Detection of oxacillin-susceptible mecA-positive Staphylococcus aureus isolates by use of chromogenic medium MRSA ID

Changes in the behavior of Staphylococcus aureus strains in the presence of oxacillin under the effect of gamma radiation

Staphylococcus aureus (S. aureus) as a major pathogen is implicated in a wide range of foodborne and hospital-acquired infections, its methicillin resistant variants contribute to the spread of β-lactam antibiotic resistance. It is essentially important to destroy these pathogens, their resistance genes and the antibiotics in wastewaters. For this purpose reactions of reactive radicals (advanced oxidation processes), first of all hydroxyl radicals (•OH), are suggested. Here the radiolysis of water supplied these radicals. In the experiments B.01755 oxacillin sensitive and B.02174 resistant S. aureus strains were used to study their behaviorr in suspensions under the effect of irradiation in presence and absence of oxacillin. Oxacillin inactivation depended on concentration of the antibiotic used (0.042 and 1 g dm-3), higher concentration required a higher dose. When 106-109 CFU cm-3 S. aureus suspensions were irradiated with γ-radiation the bacteria were inactivated at low absorbed doses: 4 orders of magnitude decrease ocurred in the number of culturable cells at ∼0.6 kGy dose. Both cell membrane and DNA suffered considerable damages during irradiation. Due to the membrane damage the cells could not be stained, and the DNA content of cells in several days period was released into the solution. In DNA damage the oxacillin resistance mecA gene was also modified, it did not multiply in PCR test. These findings are important from the point of view of applying irradiation technology to stop the spread of antibiotic resistance.

Effects of Piper betle Extracts against Biofilm Formation by Methicillin-Resistant Staphylococcus pseudintermedius Isolated from Dogs

Emergence of methicillin-resistant Staphylococcus pseudintermedius (MRSP) isolated from dogs with cutaneous and wound infections has significantly impacted veterinary medicine. This study aimed to isolate S. pseudintermedius from canine pyoderma and investigate the effects of ethanolic extracts of Piper betle (PB), P. sarmentosum (PS), and P. nigrum (PN) on the bacterial growth and biofilm formation of S. pseudintermedius and MRSP. Of the isolated 152 isolates, 53 were identified as S. pseudintermedius using polymerase chain reaction, and 10 isolates (6.58%) were identified as MRSP based on the presence of mecA. Based on phenotype, 90% of MRSPs were multidrug-resistant. All MRSP had moderate (10%, 1/10) and strong (90%, 9/10) biofilm production ability. PB extracts were the most effective in inhibiting planktonic cells, and the minimum inhibitory concentration at which ≥50% of the isolates were inhibited (MIC₅₀) was 256 µg/mL (256-1024 µg/mL) for S. pseudintermedius isolates and 512 µg/mL (256-1024 µg/mL) for MRSP isolates. The MIC₉₀ for S. pseudintermedius and MRSP was 512 µg/mL. In XTT assay, PB at 4× MIC showed an inhibition rate of 39.66-68.90% and 45.58-59.13% for S. pseudintermedius and MRSP, respectively, in inhibiting biofilm formation. For PB at 8× MIC, the inhibition rates for S. pseudintermedius and MRSP were 50.74-81.66% and 59.57-78.33%, respectively. Further, 18 compounds were identified in PB using gas chromatography-mass spectrometry, and hydroxychavicol (36.02%) was the major constituent. These results indicated that PB could inhibit bacteria growth of and biofilm formation by S. pseudintermedius and MRSP isolated from canine pyoderma in a concentration-dependent manner. Therefore, PB is a potential candidate for the treatment of MRSP infection and biofilm formation in veterinary medicine.

