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ABSTRACT The emergence of oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA) has imposed further challenges to the clinical management of MRSA infections. When exposed to β-lactam antibiotics, these strains can easily acquire reduced β-lactam susceptibility through chromosomal mutations, including those in RNA polymerase (RNAP) genes such as rpoBC, which may then lead to treatment failure. Despite the increasing prevalence of such strains and the apparent challenges they pose for diagnosis and treatment, there is limited information available on the actual mechanisms underlying such chromosomal mutation-related transitions to reduced β-lactam susceptibility, as it does not directly associate with the expression of mecA. This study investigated the cellular physiology and metabolism of six missense mutants with reduced oxacillin susceptibility, each carrying respective mutations on RpoBH929P, RpoBQ645H, RpoCG950R, RpoCG498D, RpiAA64E, and FruBA211E, using capillary electrophoresis-mass spectrometry-based metabolomics analysis. Our results showed that rpoBC mutations caused RNAP transcription dysfunction, leading to an intracellular accumulation of ribonucleotides. These mutations also led to the accumulation of UDP-Glc/Gal and UDP-GlcNAc, which are precursors of UTP-associated peptidoglycan and wall teichoic acid. Excessive amounts of building blocks then contributed to the cell wall thickening of mutant strains, as observed in transmission electron microscopy, and ultimately resulted in decreased susceptibility to β-lactam in OS-MRSA.IMPORTANCEThe emergence of oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA) strains has created new challenges for treating MRSA infections. These strains can become resistant to β-lactam antibiotics through chromosomal mutations, including those in the RNA polymerase (RNAP) genes such as rpoBC, leading to treatment failure. This study investigated the mechanisms underlying reduced β-lactam susceptibility in four rpoBC mutants of OS-MRSA. The results showed that rpoBC mutations caused RNAP transcription dysfunction, leading to an intracellular accumulation of ribonucleotides and precursors of peptidoglycan as well as wall teichoic acid. This, in turn, caused thickening of the cell wall and ultimately resulted in decreased susceptibility to β-lactam in OS-MRSA. These findings provide insights into the mechanisms of antibiotic resistance in OS-MRSA and highlight the importance of continued research in developing effective treatments to combat antibiotic resistance.

Methicillin resistant Staphylococcus aureus (MRSA) is highly divergent antibiotic resistant bacteria earmarked as “High” in global pathogens’ priority list varying the severity and resistance geographically. Here, MRSA were screened using mecA gene with Cefoxitin and other 27 antibiotics of 19 classes using disc diffusion method from a highly humid climate of India. Multiple Antibiotic Resistance (MAR) index was calculated. Minimum Inhibitory Concentration (MIC) was determined against 11 classes of antibiotics. Detection of major virulence genes tst-1 and lukPV were done. A total of 95.24% Hospital Associated (HA)-MRSA, 56.14% Community Associated (CA)-MRSA and 82.53 % Livestock Associated (LA)-MRSA were detected. Cefoxitin, Oxacillin, Ciprofloxacin, Fusidic acid and Ticarcillin-Clavulinic acid resistance was observed in more than 60% of HA-MRSA, CA-MRSA and LA-MRSA. Across the hosts, Mupirocin, Gentamicin, Linezolid, Co-trimoxazole, and Rifampicin were found effective. Vancomycin Intermediate Staphylococcus aureus (VISA) detected in CA-MRSA & LA-MRSA. Multidrug Resistant (MDR) was found very high but extensively drug-resistant (XDR) was detected moderately. No pan drug-resistant (PDR) was detected. Virulence gene tst-1 and lukPV were detected in 7.69% and 32.69% MRSA isolates. The gene tst-1 is reported for the first time in pre and post-caesarian samples from Gynaecology department in this region with high MDR. This study showed S. aureus and subsequent prevalence of MRSA is higher in this region then national data. 2nd generation Cephalosporins were found effective which is very encouraging due to their limited uses. Detection of tst-1 in caesarian samples is a serious threat as neonatal transmission of MRSA from mother is reported.

