Detection of borderline oxacillin-resistant Staphylococcus aureus and differentiation from methicillin-resistant strains

Prevalence and Potential Risk Factors for the Acquisition of Antibiotic-Resistant Staphylococcus spp. Bacteria Among Pastoralist Farmers in Kajiado Central Subcounty, Kenya

Antimicrobial resistance (AMR) is a growing health problem globally. To address this challenge, there is a need to generate baseline data on the prevalence and AMR profile of the main disease-causing bacteria. Here, we interrogated the prevalence of bacteria in the nasal cavity of healthy pastoralists in Kajiado Central Subcounty, Kenya, and the occurrence of AMR in Staphylococcus isolates among the study subjects. Nasal swabs from 176 pastoralists were cultured, and the bacteria isolates identified using standard phenotypic and biochemical bacteriological methods. Among the obtained 195 isolates, the most prevalent isolates were coagulase-negative Staphylococcus (CoNS) (44.9%), followed by Enterococci spp. (43.2%) while Staphylococcus aureus prevalence was 8%. Antimicrobial sensitivity of the Staphylococcus spp. isolates to 14 antibiotics representing six antibiotic groups was undertaken using the Kirby-Bauer disk diffusion method. Among the CoNS, the highest resistance was reported in amoxicillin (78.7%) and ceftazidime (76%), while the most resistance for S. aureus was reported in ceftazidime (100%), amoxicillin (71.4%), and streptomycin (71.4%). From an administered questionnaire looking at gender, animal contact frequency, history of hospital visitation and antibiotic usage, and habitual intake of raw milk, the study showed that male participants had a higher risk of carrying multiple drug resistant (MDR) bacteria than females (p = 0.02, OR = 1.3). Likewise, habitual intake of raw milk was significantly associated MDR acquisition (p = 0.02, OR = 1.82). This study reveals a high prevalence of AMR Staphylococcus isolates in the study area laying a foundation for further analysis of molecular characterization of the observed resistance as well as the development of interventions that can reduce the occurrence of AMR in the study area.

Truncation of GdpP mediates β-lactam resistance in clinical isolates of Staphylococcus aureus

OBJECTIVES

High-level β-lactam resistance in MRSA is mediated in the majority of strains by a mecA or mecC gene. In this study, we identified 10 mec gene-negative MRSA human isolates from Austria and 11 bovine isolates from the UK showing high levels of β-lactam resistance and sought to understand the molecular basis of the resistance observed.

METHODS

Different antimicrobial resistance testing methods (disc diffusion, Etest and VITEK® 2) were used to establish the β-lactam resistance profiles for the isolates and the isolates were further investigated by WGS.

RESULTS

A number of mutations (including novel ones) in PBPs, AcrB, YjbH and the pbp4 promoter were identified in the resistant isolates, but not in closely related susceptible isolates. Importantly, a truncation in the cyclic diadenosine monophosphate phosphodiesterase enzyme, GdpP, was identified in 7 of the 10 Austrian isolates and 10 of the 11 UK isolates. Complementation of four representative isolates with an intact copy of the gdpP gene restored susceptibility to penicillins and abolished the growth defects caused by the truncation.

CONCLUSIONS

This study reports naturally occurring inactivation of GdpP protein in Staphylococcus aureus of both human origin and animal origin, and demonstrates clinical relevance to a previously reported association between this truncation and increased β-lactam resistance and impaired bacterial growth in laboratory-generated mutants. It also highlights possible limitations of genomic determination of antibiotic susceptibility based on single gene presence or absence when choosing the appropriate antimicrobial treatment for patients.

