Methicillin Resistance Phenotype Research Articles

MecA-positive methicillin-sensitive Staphylococcus aureus clinical isolates in Zenica-Doboj Canton, Bosnia and Herzegovina

Forty-four mecA-positive and eight mecA-negative Staphylococcus aureus isolates confirmed by PCR were further tested by disc-diffusion (DD) oxacillin and cefoxitin, oxacillin Epsilon (E)-test, and oxacillin and cefoxitin minimal inhibitory concentration (MIC) Strip methicillin-resistant phenotype in S. aureus (MRSA) tests. Among 44 mecA-positive S. aureus isolates, two (4·5%) were detected as MRSA by DD-oxacillin, 17 (38·6%) by DD-cefoxitin test, and seven (15·9%) by the E-test. In the cefoxitin MIC Strip MRSA test, 19 (43·2%) isolates were resistant. In the oxacillin MIC Strip MRSA test, 18 (40·9%) isolates were resistant and 26 (59·1%) were sensitive, i.e. oxacillin-sensitive MRSA (OS-MRSA) (MIC range 0·25-≤0·25 mg/l). Fifteen out of 26 OS-MRSA (57·7%) belonged to spa-CC 355/595, 78% of which belonged to the largest PFGE clone. Some discrepancies between the phenotypic methods for MRSA identification obtained in this study were caused by large proportion of OS-MRSA. Misidentification of OS-MRSA as MSSA might result in an appearance of highly resistant MRSA in patients treated with beta-lactam antibiotics.

Designing analogs of ticlopidine, a wall teichoic acid inhibitor, to avoid formation of its oxidative metabolites

The thienopyridine antiplatelet agent, ticlopidine and its analog, clopidogrel, have been shown to potentiate the action of β-lactam antibiotics, reversing the methicillin-resistance phenotype of methicillin-resistant Staphylococcus aureus (MRSA), in vitro. Interestingly, these thienopyridines inhibit the action of TarO, the first enzyme in the synthesis of wall teichoic acid, an important cell wall polymer in Gram-positive bacteria. In the human body, both ticlopidine and clopidogrel undergo a rapid P450-dependent oxidation into their respective antiplatelet-active metabolites, resulting in very low plasma concentrations of intact drug. Herein, a series of analogs of ticlopidine and clopidogrel that would avoid oxidative metabolism were designed, prepared and evaluated as inhibitors of TarO. Specifically, we replaced the P450-labile thiophene ring of ticlopidine and clopidogrel to a more stable phenyl group to generate 2-(2-chlorobenzyl)-1,2,3,4-tetrahydro-isoquinoline) (6) and (2-chloro-phenyl)-(3,4-dihydro-1H-isoquinolin-2-yl)-acetic acid methyl ester (22), respectively. The latter molecules displayed inhibitory activity against TarO and formed the basis of a library of analogs. Most synthesized compounds exhibited comparable efficacy to ticlopidine and clopidogrel. So far, it was introduction of a trifluoromethyl group to compound 6, to generate 2-(2-trifluoromethyl-benzyl)-1,2,3,4-tetrahydro-isoquinoline (13) that exhibited enhanced activity against TarO. Compound 13 represents a novel stable inhibitor of TarO with synergistic impact on β-lactam antibiotics against MRSA and low potential for P-450 metabolism.

Characterization of Hospitalized Community-Onset Staphylococcus aureus Lower Respiratory Tract Infections Among Generally Healthy Persons 50 Years of Age or Younger

