Nonradioactive DNA probe for detection of gene for methicillin resistance in Staphylococcus aureus

Phenotypic and genetic characterization of multidrug-resistant Staphylococcus aureus in the tropics of Southeast Asia

Antibiotic resistance has become a major public health problem throughout the world. The presence of antibiotic-resistant bacteria such as Staphylococcus aureus and antibiotic resistance genes (ARGs) in hospital wastewater is a cause for great concern today. In this study, 276 Staph. aureus isolates were recovered from hospital wastewater samples in Malaysia. All of the isolates were screened for susceptibility to nine different classes of antibiotics: ampicillin, ciprofloxacin, gentamicin, kanamycin, erythromycin, vancomycin, trimethoprim and sulfamethoxazole, chloramphenicol, tetracycline and nalidixic acid. Screening tests showed that 100 % of Staph.aureus isolates exhibited resistance against kanamycin, vancomycin, trimethoprim and sulfamethoxazole and nalidixic acid. Additionally, 91, 87, 50, 43, 11 and 8.7 % of isolates showed resistance against erythromycin, gentamicin, ciprofloxacin, ampicillin, chloramphenicol and tetracycline, respectively. Based on these results, 100 % of isolates demonstrated multidrug-resistant (MDR) characteristics, displaying resistance against more than three classes of antibiotics. Of 276 isolates, nine exhibited resistance to more than nine classes of tested antibiotics; these were selected for antibiotic susceptibility testing and examined for the presence of conserved ARGs. Interestingly, a high percentage of the selected MDR Staph.aureus isolates did not contain conserved ARGs. These results indicate that non-conserved MDR gene elements may have already spread into the environment in the tropics of Southeast Asia, and unique resistance mechanisms against several antibiotics may have evolved due to stable, moderate temperatures that support growth of bacteria throughout the year.

Molecular Detection of Methicillin-Resistant Staphylococcus aureus by Non-Protein Coding RNA-Mediated Monoplex Polymerase Chain Reaction

Non-protein coding RNA (npcRNA) is a functional RNA molecule that is not translated into a protein. Bacterial npcRNAs are structurally diversified molecules, typically 50-200 nucleotides in length. They play a crucial physiological role in cellular networking, including stress responses, replication and bacterial virulence. In this study, by using an identified npcRNA gene (Sau-02) in Methicillin-resistant Staphylococcus aureus (MRSA), we identified the Gram-positive bacteria S. aureus. A Sau-02-mediated monoplex Polymerase Chain Reaction (PCR) assay was designed that displayed high sensitivity and specificity. Fourteen different bacteria and 18 S. aureus strains were tested, and the results showed that the Sau-02 gene is specific to S. aureus. The detection limit was tested against genomic DNA from MRSA and was found to be ~10 genome copies. Further, the detection was extended to whole-cell MRSA detection, and we reached the detection limit with two bacteria. The monoplex PCR assay demonstrated in this study is a novel detection method that can replicate other npcRNA-mediated detection assays.

Absence of the mecA Gene in Methicillin Resistant Staphylococcus aureus Isolated from Different Clinical Specimens in Shendi City, Sudan

Absolute dependence on mecA gene as the defining standard in determining the resistance of S. aureus to methicillin became the subject of distrust by many researchers. The present study aimed to determine the frequency of mecA gene in methicillin resistant S. aureus (MRSA) isolates using polymerase chain reaction and to correlate its presence to conventional method. In this regard, two hundred S. aureus isolates were collected from patients with different diseases attending different hospitals in Shandi City, Sudan. Phenotypic Kirby-Bauer method confirmed the existence of methicillin resistant S. aureus in 61.5% of the subjected isolates with MICs ranging from 4 μg/mL to 256 μg/mL when using E-test. However, when amplifying a 310 bp fragment of the mecA gene by PCR, twelve out of the 123 MRSA isolates (9.8%) were mecA negative, whereas all the 77 methicillin sensitive S. aureus (MSSA) were mecA negative. In conclusion, this study drew attention to the credibility of the mecA gene and its usefulness in the detection of all MRSA strains without referring to the traditional methods. Hence, it is highly recommended to consider alternative mechanisms for β-lactam resistance that may compete with mecA gene in the emergence of MRSA phenomenon in the community.