Rationale for eliminating Staphylococcus breakpoints for β-lactam agents other than penicillin, oxacillin or cefoxitin, and ceftaroline

Due to the ongoing concern about the reliability of Staphylococcus breakpoints (interpretive criteria) for other β-lactam agents, the Clinical and Laboratory Standards Institute recently approved the elimination of all breakpoints for antistaphylococcal β-lactams except for penicillin, oxacillin or cefoxitin, and ceftaroline. Routine testing of penicillin and oxacillin or cefoxitin should be used to infer susceptibility for all β-lactams with approved clinical indications for staphylococcal infections. It is critical for laboratories to reject requests for susceptibility testing of other β-lactams against staphylococci and to indicate that susceptibility to these agents can be predicted from the penicillin and oxacillin or cefoxitin results. This article reviews β-lactam resistance mechanisms in staphylococci, current antimicrobial susceptibility testing and reporting recommendations for β-lactams and staphylococci, and microbiologic data and clinical data supporting the elimination of staphylococcal breakpoints for other β-lactam agents.

Molecular docking and molecular dynamics studies on β-lactamases and penicillin binding proteins

Bacterial resistance to β-lactam antibiotics poses a serious threat to human health. Penicillin binding proteins (PBPs) and β-lactamases are involved in both antibacterial activity and mediation of β-lactam antibiotic resistance. The two major reasons for resistance to β-lactams include: (i) pathogenic bacteria expressing drug insensitive PBPs rendering β-lactam antibiotics ineffective and (ii) production of β-lactamases along with alteration of their specificities. Thus, there is an urgent need to develop newer β-lactams to overcome the challenge of bacterial resistance. Therefore the present study aims to identify the binding affinity of β-lactam antibiotics with different types of PBPs and β-lactamases. In this study, cephalosporins and carbapenems are docked into PBP2a of Staphylococcus aureus, PBP2b and PBP2x of Streptococcus pneumoniae and SHV-1 β-lactamase of Escherichia coli. The results reveal that Ceftobiprole can efficiently bind to PBP2a, PBP2b and PBP2x and not strongly to SHV-1 β-lactamase. Furthermore, molecular dynamics (MD) simulations are performed to refine the binding mode of the docked complex structure and to observe the differences in the stability of free PBP2x and Ceftobiprole bound PBP2x. MD simulation supports the greater stability of the Ceftobiprole-PBP2x complex compared to free PBP2x. This work demonstrates that potential β-lactam antibiotics can efficiently bind to different types of PBPs for circumventing β-lactam resistance and opens avenues for the development of newer antibiotics that can target bacterial pathogens.

Usefulness of cefovecin disk-diffusion test for predicting mecA gene-containing strains of Staphylococcus pseudintermedius and clinical efficacy of cefovecin in dogs with superficial pyoderma

BACKGROUND

Cefovecin has been widely used to treat skin infections in dogs. The relationship of the cefovecin disk-diffusion test results to the presence of the mecA gene and the clinical efficacy of cefovecin have not been fully evaluated.

HYPOTHESIS/OBJECTIVES

To determine the usefulness of an in vitro cefovecin disk-diffusion test in predicting the presence of the mecA gene in Staphylococcus pseudintermedius, as well as the in vivo efficacy of cefovecin therapy in dogs with superficial pyoderma.

METHODS

Twenty-six S. pseudintermedius strains isolated from 22 dogs with pyoderma were used. In vitro disk-diffusion test results of cefovecin were compared with agar-dilution test results, the presence of the mecA gene, and the improvement in clinical scores of dogs with superficial pyoderma at 14 days post treatment.

RESULTS

There was a significant linear correlation (r = -0.83) between the diameter of the obvious zone of inhibition by disk diffusion and the minimal inhibitory concentration for cefovecin (P < 0.0001). Receiver operating characteristic analysis revealed that zone diameters between 25 and 27 mm exhibited better sensitivity (92.9%) and specificity (100.0%) for detection of strains carrying the mecA gene. The mean improvement in clinical scores in dogs carrying cefovecin-resistant strains was significantly lower than in dogs carrying cefovecin-susceptible strains (P < 0.01).