Background Case series have described severe lower respiratory tract infection (LRI) in healthy, community-dwelling persons caused by methicillin-resistant Staphylococcus aureus (MRSA). Evaluating populations at risk is needed. Methods Surveillance for patients aged 50 years or younger hospitalized with LRI who had S aureus isolated from blood or respiratory specimen during September 2008 to August 2010 was performed at 25 hospitals in 5 US metropolitan areas. Persons with recent health care exposure were excluded. Lower respiratory tract infection diagnosis required supporting radiographic or clinical evidence. Clinical characteristics of LRI were compared by methicillin resistance phenotype. Results In total, 94 hospitalized community-onset S aureus LRI cases were identified. Lower respiratory tract infection cases were identified in both young adults and children (60%, 35–50 years; and 19%, younger than 17 years), without any seasonality or association with influenza circulation. Among the 94 case patients with LRI, 34 patients (36%) had bacteremia, 36 patients (40%) required ICU admission, and 6 patients (6%) died; proportions were similar between cases with methicillin-susceptible S aureus and MRSA. Lower respiratory tract infection cases with MRSA had longer median length of stay compared with those with methicillin-susceptible S aureus (9 vs. 6 days; P = 0.04). Lower respiratory tract infection cases with evidence of influenza infection had higher mortality compared with LRI cases without influenza infection (31% vs. 2%; P = 0.003). During influenza circulation, 35 (55%) of 64 case patients with LRI were tested for influenza, and 9 (26%) of the 35 case patients tested positive. Conclusions S aureus LRI occurred in both adults and children, without any seasonality or association with MRSA and with and without evidence of influenza infection, although case fatality was higher among those with evidence of influenza infection.

Evaluation of Tigecycline and Other Antimicrobials in Latin America Against Multi-drug Resistant (MDR) Staphylococcus aureus

 Results from this study show 34.5% (403/1171) of Staphylococcus aureus from Latin America are resistant to three or more drug classes, while 21% (245/1171) are resistant to at least four drug classes.  Tigecycline inhibited 100% of all multi-drug resistant (MDR) Staphylococcus aureus from Latin America at or below an MIC of 0.5 mcg/mL. The in vitro activity of tigecycline was equivalent to linezolid and vancomycin against all S. aureus.  All of the multi-drug resistant S. aureus were resistant to oxacillin (methicillin-resistant phenotype).  The in vitro activity of tigecycline in this study suggests that tigecycline should be seriously considered for the treatment of S. aureus that are resistant to multiple drugs and multiple drug classes including the methicillin-resistant phenotype. BACKGROUND

Evaluation of Multidrug Resistant (MDR) Isolates Against Tigecycline: A Canadian Perspective

 Tigecycline's MIC 90 of 1 mcg/mL was comparable to imipenem and 2 to 64 fold better than the beta-lactams, betalactam/beta-lactamase inhibitor combinations and levofloxacin against all Enterobacteriaceae tested.  Tigecycline demonstrated potent in vitro activity against both ESBL and non-ESBL producing E. coli, K. oxytoca and K. pneumoniae.  Tigecycline showed potent activity against A. baumannii with low MIC 50/90 values of 0.25 and 1 mcg/mL, respectively.  Tigecycline's limited activity against P. aeruginosa is similar to other tetracyclines and their analog derivatives.  Tigecycline showed potent inhibitory activity against S. aureus regardless of methicillin-resistant phenotype.  At 0.12 mcg/mL tigecycline had the lowest MIC 90 value of all comparative agents against both vancomycinsensitive and -resistant strains of E. faecium and E. faecalis.  Tigecycline presented a MIC 90 value of 0.5 mcg/mL against H. influenzae and was unaffected by the presence of beta-lactamase.  The in vitro activity of tigecycline in this study suggests that tigecycline is a promising compound in the treatment of serious infections caused by the most commonly encountered nosocomial and community acquired pathogens. BACKGROUND

Methicillin-susceptible, non-multiresistant methicillin-resistant and multiresistant methicillin-resistant Staphylococcus aureus infections: a clinical, epidemiological and microbiological comparative study