Guidelines for the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA)

These evidence-based guidelines have been produced after a literature review of the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA). We have considered the detection of MRSA in screening samples and the detection of reduced susceptibility to glycopeptides in S. aureus. Recommendations are given for the identification of S. aureus and for suitable methods of susceptibility testing and screening for MRSA and for S. aureus with reduced susceptibility to glycopeptides. These guidelines indicate what tests should be used but not when the tests are applicable, as aspects of this are dealt with in guidelines on control of MRSA. There are currently several developments in screening media and molecular methods. It is likely that some of our recommendations will require modification as the new methods become available.

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

Antibiotic susceptibility and genotypic characterization of methicillin-resistant Staphylococcus aureus strains in eastern France

We studied changes over four years in the characteristics of methicillin-resistant Staphylococcus aureus (MRSA) strains isolated in a French university hospital by analysis of antibiotic susceptibility and restriction fragment length polymorphism using pulsed field gel electrophoresis. Changes were considered in relation to regional and national frameworks. The proportion of gentamicin-susceptible methicillin-resistant S. aureus (GS-MRSA) among MRSA increased from 22.6% in 1994 to 86.8% in 1998. We first isolated strains of GS-MRSA susceptible to tobramycin and amikacin (TKS-MRSA) in 1996 from patients in eight wards. The number of TKS-MRSA strains per 100 MRSA was 6.1 in 1997, 14.5 in 1998, and 18.9 in 1999. Pattern A, the major DNA pattern identified, accounted for 78.6% of isolates in 1998. This pattern was found in TKS-MRSA and TKR-MRSA strains, but not in Gentamicin-resistant MRSA strains (GR-MRSA). Fitness analysis showed that GR-MRSA strains had much lower replication rates than GS-MRSA strains, but there was no difference between TKS-MRSA and TKR-MRSA strains. Aminoglycoside consumption remained constant between 1994 and 1998. The spread of TKS-MRSA in our hospital since 1996 may relate to the "total use threshold", as the level of tobramycin/amikacin use is below that required for selection of TKR-MRSA.

Detection of methicillin resistance in coagulase-negative staphylococci initially reported as methicillin susceptible using automated methods

Reliable detection of methicillin resistance in coagulase-negative staphylococci (CNS) is required for appropriate therapy of serious infections from these pathogens. To determine the most accurate method of measuring methicillin resistance in CNS initially reported as methicillin susceptible by automated methods, we compared mecA detection by polymerase chain reaction (PCR) with phenotypic methods. One hundred eighty-eight blood culture isolates of CNS that were initially reported as susceptible to methicillin using commercial methods (Vitek or MicroScan) were tested by agar dilution, disk diffusion, oxacillin salt agar screen plate, and a multiplex PCR assay using primer sets for mecA and 16S rRNA. Sixteen isolates (8.5%) previously reported as methicillin susceptible by automated methods contained the mecA gene. MICs of these isolates ranged from 0.5 microgram/mL to > or = 128 micrograms/mL. Ten of these isolates had MICs equal to or below the NCCLS breakpoint of 2 micrograms/mL. Six of the 10 isolates (4 with MICs of 0.5 microgram/mL and 2 with MICs of 2 micrograms/mL) did not grow on any of the oxacillin screen plates after 48 h of incubation at 30 degrees C or 35 degrees C. All six isolates were induced to grow in the presence of oxacillin at 128 micrograms/mL by serial passaging on plates containing increasing concentrations of antibiotic. Retesting with MicroScan and Vitek detected methicillin resistance in 7 and 10 isolates, respectively. Disk diffusion testing with incubation for 48 h proved to be the next best method after PCR for detection of methicillin resistance (15 of 16 isolates). Commercial automated methods and some methods recommended by National Committee for Clinical Laboratory Standards may not detect methicillin resistance in CNS that carry the mecA gene and have MICs just below breakpoint.