CONCLUSIONS AND CLINICAL IMPORTANCE

The cefovecin disk-diffusion test with a cut-off value estimated in this study was valuable for predicting mecA gene carriage in S. pseudintermedius, as well as the in vivo efficacy of cefovecin therapy in dogs with superficial pyoderma caused by S. pseudintermedius.

Direct mecA polymerase chain reaction testing of blood culture bottles growing Gram-positive cocci and the clinical potential in optimizing antibiotic therapy for staphylococcal bacteremia

This study evaluated the performance of direct mecA polymerase chain reaction (PCR) from blood culture bottles growing Gram-positive cocci in clusters and its role in optimization of antibiotic therapy. A total of 266 blood cultures including 121 methicillin-resistant and 122 methicillin-susceptible Staphylococci were tested for mecA. Compared to phenotypic testing, the overall performance of direct mecA PCR was 99% for sensitivity, specificity, positive predictive value, and negative predictive value, respectively. Assessment of antibiotic therapy upon microbiology reporting of direct mecA PCR results from 38 patients prior to (phase I) and 48 patients after implementation of testing and reporting (phase II) showed that the mean time to antibiotic optimization in phase II (0.9 ± 0.9 day) was significantly shorter than that in phase I (2.2 ± 3.2 days) (P < 0.05). Methicillin-susceptible staphylococcal bacteremias had significantly higher frequency of antibiotic adjustment upon direct mecA reporting, compared to methicillin-resistant staphylococcal bacteremias. Our study indicated that direct mecA PCR improved timely antibiotic optimization.

Staphylococcus aureus methicillin-resistance mechanisms

Background/Aim. In many hospitals in the world and in our country, the spread of methicillin-resistant Staphylococcus aureus (MRSA) is so wide that nowdays vancomycin is recommended for empiric treatment of staphylococcal life threatening infections (sepsis, pneumonia) instead of beta-lactam antibiotics. The aim of this study was to determine the production of beta-lactamases in hospital and community isolates of staphyloococus aureus, i. e. hospital associated MRSA (HA-MRSA) and community associated MRSA (CA-MRSA), the presence of homogeneous and heterogeneous type of methicillin resistance, and border-line resistance in Staphylococcus aureus (BORSA). The aim of this study was also to determine if there was a statistically significant difference between mechanisms of resistance in HA-MRSA and CA-MRSA. Methods. A total 216 clinical Staphylococcus aureus isolates from the General Hospital in the town of Cuprija and 186 ambulance Staphylococcus aureus isolates from the community were examined for the presence of methicillin-resistance using disk-diffusion test with penicillin disk (10 ij), oxacillin disk (1 ?g) and cefoxitin disk (30 ?g). Betalactamases production was detected by nitrocefin disk and betalactamase tablets. Determination of oxacillin minimum inhibitory concentracion (MIC) was done by agar-dilution method. Results. The prevalence of HA-MRSA was 57.4%, and CA-MRSA was 17.7% (p < 0.05). There was a higher rate of heterogeneous type of resistance among clinical MRSA isolates (11.1%) compared with ambulance ones (3.8%) (p < 0.05). The rates of beta-lactamases production were similar among hospital associated isolates (97.5%), as well as in the community associated isolates (95.5%) (p > 0.05). There were 4.6 % of BORSA hospital isolates and 3.3 % of BORSA ambulance isolates (p > 0.05). Conclusion. The frequency of MRSA isolates in hospital was significantly higher than in community, as well as the heterogeneous type of resistance. The frequency of BORSA isolates and production of betalactamases were higher among hospital Staphylococcus aureus isolates, but the difference is not significant.

Rapid detection of methicillin-resistant staphylococci by real-time PCR directly from positive blood culture bottles

For the rapid detection of methicillin-resistant staphylococci directly from blood cultures containing gram-positive cocci in clusters, we implemented a real-time (LightCycler) polymerase chain reaction (PCR) specific for the Staphylococcus aureus nuc gene encoding nuclease and the mecA gene encoding methicillin resistance. For the 475 positive blood cultures tested, the assay turned out to have 100% sensitivity and 100% specificity for the identification of methicillin-susceptible (n = 108) and methicillin-resistant (n = 34) S. aureus. When coagulase-negative staphylococci (CoNS) were included, the overall sensitivity for the detection of methicillin resistance was 93% and the specificity was 99%. Real-time PCR for nuc and mecA from blood culture bottles with staphylococci yields therefore a rapid (2-3 h) identification of S. aureus and CoNS including methicillin resistance.