Non-multiresistant methicillin-resistant Staphylococcus aureus (nmMRSA) infections are emerging worldwide and are often community-associated. This prospective case-cohort study compares features of 96 nmMRSA clinical isolates with 96 matched multiresistant MRSA (mMRSA) and 192 matched methicillin-susceptible S. aureus (MSSA) clinical isolates. Seventy-four percent of nmMRSA infections were healthcare-associated. nmMRSA infections were much more likely to involve skin and soft tissue (skin and soft tissue infections; SSTIs) and were much less likely to be treated appropriately with antibiotics than MSSA or mMRSA infections. Panton-Valentine leukocidin (PVL) genes were detected in 55% of nmMRSA, 16% of MSSA and 2% of mMRSA isolates. Independent of the methicillin-resistance phenotype, 59% of PVL-positive SSTIs presented as furunculosis compared to only 10% of PVL-negative SSTIs. Patients with PVL-positive infections were much younger than patients with PVL-negative infections. The proportion of PVL-positive infections peaked in the 10-29 years old age group, followed by a linear decline.

Resistance development in community-acquired strains of methicillin-resistant Staphylococcus aureus: an in vitro study

This study compares in vitro antimicrobial resistance development between strains of Staphylococcus aureus including newly described community-acquired methicillin-resistant strains (CA-MRSA). High-level resistance developed in all strains of S. aureus after exposure to rifampicin and gentamicin and in some strains after fusidic acid exposure, independent of methicillin resistance phenotype. Resistance did not develop after exposure to clindamycin, cotrimoxazole, ciprofloxacin, linezolid, or vancomycin. These results have important implications for therapy of CA-MRSA infections.

MP070 Differential expression of methicillin resistance phenotype in Staphylococcus epidermidis mediated by the alternative sigma factor óB

Xylooligosaccharides (XOs) are small oligomers constituted by 2−10 units of xylan monomers, with important nutraceutical properties. They can be produced through hydrolysis of xylan catalyzed by an endoxylanase. The use of immobilized and stabilized enzymes may decrease the industrial process costs. In this study, XynA, a recombinant enzyme from B. subtilis, was immobilized in three different supports: agarose and chitosan activated with glyoxal groups and chitosan activated with glutaraldehyde. High immobilization yields were obtained, 100% for agarose-glyoxal and chitosan-glutaraldehyde, 82% for chitosan-glyoxal, with recovered activities of 42.7 (±1.3), 10.7 ± 0.8 and 53.6% (± 1.7), respectively. A great increase in the thermal stability of the enzyme (at 56 °C, pH 5.5) was achieved for the glyoxal derivatives: 75-fold for chitosan and 8600-fold for agarose. The great thermal stability obtained to the derivative agarose-glyoxal can be explained by the enzyme immobilization through lysine residues located in unstable sites of the protein structure. The agarose-glyoxal derivative was tested in the production of XOs (X2, X3 and X4) from soluble and conventional birchwood xylan, reaching approximately 20% of conversion in 3 h and 23% in 24 h, without the undesirable xylose production. After 10 cycles of hydrolysis, the conversion remained almost unchanged.

Jumping the Barrier to β-Lactam Resistance in Staphylococcus aureus

ABSTRACT Although the staphylococcal methicillin resistance determinant, mecA , resides on a mobile genetic element, staphylococcus cassette chromosome mec (SCC mec ), its distribution in nature is limited to as few as five clusters of related methicillin-resistant Staphylococcus aureus (MRSA) clones. To investigate the potential role of the host chromosome in clonal restriction of the methicillin resistance determinant, we constructed plasmid pYK20, carrying intact mecA , and introduced it into several methicillin-susceptible Staphylococcus aureus strains, five of which were naive hosts (i.e., mecA not previously resident on the host chromosome) and five of which were experienced hosts (i.e., methicillin-susceptible variants of MRSA strains from which SCC mec was excised). We next assessed the effect of the recipient background on the methicillin resistance phenotype by population analysis, by assaying the mecA expression of PBP2a by Western blot analysis, and by screening for mutations affecting mecA . Each experienced host transformed with pYK20 had a resistance phenotype and expressed PBP2a similar to that of the parent with chromosomal SCC mec , but naive hosts transformed with pYK20 selected against its expression, indicative of a host barrier. Either inducible β-lactamase regulatory genes blaR1 - blaI or homologous regulatory genes mecR1 - mecI , which control mecA expression, acted as compensatory elements, permitting the maintenance and expression of plasmid-carried mecA .