The aim of this study is to describe the phenotypic and genetic properties of oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA) isolates and their beta-lactam resistant derivatives obtained after selection with oxacillin. A collection of hospital- (HA-) and community-acquired (CA-) MRSA was screened for oxacillin susceptibility. Antibiotic susceptibility testing, population analysis profile (PAP), mecA expression analysis, and whole genome sequencing (WGS) were performed for 60 mecA-positive OS-MRSA isolates. Twelve high-level beta-lactam resistant derivatives selected during PAP were also subjected to WGS. OS-MRSA were more prevalent among CA-MRSA (49/205, 24%) than among HA-MRSA (11/575, 2%). OS-MRSA isolates belonged to twelve sequence types (ST), with a predominance of ST22-t223-SCCmec IVc and ST59-t1950-SCCmec V lineages. OS-MRSA were characterized by mecA promoter mutations at − 33 (C→T) or − 7 (G→T/A) along with PBP2a substitutions (S225R or E246G). The basal and oxacillin-induced levels of mecA expression in OS-MRSA isolates were significantly lower than those in control ST8-HA-MRSA isolates. Most of the OS-MRSA isolates were heteroresistant to oxacillin. High-level beta-lactam resistant OS-MRSA derivatives selected with oxacillin carried mutations in mecA auxiliary factors: relA (metabolism of purines), tyrS, cysS (metabolism of tRNAs), aroK, cysE (metabolism of amino acids and glycolysis). Cefoxitin-based tests demonstrated high specificity for OS-MRSA detection. The highest positive predictive values (PPV > 0.95) were observed for broth microdilution, the VITEK® 2 automatic system, and chromogenic media. Susceptibility testing of CA-MRSA requires special attention due to the high prevalence of difficult-to-detect OS-MRSA among them. Mis-prescription of beta-lactams for the treatment of OS-MRSA may lead to selection of high-level resistance and treatment failures. [ABSTRACT FROM AUTHOR]

Methicillin resistant Staphylococcus aureus (MRSA) is highly divergent antibiotic resistant bacteria earmarked as “High” in global pathogens’ priority list varying the severity and resistance geographically. Here, MRSA were screened using mecA gene with Cefoxitin and other 27 antibiotics of 19 classes using disc diffusion method from a highly humid climate of India. Multiple Antibiotic Resistance (MAR) index was calculated. Minimum Inhibitory Concentration (MIC) was determined against 11 classes of antibiotics. Detection of major virulence genes tst-1 and lukPV were done. A total of 95.24% Hospital Associated (HA)-MRSA, 56.14% Community Associated (CA)-MRSA and 82.53 % Livestock Associated (LA)-MRSA were detected. Cefoxitin, Oxacillin, Ciprofloxacin, Fusidic acid and Ticarcillin-Clavulinic acid resistance was observed in more than 60% of HA-MRSA, CA-MRSA and LA-MRSA. Across the hosts, Mupirocin, Gentamicin, Linezolid, Co-trimoxazole, and Rifampicin were found effective. Vancomycin Intermediate Staphylococcus aureus (VISA) detected in CA-MRSA & LA-MRSA. Multidrug Resistant (MDR) was found very high but extensively drug-resistant (XDR) was detected moderately. No pan drug-resistant (PDR) was detected. Virulence gene tst-1 and lukPV were detected in 7.69% and 32.69% MRSA isolates. The gene tst-1 is reported for the first time in pre and post-caesarian samples from Gynaecology department in this region with high MDR. This study showed S. aureus and subsequent prevalence of MRSA is higher in this region then national data. 2nd generation Cephalosporins were found effective which is very encouraging due to their limited uses. Detection of tst-1 in caesarian samples is a serious threat as neonatal transmission of MRSA from mother is reported. [ABSTRACT FROM AUTHOR]