Phenotypic and Genotypic Characterization of Methicillin Resistance in Staphylococci Isolated from an Egyptian University Hospital

Methicillin-resistant in Staphylococci is a serious public health issue. It is mostly encoded by the mecA gene. The mecC gene is a new mecA analog responsible for resistance to methicillin in some Staphylococcal clinical isolates. This mecC gene is still underestimated in Egypt. The aim of the current study was to detect mecA and mecC genes in clinical Staphylococci isolates from a tertiary care university hospital in Egypt compared to the different phenotypic methods. A total of 118 Staphylococcus aureus (S. aureus) and 43 coagulase-negative Staphylococci (CoNS) were identified from various hospital-acquired infections. Methicillin resistance was identified genotypically using the PCR technique and phenotypically using the cefoxitin disc diffusion test, oxacillin broth microdilution and the VITEK2 system in all Staphylococcal isolates. The mecA gene was detected in 82.2% of S. aureus and 95.3% of CoNS isolates, while all of the isolates tested negative for the mecC gene. Interestingly, 30.2% of CoNS isolates showed the unique character of inducible oxacillin resistance, being mecA-positive but oxacillin-susceptible (OS-CoNS). The dual use of genotypic and phenotypic methods is highly recommended to avoid missing any genetically divergent strains.

The Cell Wall, Cell Membrane and Virulence Factors of Staphylococcus aureus and Their Role in Antibiotic Resistance

Antibiotic resistant strains of bacteria are a serious threat to human health. With increasing antibiotic resistance in common human pathogens, fewer antibiotics remain effective against infectious diseases. Staphylococcus aureus is a pathogenic bacterium of particular concern to human health as it has developed resistance to many of the currently used antibiotics leaving very few remaining as effective treatment. Alternatives to conventional antibiotics are needed for treating resistant bacterial infections. A deeper understanding of the cellular characteristics of resistant bacteria beyond well characterized resistance mechanisms can allow for increased ability to properly treat them and to potentially identify targetable changes. This review looks at antibiotic resistance in S aureus in relation to its cellular components, the cell wall, cell membrane and virulence factors. Methicillin resistant S aureus bacteria are resistant to most antibiotics and some strains have even developed resistance to the last resort antibiotics vancomycin and daptomycin. Modifications in cell wall peptidoglycan and teichoic acids are noted in antibiotic resistant bacteria. Alterations in cell membrane lipids affect susceptibility to antibiotics through surface charge, permeability, fluidity, and stability of the bacterial membrane. Virulence factors such as adhesins, toxins and immunomodulators serve versatile pathogenic functions in S aureus. New antimicrobial strategies can target cell membrane lipids and virulence factors including anti-virulence treatment as an adjuvant to traditional antibiotic therapy.

Comparison between Some Phenotypic and Genotypic Methods for Assessment of Antimicrobial Resistance Trend of Bovine Mastitis Staphylococcus aureus Isolates from Bulgaria

Simple Summary

The aim of the study was to assess the resistance of bovine mastitis Staphylococcus aureus isolates to antimicrobials by comparison of laboratory methods and statistical analysis. For this, 546 mastitis milk samples at 14 farms from 9 districts in Bulgaria were examined. A total of 92 Staphylococcus aureus strains were isolated and identified. The results showed high resistance to sulfadimethoxine (87%), followed by resistance to penicillin (33.7%), erythromycin (13%), streptomycin (8.7%), tetracycline (6.5%) and gentamicin (1.1%). The comparison of the methods demonstrated more than 90% agreement for 9 tested antimicrobial drugs, hence proving reliability of results from monitoring of resistance. Considering the detected discrepancies for some of isolates, an integral evaluation through standard and modern molecular methods for Staphylococcus aureus is recommended. The results from this study add to the microbiology laboratory experience and strategies for mastitis prevention and control programmes.

Abstract

The aim of this study was to assess the resistance of bovine mastitis S. aureus isolates from farms in Bulgaria to different classes of chemotherapeutic drugs by comparison of some phenotypic and genotypic methods by means of Cohen’s kappa statistics. The study comprised 546 milk samples from subclinical and clinical mastitis at 14 farms from 9 districts in the country. A total of 92 Staphylococcus aureus strains were isolated from tested samples and identified by nuc PCR. The results demonstrated high levels of resistance to sulfadimethoxine (87%), followed by resistance to penicillin (33.7%), erythromycin (13%), streptomycin (8.7%), tetracycline (6.5%) and gentamicin (1.1%). The comparison of both phenotypic tests with respect to 9 antimicrobials revealed strong agreement with kappa coefficient 0.836. An almost complete agreement was evidenced between phenotypic resistance to penicillin and blaZ gene presence, to methicillin with mecA gene, to tetracycline with tet genes, but the agreement between erythromycin resistance and erm genes presence was moderate. This study was the first to demonstrate discrepancy between the behaviour to cefoxitin in the disk diffusion test and oxacillin in the MIC test for an isolate shown to carry the mecA gene in the subsequent genetic analysis. Considering the detected discrepancies for some of isolates, an integral evaluation through phenotypic and molecular methods for monitoring of antimicrobial resistance of Staphylococcus aureus is recommended.