Antibacterial susceptibility testing in the clinical laboratory

This article familiarizes the clinician with the principles of bacterial susceptibility testing and reporting to facilitate communication with the clinical microbiology laboratory. The emergence of resistance in nearly all commonly isolated bacterial organisms has highlighted the need for ongoing dialogue between the laboratory and those who use its services.

Identification of methicillin-resistant isolates of Staphylococcus aureus and coagulase-negative staphylococci responsible for bloodstream infections with the Phoenix™ system

We evaluated the reliability of the new Phoenix system (Becton Dickinson Microbiology Systems, Sparks, Md.) in species-level identification and detection of oxacillin (methicillin) resistance among 493 staphylococcal isolates (Staphylococcus aureus, n = 223; coagulase-negative staphylococci, CoNS, n = 270) recovered from patients with bacteremia. Identification results were concordant with those of the ID 32 STAPH system (bioMérieux, Marcy l'Etoile, France) for 100% of S. aureus (223/223) and 97.4% (263/270) of CoNS isolates. For S. aureus isolates, Phoenix oxacillin-susceptibility results fully concurred with those of mecA polymerase chain reaction (PCR) (reference method): 96 mecA-positive isolates identified as resistant, 127 mecA-negative strains as susceptible. Two of the 210 mecA-positive CoNS isolates were misclassified as susceptible by the Phoenix (sensitivity 99%, positive predictive value 97.6%). Five of 60 mecA-negative CoNS isolates were classified as resistant by the Phoenix (specificity 91.7%; negative predictive value 96.5%). The Phoenix system can provide accurate and reliable identification of methicillin-resistant staphylococci responsible for bloodstream infections.

Community-acquired methicillin-resistant Staphylococcus aureus colonization in healthy children attending an outpatient pediatric clinic

BACKGROUND

We previously showed that children attending an inner city pediatric emergency department were sometimes asymptomatically colonized with clindamycin-susceptible community-acquired methicillin-resistant Staphylococcus aureus (MRSA) and borderline methicillin-resistant S. aureus (BRSA) as well. We wished to ascertain whether healthy children attending an outpatient clinic were colonized with these organisms. Therefore to estimate the prevalence of community-acquired MRSA and BRSA nasal colonization in a well child population, we cultured children attending an inner city pediatric outpatient clinic.

STUDY DESIGN

This was a prospective cross-sectional study conducted from January to August, 1999, at a primary care outpatient facility at the University of Chicago. The target population was 500 healthy children < or = 16 years of age who attended this facility to receive well child care.

RESULTS

One hundred twenty-two (24.4%) children were colonized with S. aureus. Three of the 122 (2.5%) S. aureus isolates were MRSA; they came from children who lacked predisposing risk factors and were susceptible to clindamycin, gentamicin, trimethoprim-sulfamethoxazole, rifampin and ciprofloxacin. Two (1.6%) additional S. aureus isolates were BRSA; both children had predisposing risk factors for MRSA colonization. The mecA gene was present in the 3 MRSA isolates and absent in both BRSA isolates.

CONCLUSIONS

These data document that a reservoir of asymptomatic MRSA colonization exists among healthy children who lack traditional risk factors for MRSA infections.