Background & Objectives: Staphylococcus spp. is a resident flora of the skin and mucosa of humans that can colonize the anterior nares of individuals. This cross-sectional study was conducted to determine the rate and antibiotic resistance pattern of nasal Staphylococcus aureus (S. aureus) carriers among the staff of Fasa hospital, southern Iran. Materials & Methods: Nasal swab samples were collected from 117 hospital staff working in 12 wards. Microbiological culture method was applied for S. aureus identification. The isolates were confirmed by tuf gene identification using PCR assay. Five isolates were randomly sequenced and phylogenetically analysed using MEGA software. The antimicrobial resistance pattern of the isolates was evaluated using the disc diffusion assay and the amplification of the methicillin resistance (mecA) gene. Results: The prevalence of S. aureus nasal carriers included 10.26% (n=12). The nasal carriers were identified in the wards of surgery ICU, gynecologic surgery, NICU, pediatric, internal surgery, and emergency. Among them, gynecologic surgery staff had the highest rate of nasal colonization (33.33%). Phylogenetic analysis showed that of five isolates, four had high similarities with each other. Also, the highest rate of resistance was related to penicillin (83.3%), followed by cefazolin (75%), and cephalexin (75%). However, the highest level of susceptibility (100%) was found for vancomycin, cefoxitin, and oxacillin. Furthermore, the methicillin resistance gene (mecA) was highly detected (75%) from the isolates, elucidating oxacillin-susceptible or cefoxitin-susceptible mecA-positive S. aureus (OS-MRSA). Conclusions: The high rates of OS-MRSA can lead to antibiotic resistance among health care workers tremendously. Moreover, the high similarity probability in phylogenetic analysis shows the possibility of cross-infection between these health care workers, warning to exert effective strategies to control infection spread, especially in the surgery ward.

Background: Oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) represents an important issue, as its oxacillin susceptibility has contributed to misidentification by conventional susceptibility tests and consequently potential therapeutic failure, but limited data on the current status of OS-MRSA infection in Chinese hospitals are available. Methods: This multicenter study performed a battery of susceptibility tests and diagnostic tests for 956 S. aureus isolates from 10 hospitals, including automated susceptibility testing on VITEK 2, broth microdilution, disk diffusion, and detection of PBB2a, mecA gene and mecC gene. For all identified OS-MRSA, multi-locus sequence typing (MLST), together with spa typing, SCCmec typing and PVL detecting, was carried out. Results: OS-MRSA, most of which were from pediatric inpatients, represented 1.8% (17/956) of total isolates. Of these 17 OS-MRSA, 10 were ST59, followed by ST965 (3/17), and 11 carried SCCmec type IV, while 5 carried SCCmec type V, but only one was Panton–Valentine leucocidin (PVL)-positive, also, 16 had one or two point mutations within mecA promoter. OS-MRSA had inducible oxacillin resistance and significantly lower MDR (Multi-Drug Resistant) rate. We observed that the VITEK 2 system exhibited some deficiency in OS-MRSA detection, whereas cefoxitin disk diffusion was shown to be a reliable and cost-saving alternative and should be supplemented in detecting S. aureus with borderline oxacillin susceptible MICs. Conclusion: This study has characterized phenotypically and molecularly OS-MRSA in China, and provided insights into more effective management of OS-MRSA.