Comparison of Phenotypic MRSA Detection Methods with PCR for mecA Gene in the Background of Emergence of Oxacillin-Susceptible MRSA

Background: Phenotypic methods for detection of methicillin resistance in Staphylococcus aureus (MRSA) can be inaccurate due to heterogeneous expression of resistance and due to environmental factors that influence the expression of resistance. This study aims to compare various phenotypic methods of detection of methicillin resistance with polymerase chain reaction (PCR) for mecA gene and to detect the presence of oxacillin-susceptible MRSA (OS-MRSA). Materials and Methods: A total of 150 S. aureus isolates were tested using cefoxitin disk diffusion, oxacillin salt agar (OSA), latex agglutination test for penicillin binding protein 2a antigen, chromogenic MRSA ID agar, and mecA PCR. Results: Using PCR as the gold standard, 91 (60.66%) of 150 clinical S. aureus strains were identified as MRSA. Three oxacillin-susceptible (minimum inhibitory concentration ≤2 μg/mL) mecA-positive isolates were classified as OS-MRSA. Among the different phenotypic MRSA detection methods studied, latex agglutination had the highest sensitivity and specificity (98.9% and 98.3%), followed by cefoxitin disk diffusion (95.6% and 98.3%), MRSA ID (97.8% and 83.05%), and OSA (86.81% and 94.92%). Conclusion: The sensitivity of cefoxitin disk diffusion method may be reduced in areas with a high prevalence of OS-MRSA where a combination of cefoxitin disk diffusion test with MRSA ID agar or latex agglutination is recommended.

Bacterial ecology and antibiotic resistance mechanisms of isolated resistant strains from diabetic foot infections in the north west of Algeria

BACKGROUND

In front of the polymorphic bacterial ecology and antibiotic resistance in diabetic patients with foot infections and good patient care, collaboration between clinicians and microbiologists is needed to improve assessment and management of patients with this pathology.

OBJECTIVE

This study was designed to characterize the bacterial ecology of diabetic foot infection (DFIs) and to determine the different mechanisms of resistance involved.

METHODS

In this study bacterial strains and antibiotic resistance profiles were determined from diabetic foot infections patients (n = 117). The identification of resistance mechanisms, such as penicillinase and/or extended-spectrum β-lactamase production (ESBL), methicillin-resistant Staphylococcus aureus (MRSA) and efflux pump over-expression were performed.

RESULTS

A high prevalence of Gram-negative bacteria (61%) with Escherichia coli, and other Enterobacteriaceae and Pseudomonas aeruginosa being the predominant isolates. Gram positive bacteria mainly represented by Staphylococcus aureus accounted for 39% of the isolates. 93.5% of the Enterobacteriaceae were resistant to, at least, one molecule in the β-lactam family, while the majority of the Staphylococci were resistant to penicillin G and tetracycline (93.3% and 71.7%). The majority of non-fermenting Gram negative bacteria were also resistant to fluoroquinolones. β-lactamase detection tests revealed the presence of extended-spectrum β-lactamase in 43.5% of the Enterobacteriaceae, while methicillin-resistant Staphylococcus aureus represented 18.2% of the isolates. Additionally, 50.9% of non-fermenting Gram negative bacteria were overproducing efflux pumps.

CONCLUSION

All Acinetobacter Baumannii were Multidrug-Resistant (MDR), as the majority of Staphylococci, and Enterobacteriaceae. These results should be taken into account by the clinician in the prescription of probabilistic antibiotic therapy in this context.