Antimicrobial resistance in Staphylococcus aureus at the University of Chicago Hospitals: a 15-year longitudinal assessment in a large university-based hospital.

To describe a longitudinal profile of resistance to beta-lactam antimicrobials among isolates of Staphylococcus aureus at a large university teaching hospital and to evaluate the impact of the methicillin resistance phenotype on resistance trends for non-beta-lactam antimicrobials. Retrospective evaluation of antimicrobial susceptibility data for all 17,287 S. aureus isolates obtained from January 1986 through December 2000. The University of Chicago Hospitals, a family of tertiary-care, university-affiliated hospitals in Chicago, Illinois, consisting of 547 adult and pediatric beds. The annual rate of resistance to methicillin increased from 13% in 1986 to 28% in 2000 (P < .001) and has not plateaued. For each non-beta-lactam antimicrobial tested, the annual rates of resistance were far higher among methicillin-resistant S. aureus (MRSA) isolates than among methicillin-susceptible S. aureus (MSSA) isolates. The annual rates of resistance to the macrolide, lincosamide, and streptogramin (MLS) antimicrobials erythromycin and clindamycin increased among MSSA isolates (P < .01), but remained lower than 20%. Resistance to the MLS antimicrobials was higher among MRSA isolates (higher than 60%), but the annual rate decreased significantly during the study (P < .01). The prevalence of methicillin resistance among S. aureus isolates has continued to increase; resistance to non-beta-lactam antimicrobials is far more common among MRSA isolates. Recent decreases in the proportion of MRSA isolates resistant to non-beta-lactam antimicrobials suggest important changes in the epidemiology of this pathogen.

Methicillin-resistant Staphylococcus aureus. Mechanisms of resistance and implications for treatment.

The frequency of methicillin-resistant Staphylococcus aureus (MRSA) continues to increase steadily, with nosocomial isolates approaching 50% of the total tested. Primarily isolated in hospitals, strains of MRSA have now spread into the community, complicating the management of this sometimes-fatal pathogen. Methicillin resistance in S aureus is mediated by the mecA gene, which encodes for a novel penicillin-binding protein (PBP), PBP-2a. In MRSA, exposure to methicillin inactivates the 4 high-binding-affinity PBPs normally present. PBP-2a, which displays a low affinity for methicillin, takes over the functions of these PBPs, permitting the cell to grow. Regulation of the methicillin-resistant phenotype and production of PBP-2a are influenced by the action of other genes. Two genes located upstream from mecA--mecR1 and mecI--control expression of PBP-2a. Antibiotics with high affinity for PBP-2a have displayed efficacy against MRSA in vivo, but none of these agents has made it beyond the investigational stage. Vancomycin remains the drug of choice for treatment of infections caused by MRSA, although it is intrinsically less active than the antistaphylococcal penicillins. Combinations of vancomycin with ss-lactam antibiotics may be synergistic in vivo against MRSA strains, including those with intermediate susceptibility to vancomycin. Given the increasing prevalence of MRSA in hospitals and in community settings, alternative approaches are needed for treatment of infections caused by MRSA.

Suppression of beta-lactam antibiotic resistance in a methicillin-resistant Staphylococcus aureus through synergic action of early cell wall inhibitors and some other antibiotics.