Mingbiao Ma,1– 3,* Minjun Chu,1,2,* Lvyan Tao,3– 5,* Jue Li,1,2 Xiaojuan Li,1,2 Hailin Huang,1,2 Kexuan Qu,6 Haiping Wang,1,2 Li Li,3– 5 Tingyi Du1– 3 1Department of Clinical Laboratory, Kunming Children’s Hospital, Kunming, Yunnan, People’s Republic of China; 2Department of Clinical Laboratory, Children’s Hospital Affiliated to Kunming Medical University, Kunming, Yunnan, People’s Republic of China; 3Yunnan Key Laboratory of Children’s Major Disease Research, Kunming Children’s Hospital, Kunming, Yunnan, People’s Republic of China; 4Yunnan Institute of Pediatrics, Kunming Children’s Hospital, Kunming, Yunnan, People’s Republic of China; 5Kunming Key Laboratory of Children Infection and Immunity, Kunming Children’s Hospital, Kunming, Yunnan, People’s Republic of China; 6Department of Blood Transfusion, Kunming Children’s Hospital, Kunming, Yunnan, People’s Republic of China*These authors contributed equally to this workCorrespondence: Li LiYunnan Institute of Pediatrics, Kunming Children’s Hospital, Kunming, Yunnan, People’s Republic of ChinaEmail lili@etyy.cnTingyi DuDepartment of Clinical Laboratory, Kunming Children’s Hospital, Kunming, Yunnan, People’s Republic of ChinaEmail dutingyi@etyy.cnPurpose: The present study investigated the prevalence characteristics of oxacillin susceptible mecA-positive Staphylococcus aureus (OS-MRSA) in a children’s hospital in Kunming from January 2019 to December 2020.Methods: A total of 499 S. aureus strains were included in the study and tested for oxacillin susceptibility using the VITEK 2 Compact automated antimicrobial susceptibility test system. All oxacillin-susceptible strains were detected mecA and mecC by polymerase chain reaction (PCR). E-test was used to compare the minimum inhibitory concentration (MIC) values of methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), and OS-MRSA for oxacillin, cefoxitin, penicillin, vancomycin, erythromycin, and clindamycin. Molecular typing of OS-MRSA was performed by MLST and SCCmec typing. Toxin genes were detected by PCR.Results: Forty-five OS-MRSA strains were detected, for an overall rate of 9.02% (45/499). The MICs of MSSA, OS-MRSA, and MRSA against oxacillin were concentrated at 0.38, 0.38, and 12 μg/mL, respectively; the cefoxitin MICs of MSSA and MRSA were concentrated at 2 and 32 μg/mL respectively; and MICs of OS-MRSA were concentrated at 2 and 8 μg/mL; penicillin, vancomycin and erythromycin MICs against MSSA, OS-MRSA, and MRSA showed same centralized points and were 32, 1, and 256 μg/mL, respectively; the MICs of clindamycin against MSSA were 0.5 μg/mL, while that against OS-MRSA and MRSA were concentrated at 256 μg/mL. Molecular typing of OS-MRSA was dominated by ST59-SCCmec IV. The carrier rates of hemolysin genes (hl-a, hl-d) and fibrinogen-binding clumping factor genes (clfA, clfB) were 100% in OS-MRSA, followed by 40% (18/45) for enterotoxin genes (sea, seb).Conclusion: OS-MRSA has a high detection rate in children, and main molecular typing is ST59-SCCmecIV in Kunming. The identification ability of automated antibacterial drug sensitivity test detection systems for OS-MRSA is very limited. A combination of phenotypic analysis and molecular detection should be used to improve OS-MRSA identification.Keywords: Staphylococcus aureus, MRSA, OS-MRSA, children, oxacillin

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. [ABSTRACT FROM AUTHOR]

Maria Stańkowska,1 Katarzyna Garbacz,1 Lidia Piechowicz,2 Marek Bronk3 1Department of Oral Microbiology, Faculty of Medicine, Medical University of Gdansk, Gdansk, Poland; 2Department of Medical Microbiology, Faculty of Medicine, Medical University of Gdansk, Gdansk, Poland; 3Laboratory of Clinical Microbiology, University Clinical Center, Gdansk, PolandCorrespondence: Katarzyna GarbaczDepartment of Oral Microbiology, Medical University of Gdansk, 25 Dębowa St, Gdansk 80-204, PolandTel +48 58 349 1900Fax +48 58 349 1668Email katarzyna.garbacz@gumed.edu.plBackground: In a recent decade, the occurrence of S. aureus isolates with low-level oxacillin resistance, have been reported increasingly. The aim of this study was to estimate the prevalence of S. aureus with low-level of oxacillin resistance and to determine their molecular characteristics, including spa types, SCCmec types and presence of toxin genes.Methods: A total of 249 S. aureus strains were analyzed. Antimicrobial susceptibility was preliminarily tested by the disk diffusion method, and further was verified with the E-test and agar dilution methods. All borderline oxacillin-resistant strains (BORSA) were screened for the mecA gene and virulence factors, including Panton-Valentine leukocidin (PVL). Staphylococcal cassette chromosome mec (SCCmec) typing and spa typing were also carried out.Results: Twelve (4.8%) borderline oxacillin-resistant strains with MIC ≤4 μg/mL were identified. Almost all strains (11/12) were oxacillin-susceptible methicillin resistant S. aureus carrying mecA gene (OS-MRSA). Among the 12 bordeline strains, five spa types (t437, t037, t015, t216, t267) and two SCCmec types (III, IV) were identified, with the most prevalent being t437-SCCmecIV pvl-positive. The second most frequent spa type, t037-SCCmecIII, was sea-positive and did not produce coagulase. The majority of borderline strains originated from skin infections and diabetic foot ulcers and were multidrug-resistant (macrolides, lincosamides and chloramphenicol).Conclusion: This study demonstrated that S. aureus with borderline resistance to oxacillin represented primarily SCCmecIV spa type t437 and coagulase-negative SCCmecIII spa type t037 and were isolated from skin infections and diabetic foot ulcers.Keywords: Staphylococcus aureus, MRSA, OS-MRSA, borderline oxacillin-resistant S. aureus, low-level oxacillin resistance