Virulence Characteristics of mecA-Positive Multidrug-Resistant Clinical Coagulase-Negative Staphylococci

Coagulase-negative staphylococci (CoNS) are an important group of opportunistic pathogenic microorganisms that cause infections in hospital settings and are generally resistant to many antimicrobial agents. We report on phenotypic and genotypic virulence characteristics of a select group of clinical, mecA-positive (encoding penicillin-binding protein 2a) CoNS isolates. All CoNS were resistant to two or more antimicrobials with S. epidermidis strain 214EP, showing resistance to fifteen of the sixteen antimicrobial agents tested. Aminoglycoside-resistance genes were the ones most commonly detected. The presence of megaplasmids containing both horizontal gene transfer and antimicrobial resistance genetic determinants indicates that CoNS may disseminate antibiotic resistance to other bacteria. Staphylococcus sciuri species produced six virulence enzymes, including a DNase, gelatinase, lipase, phosphatase, and protease that are suspected to degrade tissues into nutrients for bacterial growth and contribute to the pathogenicity of CoNS. The PCR assay for the detection of biofilm-associated genes found the eno (encoding laminin-binding protein) gene in all isolates. Measurement of their biofilm-forming ability and Spearman’s rank correlation coefficient analyses revealed that the results of crystal violet (CV) and extracellular polymeric substances (EPS) assays were significantly correlated (ρ = 0.9153, P = 3.612e-12). The presence of virulence factors, biofilm-formation capability, extracellular enzymes, multidrug resistance, and gene transfer markers in mecA-positive CoNS clinical strains used in this study makes them powerful opportunistic pathogens. The study also warrants a careful evaluation of nosocomial infections caused by CoNS and may be useful in studying the mechanism of virulence and factors associated with their pathogenicity in vivo and developing effective strategies for mitigation.

Potential bactericidal activity of S. nux-vomica-ZnO nanocomposite against multidrug-resistant bacterial pathogens and wound-healing properties

Multidrug resistance in bacterial strains has become the greatest challenge for healthcare professionals for treating non-healing ulcers such as diabetic foot infections (DFI). Plant-mediated synthesis of S. nux-vomica-ZnO nanocomposite appears as a potential new alternative therapeutic agent that might be capable of tackling antibiotic-resistant bacterial pathogens and for treating a non-healing ulcer. The aim of the study was to investigate the antibacterial potential of S. nux-vomica-ZnO nanocomposite biosynthesised from Strychnos nux-vomica against multidrug-resistant organisms (MDROs) from DFU, wound-healing properties, and cytotoxic effects. The antibacterial potential was assessed by minimum inhibitory concentration (MIC)/ minimum bactericidal concentration (MBC) assays, time-kill kinetics, protein-leakage, and flow cytometric analysis. The wound-healing properties were assessed by scratch assay on mouse L929 fibroblastic cell line to quantify cell migration towards the injured area. Cytotoxicity was assessed using 3-[4,5-dimethyl-2-thiazol-yl]-2,5-diphenyl- 2H-tetrazolium bromide (MTT) cellular viability assay on the L929 cell line and human embryonic kidney epithelial (HEK-293) cell line. Strychnos nux-vomica-ZnO nanocomposite at a size range of 10-12 nm exhibited significant bactericidal potency at a concentration of 100-200 μg/ml against MDR-Methicillin-resistant Staphylococcus aureus, MDR-Escherichia coli, MDR-Pseudomonas aeruginosa, MDR-Acinetobacter baumannii, and also against standard bacterial strains S. aureus ATCC 29213, E. coli ATCC 25922, P. aeruginosa ATCC 27853, E. faecalis ATCC 29212. S. nux-vomica-ZnO nanocomposite also exhibited wound-healing and reduced cytotoxic properties at the antimicrobially active concentrations. Our findings thus suggested remarkable bactericidal properties of S. nux-vomica-ZnO nanocomposite and can be further exploited towards for the development of an antibacterial agent against the threatening superbugs.

Evaluation of reference values for phenotypic tests to detect oxacillin resistance in coagulase-negative staphylococci

AIM

To evaluate the adequacy of the disc-diffusion test and E-test^® compared with detection of mecA for coagulase-negative staphylococci isolated from blood cultures, nasal swabs and wounds.

RESULTS

Agreement between all techniques was observed in 65.7% of cases. The greatest discrepancy between mecA/susceptible E-test was observed for non-epidermidis species. A resistance breakpoint ≤19 mm using the oxacillin disc was found to best classify all coagulase-negative staphylococci isolates; Staphylococcus epidermidis, ≤19 mm (oxacillin) and ≤27 mm (cefoxitin); Staphylococcus haemolyticus and Staphylococcus capitis, ≤21 mm (oxacillin) and ≤18 mm (cefoxitin); Staphylococcus warneri, MICs ≥0.75 mg/l.