Comparison of disk diffusion, the E test, and detection of mecA for determination of methicillin resistance in coagulase-negative staphylococci

The aim of this study was to find a reliable, fast, and simple alternative to the methicillin disk method for determination of methicillin resistance in coagulase-negative staphylococci, since results of this method are often difficult to read due to growth within the zone of inhibition. The sensitivity of 319 strains of coagulase-negative staphylococci to a 5 microgram methicillin disk on Mueller-Hinton agar using an incubation period of 48 h was compared with that of 1 microgram and 5 micrograms oxacillin disks on Mueller-Hinton agar with or without 2% NaCl, using an incubation period of 24 h. The detection of mecA (MecAgen) by the polymerase chain reaction was used as a standard. Minimum inhibitory concentrations were determined by means of the E test. Of the 225 mecA-positive strains, 190, 215, and 193 were resistant to 5 micrograms methicillin, 1 microgram oxacillin and 5 micrograms oxacillin disks on Mueller-Hinton agar, respectively, and 216, 218, and 223 were resistant on Mueller-Hinton agar with 2% NaCl. Of the 94 mecA-negative strains, 89, 93, and 94 were susceptible to 5 micrograms methicillin, 1 microgram oxacillin, and 5 micrograms oxacillin disks on Mueller-Hinton agar, respectively, and 92, 93, and 94 were susceptible on Mueller-Hinton agar with 2% NaCl. Using breakpoints of 2 micrograms/ml for oxacillin resistance and 8 micrograms/ml for methicillin resistance, the E test yielded sensitivities of 99.6 and 99.1% and specificities of 97.9 and 98.9% after 48 h of incubation. The 5 microgram oxacillin disk was faster and easier to read than the methicillin disk and correlated better with detection of mecA than the methicillin disk of the 1 microgram oxacillin disk.

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'.

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.

Detection of the Staphylococcal mecA gene by chemiluminescent DNA hybridization

A new solution-phase DNA hybridization capture assay for the rapid detection of the mecA gene in clinical isolates of Staphylococcus was compared with multiplex PCR and disk diffusion methods. The assay uses a DNA capture probe immobilized on paramagnetic particles and a second DNA probe labeled with an acridinium ester. Bacteria from 24-h cultures are lysed, and the lysates are hybridized with the DNA probes. After magnetic separation to remove unhybridized labeled probe, the mecA gene is detected by the chemiluminescence of the hybridized probe. Four hundred consecutive staphylococcal isolates were assayed, including 147 S. aureus and 253 coagulase-negative Staphylococcus isolates. Among the S. aureus isolates, 14 of 147 were MecA+ by both the hybridization capture assay and PCR; 133 of 147 were MecA negative by both assays (positive and negative predictive values, 100%). Comparison of disk diffusion results with those obtained by genotypic methods indicated that 14 of 147 S. aureus isolates judged to be resistant were positive by both methods; 119 of 147 were Oxs and negative by both genotypic methods (positive and negative predictive values, 50 and 100%, respectively). The remaining 14 S. aureus isolates were MecA- Oxr; among these, 13 were beta-lactamase hyperproducers. For coagulase-negative staphylococci, 130 of 253 were MecA+ by the hybridization capture assay; 129 of 130 of these isolates were positive by PCR (positive and negative predictive values, 99.2 and 100%, respectively). Comparison with the disk diffusion assay showed that 128 of the coagulase-negative MecA+ isolates were Oxr; 111 of 253 were MecA- and Oxs (positive and negative predictive values, 90.8 and 99.1%, respectively). Thirteen coagulase-negative isolates were MecA-Oxr; among these, three were beta-lactamase hyperproducers. Comparison of the hybridization capture assay results with PCR indicates that the DNA hybridization assay is a sensitive and specific test for the detection of the mecA gene in clinical isolates of Staphylococcus.

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.

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.

Typing of Staphylococcus aureus by amplification of the 16S-23S rRNA intergenic spacer sequences

The possibility of using polymorphisms in the spacer regions between 16S and 23S rRNA genes in order to type Staphylococcus aureus has been evaluated. To this purpose, DNA extracted from 74 independent isolates was amplified making use of a pair of primers complementary to conserved regions in the 16S and 23S genes. We have demonstrated that the method provides a good discrimination between unrelated isolates, giving better results when methicillin-sensitive strains are considered. Moreover, the amplification profiles were reproducible and all strains were typable. Given these results, and the technical simplicity of the process, we propose PCR-ribotyping to be taken into consideration as a method for typing S. aureus.

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.

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.