Comparison of the Vitek Gram-Positive Susceptibility 106 card and the MRSA-screen latex agglutination test for determining oxacillin resistance in clinical bloodstream isolates of Staphylococcus aureus

The Vitek automated susceptibility testing system with a modified Gram-Positive Susceptibility (GPS) 106 Card (bioMerieux Vitek, Inc., Hazelwood, Mo.) and a rapid slide latex agglutination test (MRSA-Screen; Denka Seiken Co., Ltd., Tokyo, Japan) were evaluated for their ability to detect oxacillin resistance in Staphylococcus aureus. The oxacillin-salt agar screen (OS) test, the reference broth microdilution method, and the detection of the mecA gene by PCR were compared with the commercial products. A total of 200 contemporary (1999) bloodstream infection isolates were collected from the SENTRY Antimicrobial Surveillance Program, representing diverse geographic areas throughout the world. Among the 99 mecA-positive isolates, 3 isolates were found negative by the MRSA-Screen. Another two isolates did not grow on OS plates and had MICs of 0.5 and 2 microg/ml with the Vitek GPS card. All 101 mecA-negative isolates were also found negative by the MRSA-Screen and were categorized as susceptible by the GPS card. Overall, the MRSA-Screen, GPS card, and OS test had sensitivities of 96.9, 98.0, and 98.0% and specificities of 100.0, 100.0, and 98.0%, respectively. MRSA-Screen was a rapid (</=15 min) and simple test to perform, and the GPS card provided results in <8 h. Both methods were sensitive and specific for detecting staphylococcal oxacillin resistance in the clinical microbiology laboratory.

Evaluation of three rapid methods for detection of methicillin resistance in Staphylococcus aureus

The probe-based Velogene Rapid MRSA Identification Assay (ID Biomedical Corp., Vancouver, British Columbia, Canada) and the latex agglutination MRSA-Screen (Denka Seiken Co., Tokyo, Japan) were evaluated for their ability to identify methicillin-resistant Staphylococcus aureus (MRSA) and to distinguish strains of MRSA from borderline oxacillin-resistant S. aureus (BORSA; mecA-negative, oxacillin MICs of 2 to 8 microgram/ml). The Velogene is a 90-min assay using a chimeric probe to detect the mecA gene. MRSA-Screen is a 15-min latex agglutination test with penicillin-binding protein 2a antibody-sensitized latex particles. We compared these assays with the BBL Crystal MRSA ID System (Becton Dickinson, Cockeysville, Md.) and with PCR for mecA gene detection. A total of 397 clinical isolates of S. aureus were tested, consisting of 164 methicillin-susceptible strains, 197 MRSA strains, and 37 BORSA strains. All assays performed well for the identification of MRSA with sensitivities and specificities for Velogene, MRSA-Screen, and BBL Crystal MRSA ID of 98.5 and 100%, 98.5 and 100%, and 98.5 and 98%, respectively. Three MRSA strains were not correctly identified by each of the Velogene and MRSA-Screen assays, but repeat testing with a larger inoculum resolved the discrepancies. The BBL Crystal MRSA ID test misclassified four BORSA strains as MRSA. Both the Velogene and the MRSA-Screen assays are easy to perform, can accurately differentiate BORSA isolates from MRSA isolates, and provide a rapid alternative for the detection of methicillin resistance in S. aureus in clinical laboratories, especially when mecA PCR gene detection is unavailable.

Rapid dissemination of Staphylococcus aureus with classic oxacillin resistance phenotype at a new university hospital

To determine the prevalence rates of oxacillin-resistant Staphylococcus aureus (ORSA) at a new university hospital since its opening, the results of disk diffusion tests on all clinical isolates, recovered between 1990 and 1998 at the National Cheng Kung University Hospital, were reviewed. In order, to investigate the mechanisms of oxacillin resistance among strains of S. aureus in Taiwan, MICs were determined by an agar dilution method, and polymerase chain reaction and colony hybridization assays were performed on 288 isolates collected during November 1998 to detect the mecA gene. The prevalence rates of ORSA increased rapidly from 14.1% in 1990 to 61.0% in 1998. The increasing rates were most rapid in the first four-year period, ranging from 11.6 to 106.7% per year, and became steady after 1994, ranging from 1.8% to 11.6%. Of 288 clinical isolates collected in November 1998, 206 (71.5%) were resistant to oxacillin (MIC >/= 16 mg/L), and four were borderline resistant (MIC 2-8 mg/L). All 210 strains possessed the mec A gene (classic resistance). The present study demonstrated that ORSA could disseminate in a new hospital with great speed, and indicated that all ORSA strains in Taiwan revealed classic resistance phenotype.