CONCLUSION

Although no longer recommended by the Clinical Laboratory Standards Institute, we observed some cases in which only the oxacillin disc-diffusion test detected resistance. The discrepancy between phenotypic tests and mecA is probably due to heterogeneity and borderline resistance.

Enhanced antibacterial effects of green synthesized ZnO NPs using Aristolochia indica against Multi-drug resistant bacterial pathogens from Diabetic Foot Ulcer

BACKGROUND

Increased incidence of Multi-drug resistance in microorganisms has become the greatest challenge in the treatment of Diabetic Foot Ulcer (DFU) and urges the need of a new antimicrobial agent. In this study, we determined the bactericidal effects of ZnO nanoparticles (ZnO NPs) green synthesized from Aristolochia indica against Multi-drug Resistant Organisms (MDROs) isolated from pus samples of DFU patients attending in a tertiary care hospital in South India.

METHODS

ZnO NPs were characterized by UV-vis-DRS spectroscopy, Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and for its zeta potential value. MIC/MBC assays were performed to determine bactericidal or bacteriostatic effects. Time-kill assays, Protein leakage and Flow cytometric analysis evaluated bacterial cell death at 1x MIC and 2x MIC concentrations of ZnO NPs.

RESULTS

ZnO NPs of size 22.5nm with a zeta potential of -21.9±1mV exhibited remarkable bactericidal activity with MIC/MBC ranging from 25 to 400μg/ml with a significant reduction in viable count from 2h onwards. Protein leakage and Flow cytometric analysis confirmed bacterial cell death due to ZnO NPs.

CONCLUSION

This study concluded that green synthesis protocol offers reliable, eco-friendly approach towards the development of antimicrobial ZnO NPs to combat antibiotic drug resistance.

Synthesis and characterization of ZnO phytonanocomposite using Strychnos nux-vomica L. (Loganiaceae) and antimicrobial activity against multidrug-resistant bacterial strains from diabetic foot ulcer

Nanobiotechnology has been emerged as an efficient technology for the development of antimicrobial nanoparticles through an eco-friendly approach. In this study, green synthesized phytonanocomposite of ZnO from Strychnos nux-vomica leaf aqueous extract was characterized by X-ray diffraction analysis (XRD), UV–visible-spectroscopy, Photoluminescence spectroscopy (PL), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), High-resolution Transmission Electron Microscopy (HR-TEM), and Energy dispersive X-ray analysis (EDX). Antibacterial activity was investigated against multidrug-resistant bacteria (MDR) isolated from diabetic foot ulcers (DFUs), such as MDR–methicillin resistant Staphylococcus aureus (MRSA), MDR–Escherichia coli, MDR–Pseudomonas aeruginosa, MDR–Acinetobacter baumannii, as well as against standard bacterial strains, S. aureus ATCC 29213, E. coli ATCC 25922, P. aeruginosa ATCC 27853, and E. faecalis ATCC 29212 through disc diffusion assays on Muller Hinton Agar. The characterization studies revealed a size-controlled synthesis of quasi-spherical hexagonal wurtzite structured ZnO phytonanocomposite with an average size of 15.52 nm. Additionally, remarkable bactericidal activities against MDR clinical as well as ATCC bacterial strains were exhibited, with a maximum zone of inhibition of 22.33 ± 1.53 mm (against S. aureus ATCC 29213) and 22.33 ± 1.16 mm (MDR–MRSA) at a concentration of 400 µg/mL. This study thus established the possibility of developing antimicrobial ZnO nanocomposite of Strychnos nux-vomica leaf extract to combat developing drug resistance currently being experienced in health care facilities.

