the period 1989-1991 and to describe the infection control measures used locally for limiting its spread. Questionnaires were returned by 144 acute care hospitals, with a coverage of 41 to 72% of hospitals by province. Methods used for detection of MRSA included disk diffusion (91%), microdilution panels (8%) and oxacillin agar screen (9%). Only 34% of laboratories performed disk diffusion testing under optimal conditions for detection of heterogenous resistance. Among 36 hospitals reporting complete susceptibility data of Staphylococcus aureus isolates tested during the study period (n = 24,153), a mean MRSA prevalence of 14% was found (range: 0-70%). The median prevalence increased from 9.5% in 1989 to 13.7% in 1991 and showed a significant linear increase during this period in 30% of these hospitals (p < 0.01). Precautions used for controlling spread of MRSA included hand decontamination using either soap and water or antimicrobial preparations (68% of hospitals), room decontamination (62%), patient isolation (55%) and various barrier precautions (24-49%). Carrier screening was performed in 37% of hospitals, but antibiotic decolonization was attempted in only 24%. This survey identified areas for improvement in MRSA detection methods and underscored the need for multicentric surveillance of MRSA prevalence and a reappraisal of MRSA control strategies in Belgian hospitals.

Update on clinical significance of coagulase-negative staphylococci

The clinical significance of coagulase-negative Staphylococcus species (CNS) continues to increase as strategies in medical practice lead to more invasive procedures. Hospitalized patients that are immunocompromised and/or suffering from chronic diseases are the most vulnerable to infection. Since CNS are widespread on the human body and are capable of producing very large populations, distinguishing the etiologic agent(s) from contaminating flora is a serious challenge. For this reason, culture identification should proceed to the species and strain levels. A much stronger case can be made for the identification of a CNS etiologic agent if the same strain is repeatedly isolated from a series of specimens as opposed to the isolation of different strains of one or more species. Strain identity initially can be based on colony morphology, and then one or more molecular approaches can be used to gain information on the genotype. Many of the CNS species are commonly resistant to antibiotics that are being indicated for staphylococcal infections, with the exception of vancomycin. The widespread use of antibiotics in hospitals has provided a reservoir of antibiotic-resistant genes. The main focus on mechanisms of pathogenesis has been with foreign body infections and the role of specific adhesins and slime produced by Staphylococcus epidermidis. Slime can reduce the immune response and opsonophagocytosis, thereby interfering with host defense mechanisms. As we become more aware of the various strategies used by CNS, we will be in a better position to compromise their defense mechanisms and improve treatment.

Two percent sodium chloride is required for susceptibility testing of staphylococci with oxacillin when using agar-based dilution methods

The need to add NaCl to agar media to ensure accuracy of results when testing staphylococci with oxacillin was investigated. The results of four antimicrobial susceptibility testing methods (agar and broth dilution, E test, and disk diffusion) in which the growth medium contained 0, 2, 4, or 5% NaCl were compared with the results of a hybridization assay using a mec gene probe. We tested 223 strains of staphylococci, 128 of which were mec gene positive. A total of 7 of the 128 positive strains were coagulase-negative staphylococci with 24-h oxacillin MICs of < or = 2 micrograms/ml. Ninety-five isolates were mec gene negative, including seven strains of Staphylococcus aureus with oxacillin MICs of > or = 4 micrograms/ml. The oxacillin MICs for mec gene-positive, oxacillin-resistant strains of staphylococci increased two- to fourfold with the addition of NaCl to the test medium, while the MICs for mec gene-negative strains did not change in the presence of added salt. Very major error rates for the agar dilution and E test methods in the absence of salt ranged from 18.2 to 20.2%. Major error rates for mec gene-negative S. aureus isolates were > 17% for all test methods when 4 or 5% NaCl was added to the test medium. The addition of 2% NaCl to Mueller-Hinton agar for testing of oxacillin resulted in very major error rates of < 1% for the agar dilution and E test methods although the major error rates for the two methods with added NaCl were 8.5 and 6.9%, respectively. The disk diffusion test did not perform well in this study, showing essential error rates of > or = 18.3%. We recommended the addition of 2% NaC1 to Mueller-Hinton agar when testing staphylococci with oxacillin by either the agar dilution or E test method. NaC1 should not be added for the disk diffusion test.