Borderline susceptibility to methicillin in Staphylococcus aureus: a new mechanism of resistance?

Staphylococcus aureus strains with borderline levels of susceptibility or resistance to antistaphylococcal penicillinase-resistant penicillins (PRPs) were initially reported as neither heteroresistant nor multiply resistant organisms, producing large amounts of beta-lactamase, and becoming fully susceptible to PRPs in the presence of beta-lactamase inhibitors. This borderline susceptibility or low-level resistance was suggested to be due to beta-lactamase hyperproduction: according to this hypothesis, the staphylococcal beta-lactamase, when hyperproduced, would succeed in partially hydrolyzing methicillin and related PRPs. However, later studies demonstrated that borderline PRP susceptibility cannot be explained soley on this basis, beta-lactamase hyperproduction being neither sufficient nor necessary to determine the borderline phenotype. Intrinsic mechanisms have also been reported to be associated with some borderline PRP susceptible S. aureus strains. The more recent discovery of a PRP-hydrolyzing beta-lactamase (methicillinase) produced, in addition to the conventional penicillinase, by borderline S. aureus strains suggests that this second, more specific beta-lactamase is more likely to be responsible for the borderline phenotype than an increased amount of the penicillinase. The slow kinetics of PRP hydrolysis by methicillinase is consistent with its association with reduced susceptibility rather than true resistance to PRPs. The combined effect of methicillinase plus penicillinase on some common substrates might explain the increased beta-lactamase activity often observed in borderline S. aureus strains.

Community-acquired and clindamycin-susceptible methicillin-resistant Staphylococcus aureus in children

BACKGROUND

Recognition of children with community-acquired (CA) infections caused by clindamycin-susceptible, methicillin-resistant Staphylococcus aureus (MRSA) prompted a retrospective study in two Chicago hospitals conducted from 1987 through 1997.

METHODS

Laboratory records of MRSA isolates, antibiotic susceptibilities and information from patient medical records were reviewed.

RESULTS

One hundred eleven MRSA isolates from 103 children were studied with 51 isolates CA and 77 susceptible to clindamycin. The CA infections were less frequently associated with prior surgery (P = 0.0039) or a foreign body (P = 0.0001), and clindamycin-susceptible MRSA infections were less frequently associated with a foreign body (P = 0.001) compared with nosocomially acquired or clindamycin-resistant MRSA infections. Clindamycin-susceptible MRSA sources were mostly skin, wound or abscess (69%). Soft tissue diagnoses predominated (70%), but 16% were serious invasive infections. Ninety percent of clindamycin-susceptible MRSA were susceptible to erythromycin and/or trimethoprim-sulfamethoxazole. Antibiotic undertreatment (45%) or overtreatment (17%) of children with the clindamycin-susceptible MRSA occurred, but clindamycin appeared to be effective when used.

CONCLUSION

The impact of these organisms could be substantial if they become more frequent or widespread. S. aureus is a potential pathogen in large numbers of pediatric patients; microbiologic evaluation and both presumptive and definitive treatment of all these children may need to be changed.

Pseudo-outbreak of methicillin-resistant Staphylococcus aureus

OBJECTIVE

To determine whether a high rate of methicillin-resistant Staphylococcus aureus at our institution was due to laboratory misclassification and to evaluate the effect of this misclassification.