Genotypic and phenotypic β-lactam resistance and presence of PVL gene in Staphylococci from dry bovine udder

Dairy cows affected with subclinical mastitis can be sources of virulent, antimicrobial-resistant Staphylococci to humans because of the excretion of the bacteria through their milk. This study focussed on the phenotypic and genotypic antibiotic resistance patterns of Staphylococci isolated from dairy cows in early dry period. Among 96 isolates of Gram positive cocci from 157 cows, 76 were identified as Coagulase Negative Staphylococci and the remaining 20 were Staphylococcus aureus. Typical amplicons of coagulase gene were obtained for all 20 samples of S. aureus with three major coagulase types being identified as giving 627 bp (40%), 910 bp (35%) and 710 bp (25%) long PCR products. The groEL gene was amplified in PCR of all 76 isolates of Coagulase Negative Staphylococci, and incubation of PCR products with restriction enzyme PvuII yielded three distinct PCR-RFLP fragment patterns bearing resemblance to S. chromogenes and S. hyicus. Highest sensitivity of Coagulase Negative Staphylococci was noted for Azithromycin (92.5%) and the least to Tetracyclines (76.3%), whereas for S. aureus, it was Cefoperazone (95%) and Azithromycin (72.2%) respectively. Phenotypic resistance to Oxacillin (25 isolates), and Cefoxitin (11 isolates) was detected by dilution method with a commercial strip (Ezy MIC^TM). Genotypic resistance to β-Lactam antibiotics was found in 65 (34 with mecA gene and 31 with blaZ gene) isolates. Eighteen isolates possessed both the genes, with the PVL gene for virulence being detected in five of them. Nine isolates which had mecA gene were phenotypically susceptible to oxacillin while phenotypic resistance to oxacillin was observed in seven isolates that did not have either mecA or blaZ gene. This is the first report of persistent Staphylococcal infections possessing PVL gene and high level of genotypic resistance to β-Lactam antibiotics in small- holder dairy cattle from India.

Characterisation of clinical isolates of oxacillin-susceptible mecA-positive Staphylococcus aureus in China from 2009 to 2014

OBJECTIVES

The aim of this study was to investigate the prevalence and molecular characteristics of clinical oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) isolates in China from July 2009 to June 2014.

METHODS

A total of 2068 non-duplicate S. aureus isolates were collected from 21 hospitals. Antimicrobial susceptibility testing was performed by the agar dilution method. All OS-MRSA strains were screened for the presence of the genes mecA, mecC and nuc as well as the Panton-Valentine leukocidin gene (pvl). Staphylococcal cassette chromosome mec (SCCmec) typing, staphylococcal protein A (spa) typing and multilocus sequence typing (MLST) were performed to analyse the isolate genotypes.

RESULTS

A total of 34 S. aureus isolates were mecA-positive but were susceptible to oxacillin [minimum inhibitory concentration (MIC)≤2mg/L]. All OS-MRSA isolates were resistant to cefoxitin and most were also multiresistant to other antibiotics besides β-lactams. Among the 34 OS-MRSA isolates, nine spa and three SCCmec types were detected and, combined with MLST, ST338/59-t437-SCCmecV (47%; 16/34) was the predominant clone. In addition, 17 strains (50%) carried the pvl gene.

CONCLUSIONS

The most frequent clone of OS-MRSA isolates in China was ST338-t437-SCCmecV. Most of the OS-MRSA isolates were susceptible to the majority of antibacterial agents except macrolides, clindamycin and chloramphenicol.

Prevalence and Characterization of Oxacillin Susceptible mecA-Positive Clinical Isolates of Staphylococcus aureus Causing Bovine Mastitis in India

Bovine mastitis caused by multidrug resistant Staphylococcus aureus is a huge problem reported worldwide, resulting in prolonged antibiotic treatment and death of livestock. The current study is focused on surveillance of antibiotic susceptibility along with genotypic and phenotypic characterization of the pathogenic S. aureus strains causing mastitis in India. One hundred and sixty seven milk samples were collected from mastitis-affected cows from different farms in India resulting in thirty nine isolated S. aureus strains. Antibiotic sensitivity profiling revealed the majority of the strains (n = 24) to be multidrug resistant and eleven strains showed reduced susceptibility to vancomycin (MICs = 2μg/ml). All strains were oxacillin sensitive, but 19 strains were positive for the mecA gene, which revealed the occurrence of oxacillin susceptible mecA positive strains (OS-MRSA) for the first time from India. Additionally, 32 strains were positive for the pvl gene, a virulence determinant; of these 17 were also OS-MRSA strains. Molecular characterization based on multilocus sequence typing (MLST), spa typing, agr typing and SCCmec classification revealed strains belonging to different groups. Moreover, strains showed spa types (t2526, t9602) and MLST sequence types, ST-72, ST-88 and ST-239 which have been earlier reported in human infections. The prevalence of OS-MRSA strains indicates the importance of including both the genetic and phenotypic tests in characterizing S. aureus strains. Increased genotypic variability with strain related to human infections and pvl positive isolates indicates a worrisome situation with the possibility of bilateral transfer.