We tested the effect of a number of mechanistically distinct antibacterial agents on the expression of methicillin resistance in a highly and homogeneously resistant strain of methicillin-resistant Staphylococcus aureus. The antibiotics, used at 0.25 x MIC, included inhibitors of early steps in peptidoglycan synthesis (fosfomycin, beta-chloro-D-alanine, D-cycloserine); bacitracin; teicoplanin and vancomycin; beta-lactam inhibitors chosen on the basis of their relatively selective affinities for penicillin-binding proteins 1, 2, 3 and 4 of S. aureus (imipenem, cefotaxime, cephradine and cefoxitin); compounds that inhibit various steps in protein synthesis (tetracycline, chloramphenicol, gentamicin, erythromycin and quinupristin/dalfopristin) and an inhibitor of DNA gyrase (temafloxacin). All inhibitors of early cell wall synthesis caused reduction of methicillin resistance and change from the homogeneous to the heterogeneous methicillin-resistant phenotype. Similar effects were obtained with only cephradine out of the four beta-lactams tested, and with erythromycin and quinupristin/dalfopristin as well. The other inhibitors of protein synthesis and DNA gyrase had no effect.

Methicillin-resistant Staphylococcus aureus isolates recovered from a New York City hospital: analysis by molecular fingerprinting techniques.

Fifty-five methicillin-resistant Staphylococcus aureus (MRSA) isolates collected at New York Hospital Medical Center of Queens in 1989 were analyzed by molecular fingerprinting techniques. Close to 70% of these isolates (38 of 55) shared a common pulsed-field gel electrophoretic pattern, carried the same mecA gene polymorph type II, were free of the transposon Tn554, and would not react with a mecI-specific gene probe. An additional five isolates shared all properties of the major MRSA clone except that they carried mecA gene polymorph type III. All these isolates had an extremely heterogeneous methicillin resistance phenotype that belonged to population analysis profile class 1 or 2. The rest of the 12 MRSA isolates showed a variety of chromosomal pulsed-field gel electrophoretic patterns that carried different mecA polymorphs and that also gave positive reactions with DNA probes for Tn554 and for the mecI gene. The molecular features of the majority MRSA clone suggest that it is an archaic MRSA isolate similar in features to early MRSA isolates recovered in the 1960s.

Lack of mecA transcription in slime-negative phase variants of methicillin-resistant Staphylococcus epidermidis

Five phase variants (PV1 to PV5) of the well-characterized, slime-producing, methicillin-resistant, pathogenic strain of Staphylococcus epidermidis sensu strictu RP62A (ATCC 35984) were isolated by the Congo red agar method. In comparison with the parent strain, the phase variants showed a different colonial morphology on Congo red agar, a strongly reduced adherence capacity, and decreased levels of resistance to methicillin, oxacillin, and penicillin. All phase variants yielded biochemical reaction patterns and profiles in pulsed-field gel electrophoresis identical to those of parent strain RP62A, indicating a common origin. All phase variants proved to have the capacity to shift back to the original phenotype of parent strain RP62A. A search for the resistance mechanisms of strain RP62A revealed beta-lactamase production and the presence of mecA in PV1 to PV5 as well as parent strain RP62A. In Northern blots of total staphylococcal RNA, the phase variants showed no detectable mecA-specific transcription product, whereas parent strain RP62A revealed a strong signal, indicating that mecA transcription is not the mechanism responsible for the decreased methicillin resistance phenotype of phase variants PV1 to PV5.

In Vivo Stability of Heterogeneous Expression Classes in Clinical Isolates of Methicillin-Resistant Staphylococci

To define the stability of methicillin-resistant Staphylococcus aureus (MRSA) in vivo, 22 isolates collected at one New York institution in 1989 and 1990 were studied. All 22 belonged to one of two distinct methicillin-resistant phenotypes (class 3 or 2), which were precisely identified as belonging to two distinct genotypes. Genotypic classification was based on restriction analysis of chromosomal DNA with EcoRI and HindIII and Southern analysis of ClaI digests using two DNA probes. One was specific for the mec gene; the other was specific for transposon Tn554. The findings suggest that the MRSA isolates studied were representative of two genetically distinct MRSA "clones," each with a unique strain-specific methicillin-resistant phenotype that is stable under the conditions of invasive disease, carriage, and spread from patient to patient.