A factor found in aged tungstate solution enhanced the antibacterial effect of beta-lactams on methicillin-resistant Staphylococcus aureus

We have found a factor (Factor T) in aged mixtures of tungstate and phosphate which greatly enhances the antibacterial effects of beta-lactams on both inducible and constitutive methicillin-resistant Staphylococcus aureus, but not on methicillin-susceptible S. aureus. Factor T alone did not strongly inhibit bacterial growth. There was no synergism of Factor T with other classes of antibiotics, nor with other groups of bacteria, and it reduced the efficacy of amino-glycosides and tetracycline. Upon preparation of Factor T, acidifying and heating the mixture of tungstate and phosphate resulted in a high yield and reproducibility, and no enhancing effect was observed when other anions such as sulfate or molybdate were used instead. Factor T is heat- and acid-stable but labile to alkalization, and is probably a complex of phosphate and tungstate.

Multiplex polymerase chain reaction for detection of genes for Staphylococcus aureus thermonuclease and methicillin resistance and correlation with oxacillin resistance

A multiplex polymerase chain reaction (mPCR) was used for simultaneous amplification of the staphylococcal nuc gene, encoding the thermostable nuclease (TNase), and the mecA gene, encoding the penicillin-binding protein 2a which is associated with staphylococcal methicillin resistance. A total of 219 staphylococcal strains were tested and the mPCR data were compared with coagulase production and in vitro oxacillin susceptibility. The agreement was 100% for coagulase production and nuc amplification, and 97.7%, 96.8 and 97.3% for mecA amplification and oxacillin resistance tested with MIC determination, disk diffusion and agar screen methods, respectively. Discrepant results were due to non-S. aureus isolates with borderline MICs of oxacillin (1-8 micrograms/ml). In a pilot test the mPCR simultaneously amplified both genes of staphylococci in blood cultures. This mPCR is a rapid and reliable method for single-step identification of cultures of MRSA and may prove to be useful for direct application on clinical specimens.

Concordant clonal delineation of methicillin-resistant Staphylococcus aureus by macrorestriction analysis and polymerase chain reaction genome fingerprinting

Pulsed-field gel electrophoresis of DNA macrorestriction fragments (macrorestriction analysis) allows epidemiologic typing and delineation of genetic relatedness of methicillin-resistant Staphylococcus aureus (MRSA) by indexing variations in the global chromosome architecture. Polymerase chain reaction (PCR)-mediated genome fingerprinting can also discriminate MRSA strains by detecting locally variable DNA motifs. To assess the correlation between these methods, 48 epidemic MRSA strains collected from 20 hospitals over a 10-year period were tested in a blind comparison by (i) macrorestriction analysis with SstII or SmaI endonuclease and (ii) PCR fingerprinting with four primer sets aimed at the mecA gene, enterobacterial repetitive intergenic consensus sequences, and arbitrary sequences. Isolates were discriminated into 22 macrorestriction patterns and 15 PCR fingerprints. MRSA strains belonging to 12 distinct clones by macrorestriction analysis showed 11 distinct PCR genotypes distinguished by multiple band differences. In contrast, 34 of 37 MRSA strains found to be clonally related by macrorestriction analysis clustered in two highly related PCR genotypes that differed by a single DNA fragment (P < 0.0001). These data demonstrate concordant clonal delineation of epidemic MRSA by macrorestriction analysis and PCR fingerprinting and thereby indicate that the rapid PCR assay may be an efficient epidemiologic typing system.

Strains of methicillin-resistant Staphylococcus aureus isolated in Australian hospitals from 1986 to 1990. Australian Group for Antimicrobial Resistance

Major teaching hospitals in each state of Australia participated in five annual surveys (1986 to 1990) of clinically significant isolates of Staphylococcus aureus. All isolates of methicillin-resistant S. aureus (MRSA) were phage typed with the Basic International Set and an Australian experimental set of typing phages. One or two predominant strains were isolated in individual states during each of the survey periods. Less than 3% (33 of 1243) of MRSA isolates were not typable and more than 86% (1070 of 1243) belonged to strains that were isolated on at least five occasions during a single survey period. Strains of phage types 83A/85/95/90/88@47T/90A/87M/13M and 85/90/88@47T/90A/87A were the most prevalent, but each was identified in only four of the five surveys. Isolates of phage type (83A/85/95) weak/88@87M persisted throughout the survey period.