Genetic Characterization of Staphylococcus aureus Isolated from Retail Meat in Riyadh, Saudi Arabia

Limited data exist from the Gulf Cooperation Council states on the prevalence and population dynamics of Staphylococcus aureus colonizing livestock or contaminating retail meat. This study was designed to determine the presence and genetic characteristics of Staphylococcus aureus isolated from raw retail meat sold in Riyadh, Saudi Arabia. Over a period of 9 months, different raw retail meat types were aseptically processed using the double broth enrichment technique, characteristic colonies from chromogenic and mannitol salt agar were further identified using conventional methods. Susceptibility to 9 antibiotics was determined using the disc diffusion technique. Interpretation of inhibition zone was done according to Clinical and Laboratory Standards Institute guidelines. Molecular characterization was carried out using the StaphyType DNA microarray technology. Twenty-five meat samples yielded Staphylococcus aureus isolates. Camel meat had the highest contamination rate with Methicillin resistant Staphylococcus aureus (MRSA) (20%) and Methicillin susceptible Staphylococcus aureus (28%), while poultry meat had the least contamination rate with MRSA (4%). The MRSA isolates were grouped into 4 clonal complexes (CCs) namely CC1-MRSA-IV/SCCfus (n = 2), CC15-MRSA-V/SCCfus (n = 4), CC80-MRSA-IV/PVL+ (n = 5), and CC88-MRSA-IV/PVL+ (n = 2). All CC15-MRSA-V/SCCfus isolates were obtained from camel meat. This is the first study to demonstrate the novel CC15-MRSA-V/SCCfus in retail camel meat. We recommend that surveillance studies should be incorporated in public health and food hygiene programs.

First report of lukM-positive livestock-associated methicillin-resistant Staphylococcus aureus CC30 from fattening pigs in Northern Ireland

The increasing number of reports of livestock-associated meticillin-resistant Staphylococcus aureus (LA-MRSA) world-wide attests to the public health concern surrounding this pathogen in animal husbandry and in-contact humans. In Europe, LA-MRSA CC398 is predominant and generally regarded as being of low virulence for animals. Herein we report the recovery of a lineage of LA-MRSA, belonging to CC30, from three pigs in Northern Ireland and which encodes a marker of virulence (lukM and lukF-P83) restricted to animal-associated clones of S. aureus.

Reversion From Methicillin Susceptibility to Methicillin Resistance in Staphylococcus aureus During Treatment of Bacteremia

Approximately 3% of Staphylococcus aureus strains that, according to results of conventional phenotypic methods, are highly susceptible to methicillin-like antibiotics also have polymerase chain reaction (PCR) results positive for mecA. The genetic nature of these mecA-positive methicillin-susceptible S. aureus (MSSA) strains has not been investigated. We report the first clearly defined case of reversion from methicillin susceptibility to methicillin resistance among mecA-positive MSSA within a patient during antibiotic therapy. We describe the mechanism of reversion for this strain and for a second clinical isolate that reverts at a similar frequency. The rates of reversion are of the same order of magnitude as spontaneous resistance to drugs like rifampicin. When mecA is detected by PCR in the clinical laboratory, current guidelines recommend that these strains be reported as resistant. Because combination therapy using both a β-lactam and a second antibiotic suppressing the small revertant population may be superior to alternatives such as vancomycin, the benefits of distinguishing between mecA-positive MSSA and MRSA in clinical reports should be evaluated.

Whole-genome analysis of an oxacillin-susceptible CC80 mecA-positive Staphylococcus aureus clinical isolate: insights into the mechanisms of cryptic methicillin resistance

OBJECTIVES

The mec and bla systems, among other genetic factors, are critical in regulating the expression of methicillin resistance in Staphylococcus aureus. We examined by WGS a naturally occurring oxacillin-susceptible mecA-positive S. aureus isolate to identify the mechanism conferring oxacillin susceptibility.