MATERIAL AND METHODS

We evaluated all S. aureus isolates identified at our institution during a 60-day period in 1997. Automated susceptibility test results (using the Vitek system) from our clinical microbiology laboratory and an independent laboratory were compared with oxacillin agar screen plate results at both laboratories. Isolates with discordant results for susceptibility to oxacillin were tested by broth microdilution minimal inhibitory concentrations and for the presence of the mecA gene.

RESULTS

Eighteen (72%) of the 25 organisms (obtained from 17 patients) found to be resistant to oxacillin by the Vitek system at our institution were susceptible by the oxacillin agar screen. Discordant isolates tested by broth microdilution minimal inhibitory concentrations and for the mecA gene were found to be oxacillin susceptible and mecA gene negative. Thus, at our hospital, almost three fourths of the organisms initially identified as methicillin-resistant S. aureus by the Vitek system were actually susceptible to oxacillin. This misclassification resulted in needless infection control measures and unnecessary vancomycin use.

CONCLUSION

Hospitals that use only automated susceptibility testing for S. aureus should periodically validate their results with additional testing.

Disk with high oxacillin content discriminates between methicillin-resistant and borderline methicillin-susceptible Staphylococcus aureus strains in disk diffusion assays using a low salt concentration

A separation between mecA+ strains of Staphylococcus aureus and strains lacking mecA was achieved by the disk diffusion assay and the agar dilution method, utilizing disks containing 5 microg of oxacillin and inocula of approximately 5 x 10(5) CFU/spot, respectively, provided that agar with 0 to 0.5% NaCl and incubation at 30 degrees C were employed. The 5-microg oxacillin disks clearly discriminated between borderline methicillin-susceptible and mecA+ strains. The oxacillin MICs were more affected by the inoculum density and salt concentration than were the methicillin MICs, and oxacillin MICs of 4 to 16 microg/ml were obtained for strains lacking mecA. Significantly higher levels of beta-lactamase production and reduced oxacillin susceptibilities were recorded for strains lacking mecA, in particular strains of phage group V, when agar with >/=2% NaCl was used than when agar with 0 to 0.5% NaCl was employed. The results indicate that the borderline methicillin-susceptible phenotype is a salt-dependent in vitro phenomenon of questionable clinical relevance.

Antimicrobial resistance in staphylococci. Epidemiology, molecular mechanisms, and clinical relevance

Staphylococcal infections continue to pose important clinical problems in children and adults. Antibiotic resistance among the staphylococci has rendered therapy of these infections a therapeutic challenge. Despite early, uniform susceptibility to penicillin, staphylococci acquired a gene elaborating beta-lactamase that rendered penicillin inactive and that is borne by nearly all clinical isolates. "Penicillinase-resistant beta-lactams," such as methicillin, were introduced in the early 1960s, but resistance to them has become an increasing concern. The mechanism of the so-called "methicillin resistance" is complex. Moreover, once confined to the ecology of hospitals and other institutions, a recent increase in community-acquired methicillin-resistant S. aureus infections has been observed. Glycopeptides, until now the only uniformly reliable therapeutic modality, have been increasingly used for therapy of staphylococcal infections. The recent recognition of clinical isolates with reduced susceptibility to glycopeptides is of concern.

Antimicrobial resistance in staphylococci

Staphylococci have developed a variety of strategies for dealing with the presence of antibiotics encountered in clinical environments. Resistance to beta-lactams and other antimicrobial agents has been accomplished by a diverse array of molecular mechanisms. Options available to treat infections caused by staphylococci resistant to methicillin are limited, and the next generation of antibiotics to be introduced, should glycopeptide resistance become an important clinical problem, is not yet on the horizon.

Characterization of two different clusters of clonally related methicillin-resistant Staphylococcus aureus strains by conventional and molecular typing

The DNA fragments of 28 distinct isolates of methicillin-resistant Staphylococcus aureus (MRSA) originating from different hospitals in Warsaw and Lodz, were studied. They were obtained by cleavage with restriction endonuclease SmaI and subsequently analysed by pulsed-field electrophoresis. Sixteen different patterns were seen and clusters of related strains were clearly distinguishable. Minor differences in fragment patterns within these clusters and among epidemiologically related strains, revealed genomic rearrangements in the course of clonal dissemination of particular strains. The isolates were also checked for the expression of methicillin resistance. Isolates with heterogenous and homogeneous phenotypes, fell into clearly distinct clusters and thus formed two clonally related MRSA strains. Differences were also seen with phage and biochemical typing, and antimicrobial resistance patterns.