We report for the first time an oxacillin-susceptible mecA -positive Staphylococcus aureus (OS-MRSA) associated with a processed food product in Europe. One isolate (MRSA-ST5-type V SCC mec) was found in cheese among 600 food samples confiscated from air passengers from international flights in Vienna Airport (Austria). Type V SCC mec strains do not harbor functional mecI-mecR1 genes and in such strains mecA expression is regulated by the bla system (blaI-blaR1-blaZ). It has been recently reported that malfunctions in the bla system lead to the constitutive expression of mecA. The OS-MRSA reported in this study harbored the bla system on a plasmid and one deletion occurred in the blaR1 gene causing a frameshift variant that lead to an incomplete BlaR1 protein. This finding highlights the potential role of food as a neglected route of dissemination of emerging MRSA variants. • First characterization of OS-MRSA associated with processed food products in Europe. • Novel amino acid substitution identified in the transpeptidase domain of PBP2a. • Description of bla system carried on a plasmid. • Deletion in blaR1 gene lead to an incomplete BlaR1 protein. [ABSTRACT FROM AUTHOR]

Abstract mecA -positive oxacillin phenotypically susceptible Staphylococcus aureus (OS-MRSA) is increasingly reported worldwide. This bacterium poses a therapeutic threat, as it can be misidentified as an oxacillin-susceptible organism by phenotypic methods that are routinely used in the majority of clinical microbiology laboratories. Herein, we report the first case of fatal sepsis in a 43-year-old female patient caused by an OS-MRSA SCC mec type IVa/ST1/CC1 in a tertiary hospital in southern Brazil, which highlights the difficulties involved in diagnosing this bacterium. Blood cultures and phenotypic susceptibility tests on admission yielded a penicillin-resistant S. aureus. Although vancomycin therapy was initiated, this antibacterial was replaced by oxacillin, based on the susceptibility result. However, the clinical conditions of the patient deteriorated rapidly evolving to fatal septic shock. Clinical microbiology laboratories should consider the use of additional tests to accurately distinguish between various antimicrobial phenotypes of S. aureus. [ABSTRACT FROM AUTHOR]

Staphylococcus aureus isolates with discordant susceptibility results between oxacillin and cefoxitin obtained using automated microbiology systems are infrequently observed. From April 2013 to December 2014, 1956 methicillin-resistant S. aureus (MRSA) and 1761 methicillin-susceptible S. aureus isolates were obtained from different patients. Forty isolates (1.1% and 2% in case of S. aureus and MRSA, respectively) with discordant susceptibility results (oxacillin susceptible and cefoxitin resistant) and carrying mecA gene were obtained. Except 2 SCC mec type IV isolates, 38 MRSA isolates were all SCC mec type V (V T or non-V T ), which were further divided into V T (n = 22) and non-V T (n = 16). The most common spa type in V T and non-V T isolates were t437 (n = 19) and t1081 (n = 13), respectively. Only 55% of patients received effective antimicrobial agents; 2 mortalities were not attributable to MRSA infection. Using standard agar dilution, 17 MRSA isolates (0.46% and 0.87% in case of S. aureus and MRSA, respectively) had oxacillin MIC in the susceptible ranges (oxacillin-susceptible MRSA [OS-MRSA]); all carried SCC mec type V (V T , n = 8; non-V T , n = 9). The most common spa -MLST types of OS-MRSA in V T and non-V T were t437-ST59 (n = 4) and t1081-ST45 (n = 7), respectively. Concomitant testing by both cefoxitin- and oxacillin-based methods is a practical strategy for OS-MRSA detection in the clinical laboratories. Continuous monitoring of OS-MRSA isolates is necessary to elucidate their impact in clinical infectious diseases. [ABSTRACT FROM AUTHOR]