METHODS

The mecA-positive oxacillin-susceptible S. aureus isolate GR2 (penicillin and oxacillin MICs 0.094 and 1 mg/L, respectively), belonging to clonal complex 80, was characterized. DNA fragment libraries were sequenced on Roche 454 and Illumina MiSeq sequencers and de novo assembly of the genome was generated using SeqMan NGen software. Plasmid curing was conducted by SDS treatment. Expression of mecA was quantified without/with β-lactam pressure.

RESULTS

The genome of GR2 consisted of a 2 792 802 bp chromosome and plasmids pGR2A (28 895 bp) and pGR2B (2473 bp). GR2 carried SCCmec type IV, with a truncated/non-functional mecR1 gene and no mecI. A single copy of the bla system, with an organization unique for S. aureus, was found, harboured by plasmid pGR2A. Particularly, the blaZ gene was orientated like its regulatory genes, blaI and blaR1, and a gene encoding transposase IS66 was integrated between blaZ and the regulatory genes deleting the 5'-end of blaR1; blaI, encoding blaZ/mecA repressor, was intact. After plasmid loss, GR2 became penicillin and oxacillin resistant (MICs 0.5 and 6 mg/L, respectively).

CONCLUSIONS

We can conclude that after exposure to β-lactams, the non-functional BlaR1 does not cleave the mecA repressor BlaI, derepression does not occur and mecA is not efficiently expressed. Removal of the bla system after curing of pGR2A allows constitutive expression of mecA, resulting in oxacillin and penicillin resistance.

The use of molecular methods for the detection and identification of methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major pathogen in many hospitals and long-term care facilities as well as in the community. To limit the spread of MRSA, early detection and proper treatment are essential. Because conventional culture as gold standard is time consuming, new techniques such as PCR-based and hybridization assays have emerged for the rapid detection of MRSA. This review will focus on the currently available molecular-based assays and on their utility and performance for detection of S. aureus, of its virulence factors and of the markers for acquired resistance.

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.

Staphylococcus aureus nasal carriage among patients and health care workers in São Tomé and Príncipe

Methicillin-resistant Staphylococcus aureus (MRSA) is a major human pathogen worldwide. However, data on MRSA prevalence in the African continent are scarce and nonexistent for São Tomé and Príncipe. In November 2010 and April 2012, a total of 332 individuals (258 patients and 74 health care workers [HCW]) from Hospital Dr. Ayres Menezes in São Tomé and Príncipe, were screened for S. aureus and MRSA carriage. Fifty-two persons (15.7%) were S. aureus nasal carriers out of which 14 (26.9%) were colonized with MRSA. MRSA isolates belonged to three clonal complexes: CC8 (PFGE type B-ST8-t064/t451-IVg/V), CC88 (PFGE E-ST88-t186/t786-IVa), and CC5 (PFGE K-ST5-t105-IVa/PFGE K-ST105-t002-II). A higher genetic diversity was found among methicillin-susceptible S. aureus (MSSA) isolates where 58.5% (n=24) belonged to four major lineages: PFGE type A-ST15-t084; PFGE C-ST508-t861 or related; PFGE D-ST152-t355 or related; and PFGE G-ST121-t159/t2304. Despite the common nonmultiresistant profile, 98% of the isolates harbored two or more virulence factors. Panton-Valentine leukocidine was detected in 36% of the isolates, all MSSA. S. aureus cross-transmission between HCW and patients in the pediatric and medicine wards and the detection of identical MRSA strains among patients in two different wards evidenced the need of implementation of additional infection control measures in this hospital.

Prevalence and molecular typing of oxacillin-susceptible mecA-positive Staphylococcus aureus from multiple hospitals in China

Among 1588 non-duplicated Staphylococcus aureus isolates from 10 cities in China, 60 isolates were susceptible to oxacillin (MIC50: 1 μg/mL; MIC90: 2 μg/mL) but were mecA-positive. Twenty-one spa and 5 staphylococcal cassette chromosome mec (SCCmec) types were detected, and combined with multilocus sequence typing method, ST59-t437-SCCmecIV/V was the predominant clone (26.7%, 16/60).