Use of chemiluminescent DNA probes in the rapid detection of oxacillin resistance in clinically isolated strains of Staphylococcus aureus

Chemiluminescent DNA probes (AccuProbe, species specific; and FlashTrack, bacterial generic) were used to determine oxacillin resistance in Staphylococcus aureus. Ribosomal RNA was measured at designated intervals in the presence and absence of antibiotic. A total of 48 (AccuProbe assay) and 24 (FlashTrack) S. aureus isolates with known oxacillin susceptibility patterns were inoculated into Bactec 6A bottles both with and without 4 micrograms/ml oxacillin and incubated at 35 degrees C for 4 h. Aliquots were removed at 0 and 4 h, and pellets of bacteria were obtained via selective centrifugation. Probe assay counts (relative light units, RLUs) were performed. Of 21 oxacillin-resistant S. aureus (ORSA) strains, 20 showed a > 5-fold RLU increase during the incubation period (Accu-Probe assay): 25 of 27 oxacillin-susceptible strains demonstrated a < or = 4-fold increase. AccuProbe test sensitivity and specificity were 95% and 92%, respectively. With the generic FlashTrack probe assay, all nine ORSA isolates showed a > or = 4- to 10-fold increase in RLUs, and all 15 oxacillin-susceptible strains showed a < or = 4-fold increase in RLUs during the 4-h incubation. The FlashTrack test sensitivity and specificity were both 100%. Probe assays were completed within 5 h. This study suggests that rapid and reliable determination of oxacillin resistance in S. aureus clinical isolates can be accomplished using commercially available DNA probes.

Rapid detection of the mecA gene in methicillin-resistant staphylococci by enzymatic detection of polymerase chain reaction products

In order to identify methicillin-resistant staphylococci from clinical sources with ease and reliability, enzymatic detection of polymerase chain reaction (ED-PCR) was applied. ED-PCR is based on the capture of amplified products via biotin-streptavidin affinity and the detection of an incorporated hapten in amplified products with an enzyme-linked antibody. In order to identify methicillin-resistant staphylococci of all species, a 150-bp fragment of the mecA gene was targeted for ED-PCR. After PCR was performed with a pair of biotin and dinitrophenol 5'-labeled primers, the reaction mixture was applied to a microtiter well precoated with streptavidin. Thereafter, bound PCR products were detected colorimetrically with alkaline phosphatase-conjugated anti-dinitrophenol antibody. The extraction of DNA from staphylococcal cells for PCR was simplified so that it could be performed within one tube. The total assay, including PCR, took less than 3 h. The sensitivity of mecA gene detection ranged from greater than 5 x 10(2) CFU per tube for Staphylococcus aureus to greater than 5 x 10(3) CFU per tube for Staphylococcus epidermidis. Genotyping results obtained by ED-PCR of 161 tested strains from the colonies (97 strains of S. aureus and 64 strains of coagulase-negative staphylococci) were compared with the phenotypic susceptibilities of the strains to oxacillin. The results of ED-PCR showed excellent agreement with the MICs of oxacillin with very few exceptions; only one strain of S. aureus and two strains of coagulase-negative staphylococci were found to possess the mecA gene, which was discrepant with their phenotypes. Fifty-five blood culture samples were also tested by ED-PCR. For staphylococcal isolates in 33 of the cultures, oxacillin MICs were >4 microgram/ml; 31 of the 33 staphylococcal isolates were determined by ED-PCR to be mecA gene positive. These results suggest that ED-PCR can be used with reasonable confidence in the clinical microbiological laboratory.

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.

Genotypic identification of methicillin-resistant coagulase-negative staphylococci by polymerase chain reaction

A rapid method for the detection of methicillin resistance in staphylococci was developed. The method was based on the polymerase chain reaction (PCR) and primers that targeted the internal region of the coding frame of the mec gene. The amplification reaction was carried out with crude cell lysates as a source of target DNA and provided data in less than 5 h. Seventy-four isolates of coagulase-negative staphylococci were tested by PCR, DNA hybridization with a probe derived from the mec gene, and an agar dilution susceptibility assay. PCR results showed a 100% correlation with the susceptibility assay carried out with high inocula (10(8) CFU) and incubation at 32 degrees C for 48 h. PCR was more sensitive and specific than DNA hybridization in detecting methicillin resistance in coagulase-negative staphylococci. The former technique identified the mec gene in all the strains which were phenotypically resistant but which did not hybridize with the probe. Identification of methicillin-resistant strains by PCR offers a very specific, sensitive, and rapid alternative to traditional susceptibility tests and DNA hybridization as a guide for the treatment of infections caused by staphylococci.