Background: Staphylococcus aureus is the major cause of global and nosocomial infections with a significant impact in hospitals worldwide. Our objective was to investigate clinical and molecular characteristics of S. aureus isolates causing infections in patients admitted to hospitals from Recife city, Brazil, and investigate the prevalence of oxacillin-susceptible mecA-positive S. aureus (OS-MRSA) in the region, as well as genetically characterize the isolates and compare with epidemic clones. Results: We characterized 89 isolates in total, 31 clinical methicillin-resistant S. aureus (MRSA) and 58 methicillin-sensitive (MSSA) isolates by PFGE, MLST, spa typing and SCCmec genotyping. Isolates belonging to international MRSA clones were present: Brazilian epidemic clone (BEC) (61 % of MRSA isolates), Paediatric (36 %), New York/Japan (3 %). Some MSSA isolates were related to MRSA clones: USA400-related (10 % of MSSA isolates), Berlin clone (2 %), Paediatric (14 %), New York/Japan (2 %) and Southwest Pacific clone (17 %). MLST revealed new sequence types (ST's): ST2381, ST2382, and ST2383 and new spa types: 10548 and 10550. Among isolates phenotypically identified as MSSA by antimicrobial susceptibility assays, we verified 30 oxacillin-susceptible isolates, which exhibited the mecA gene, without mec complex amplification and were thus classified as OS-MRSA. We observed clonal spread of MRSA and MSSA, including OS-MRSA, within several areas of the main hospital investigated and closely related isolates between hospitals analyzed. Conclusions: The results of this study suggest a possible spread of the strains in hospital environment that could be responsible for nosocomial infections. We documented the presence of several MRSA clones, as well as new MLST and spa types, that were responsible for severe infections in hospitalized patients. The finding of OS-MRSA isolates could have implications for therapy, because testing for mecA and PBP2a is not a routine procedure performed by clinical microbiology laboratories in Brazil and, as consequence, these isolates could be misclassified as MSSA. Our data alert to the necessity to develop more effective strategies for epidemiological control of S. aureus in order to avoid an increase of hospital infections provoked by this pathogen. We reinforce the use of genetic methods, in addition to phenotypic tests, for a precise identification of MRSA. [ABSTRACT FROM AUTHOR]

Purpose: Methicillin-resistant Staphylococcus aureus (MRSA) is defined as S. aureus genetically having the mecA or mecC genes or phenotypically showing minimum inhibitory concentration (MIC) of oxacillin higher than 2 mg/L. However, recently, cefoxitin/oxacillin-susceptible mecA-positive S. aureus (OS-MRSA) has been reported worldwide. Little is known about the prevalence and virulence of these strains among clinically significant isolates in the UK. The aims were to (1) investigate the prevalence of OS-MRSA in seven major hospitals in the Wessex region/UK from a cohort of 500 clinically significant phenotypically identified MSSA isolates, (2) genetically characterise OS-MRSA strains by pulsed-field gel electrophoresis (PFGE) and compare these to common UK epidemic strains; and (3) to determine Panton-Valentine leukocidin (PVL; lukFS) gene carriage rates among these isolates. Results: OS-MRSA was found in six isolates (1.2 %) of phenotypically identified and reported MSSA isolates by conventional methods. PFGE showed OS-MRSA strains to be genetically diverse and distinct from the common UK epidemic strains EMRSA-15 and EMRSA-16. None of these OS-MRSA stains carried the genes encoding PVL; however, overall positivity rate for PVL was 4.4 %, much higher than the nationally reported rates of 2 % in the UK. Conclusion: There are still many unknowns regarding phenotypic and/or genetic characterization of the emerging OS-MRSA isolates in the UK and worldwide. Data regarding their epidemiology and optimal therapy for infection are limited and need further investigation not only in the UK, but also worldwide, as it is likely to have an impact on the empirical treatment of S. aureus infections. [ABSTRACT FROM AUTHOR]