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.

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.

Survey of clinical isolates of Staphylococcus aureus for borderline susceptibility to antistaphylococcal penicillins

On the basis of the MICs of methicillin and oxacillin, 975 clinical isolates of Staphylococcus aureus were categorized as having resistance, borderline susceptibility or full susceptibility to penicillinase-resistant penicillins (PRPs). The borderline phenotype accounted for 122 isolates (12.5%), whereas 562 isolates were fully susceptible and 290 resistant; one remaining isolate had resistance to methicillin and borderline susceptibility to oxacillin. Reductions in the MICs of methicillin and oxacillin in the presence of sulbactam were greater in strains with borderline PRP susceptibility than in fully susceptible or resistant isolates. Over 99% of fully PRP-susceptible strains, 93% with borderline susceptibility and 71% of resistant strains were susceptible to ampicillin/sulbactam. The production of beta-lactamase, assayed in all strains using nitrocefin as substrate, could be detected without prior induction in 729 strains and after induction only in another 156 strains. With only two exceptions, the beta-lactamase negative strains were part of the fully PRP-susceptible group of organisms (88 of 562 isolates). Among the borderline isolates, strong beta-lactamase reactions were encountered with particular frequency, but not in all strains and not exclusively in borderline strains. Although associated with the majority of borderline strains, beta-lactamase hyperproduction thus did not appear to be an essential feature of the borderline phenotype. The results obtained may have implications for laboratory and clinical medicine, also in the light of recent findings suggesting that other mechanisms besides beta-lactamase hyperproduction may account for borderline susceptibility to PRPs.

Comparison of ampicillin/sulbactam and amoxicillin/clavulanic acid for detection of borderline oxacillin-resistant Staphylococcus aureus

Borderline oxacillin-resistant Staphylococcus aureus (BORSA) may be misidentified as intrinsically methicillin-resistant Staphylococcus aureus (MRSA) in the clinical laboratory. Under disk diffusion testing conditions designed to maximize detection of MRSA (incubation at 32 degrees C, pre-induction with methicillin, or plating on 4% NaCl-supplemented agar), BORSA strains also tend to appear resistant to semisynthetic penicillins. Under these conditions, ampicillin/sulbactam appears to be more accurate than amoxicillin/clavulanic acid for differentiating BORSA from MRSA.

Comparison of conventional susceptibility tests with direct detection of penicillin-binding protein 2a in borderline oxacillin-resistant strains of Staphylococcus aureus

Six selected strains of Staphylococcus aureus classified as borderline oxacillin-resistant, according to standard disk diffusion and microdilution susceptibility test methods, and seven methicillin-resistant and seven methicillin-susceptible control strains were examined for the presence of penicillin-binding protein 2a (PBP 2a) by fluorography and immunoblotting and for DNA hybridization with a mec-specific probe in a dot blot assay. Oxacillin agar screen tests with and without NaCl supplementation were also performed with all strains. PBP 2a was detected both by fluorography and by immunoblotting in all seven methicillin-resistant control strains and in none of the susceptible controls. PBP 2a was detected in two borderline strains. Results of agar screen tests performed without NaCl supplementation were completely concordant with susceptibility determined by PBP 2a and mec detection methods. Agar screening with NaCl supplementation was less accurate. These findings were confirmed with 20 additional borderline strains. Direct detection methods for the presence of PBP 2a or mec, the gene encoding it, allow accurate and definitive classification of borderline strains. Further efforts to develop a rapid, clinically useful, antibody detection system for PBP 2a are warranted.