Comparison of phage typing and DNA fingerprinting by polymerase chain reaction for discrimination of methicillin-resistant Staphylococcus aureus strains

A typing procedure for methicillin-resistant Staphylococcus aureus (MRSA) based on the polymerase chain reaction (PCR) amplification of both mecA sequences and variable DNA sequences as present in the prokaryotic genome has been developed. Two primers based on the sequences of DNA repeats as discovered in gram-negative members of the family Enterobacteriaceae allow detection of variable regions in the genome of a gram-positive bacterium such as S. aureus, as does a newly described arbitrary primer. This procedure, enabling the detection of 23 different genotypes in a collection of 48 MRSA isolates, was validated by comparisons with phage typing studies. It appeared that within the same group of isolates only 13 different phagovars could be identified. Combination of the results from both phage typing and genotyping allowed the discrimination of 34 of 48 isolates. However, depending on the primer-variable complexity of the PCR fingerprints, which could also be modulated by combination of PCR primers, clear homologies between the groups defined by either phage typing or fingerprinting were observed. An analysis of an MRSA outbreak in a geriatric institution showed a collection of genetically homogeneous isolates. In agreement with phage typing, PCR fingerprinting revealed the identical natures of the MRSA strains isolated from all patients.

Assessment of oxacillin salt agar for detection of MRSA identified by presence of the mecA gene

To assess the screening method for detection of methicillin-resistant Staphylococcus aureus recommended by Thornsberry and McDougal J. Clin Microbiol 1983; 18: 1084-1901, the growth of S. aureus and Staphylococcus epidermidis strains on Mueller Hinton agar containing 4% NaCl and 6 mg l-1 oxacillin, after 24 h incubation at 35 degrees C, was investigated. The strains used for this study were characterized for possession of the mecA gene by the polymerase chain reaction. All 39 strains of S. aureus with the mecA gene grew on this agar, and all 12 strains of S. aureus without the mecA gene did not grow. On the other hand, three of 12 strains of S. epidermidis did not grow on this agar, although all these strains possessed the mecA gene. These results suggest that Thornsberry and McDougal's screening method is suitable for detecting S. aureus strains with mecA gene. Introduction of this method, to detect methicillin-resistant S. aureus more precisely, is simple and cheap for any laboratory in any part of the world.

Polymerase chain reaction for diagnosis of infectious diseases

The polymerase chain reaction (PCR) has been utilized and demonstrated to be useful for detecting minute amounts of a wide variety of infectious agents. In such studies, one must keep in mind that the most appropriate conditions for amplification vary with organisms of interest. In this study, PCR was used as a rapid and sensitive method for detecting infectious agents for which three assay systems were devised comprising the method for the amplification of human T cell leukemia virus type I, Mycobacterium tuberculosis and Mycoplasma pneumoniae. The possibility of vertical transmission of human T cell leukemia virus type I through cord blood was demonstrated using cord blood mononuclear cells from carrier mothers of this virus. In the study of Mycobacterium tuberculosis, PCR was shown to be efficient, particularly in detecting this organism in extrapulmonary cases. Evidence of direct invasion into the central nervous system by Mycoplasma pneumoniae and the concomitant occurrence of mycoplasmaemia in the mycoplasmal central nervous system involvement was obtained using PCR. These results validated the potential of PCR in the clinical research of infectious diseases.

Detecting methicillin-resistant Staphylococcus epidermidis--disc diffusion, broth breakpoint or polymerase chain reaction?

Growth conditions are important for the expression of resistance to methicillin among staphylococci. Consequently a phenotypic susceptibility test has to be chosen carefully to avoid false susceptible results. In this study we wanted to devise rapid and simple phenotypic tests whose results completely correlate with the presence of the methicillin resistance gene, mecA. A simplified polymerase chain reaction (PCR) method not needing separate DNA extraction from the tested bacteria was used to amplify a 449 bp region of the mecA gene. One hundred and ten strains of S. epidermidis were tested. The results were in complete agreement with those from a broth tube breakpoint test, known to identify more strains as resistant than does the method recommended by NCCLS. In disc diffusion test it was possible to clearly distinguish resistant from susceptible strains by using discs containing oxacillin, cephalexin and cephradine. A 5 micrograms cephradine disc was further analysed by testing another 441 consecutive clinical isolates of staphylococci. All resistant coagulase-negative staphylococci grew out to the edge of this disc, whereas susceptible strains showed an inhibition zone at least 10 mm in diameter. The 5 micrograms cephradine disc is recommended for routine work. The PCR method and broth tube breakpoint test are both reliable reference methods.