The aim of this study was to characterize phenotypically and genotypically 27 mecA positive Staphylococcus aureus strains with oxacillin MICs of ≤2μg/ml by Vitek 2, isolated in different regions of Uruguay. Susceptibility to oxacillin and cefoxitin was studied by gradient diffusion, disk diffusion to cefoxitin, and Phoenix and MicroScan systems. PBP2a was determined. SCCmec typing was performed and the isolates were compared by PFGE. Twenty-six isolates were susceptible to oxacillin; one strain was susceptible to cefoxitin by disk diffusion and 3 strains by gradient diffusion. Phoenix and MicroScan panels detected methicillin resistance in 25 and 27 strains, respectively. Twenty-six strains tested positive for PBP2a. Twenty-six strains carried SCCmec V and 24 belonged to pulsotype A. One strain carried SCCmec IV and did not belong to pulsotype A. Cefoxitin disk diffusion test and PBP2a detection correctly identified 26 of these 27 strains as MRSA. PFGE results suggest the dissemination of a cluster of MRSA carrying SCCmec V., (Copyright © 2022 Asociación Argentina de Microbiología. Publicado por Elsevier España, S.L.U. All rights reserved.)

Purpose: The present study investigated the prevalence characteristics of oxacillin susceptible mec A-positive Staphylococcus aureus (OS-MRSA) in a children's hospital in Kunming from January 2019 to December 2020., Methods: A total of 499 S. aureus strains were included in the study and tested for oxacillin susceptibility using the VITEK 2 Compact automated antimicrobial susceptibility test system. All oxacillin-susceptible strains were detected mec A and mec C by polymerase chain reaction (PCR). E-test was used to compare the minimum inhibitory concentration (MIC) values of methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), and OS-MRSA for oxacillin, cefoxitin, penicillin, vancomycin, erythromycin, and clindamycin. Molecular typing of OS-MRSA was performed by MLST and SCC mec typing. Toxin genes were detected by PCR., Results: Forty-five OS-MRSA strains were detected, for an overall rate of 9.02% (45/499). The MICs of MSSA, OS-MRSA, and MRSA against oxacillin were concentrated at 0.38, 0.38, and 12 μg/mL, respectively; the cefoxitin MICs of MSSA and MRSA were concentrated at 2 and 32 μg/mL respectively; and MICs of OS-MRSA were concentrated at 2 and 8 μg/mL; penicillin, vancomycin and erythromycin MICs against MSSA, OS-MRSA, and MRSA showed same centralized points and were 32, 1, and 256 μg/mL, respectively; the MICs of clindamycin against MSSA were 0.5 μg/mL, while that against OS-MRSA and MRSA were concentrated at 256 μg/mL. Molecular typing of OS-MRSA was dominated by ST59-SCCmec IV. The carrier rates of hemolysin genes ( hl-a, hl-d ) and fibrinogen-binding clumping factor genes ( clf A, clf B) were 100% in OS-MRSA, followed by 40% (18/45) for enterotoxin genes ( sea, seb )., Conclusion: OS-MRSA has a high detection rate in children, and main molecular typing is ST59-SCC mec IV in Kunming. The identification ability of automated antibacterial drug sensitivity test detection systems for OS-MRSA is very limited. A combination of phenotypic analysis and molecular detection should be used to improve OS-MRSA identification., Competing Interests: The authors declare that they have no conflicts of interest for this work., (© 2021 Ma et al.)

In this study, we explored the prevalence of oxacillin-susceptible mecA -positive Staphylococcus aureus (OS-MRSA) in staphylococcal food poisoning outbreak isolates and foodborne isolates, and then investigated their molecular characteristics, classical staphylococcal enterotoxins (SEs), and drug resistance. Eight (2.9%) of 275 isolates from food poisoning outbreaks and 7 (3.8%) of 184 isolates from retail foods were identified as OS-MRSA isolates. Among the 15 OS-MRSA isolates, the most frequently detected toxin genes were hld (100%), hla (93.3%), pvl (80.0%), and hlb (46.7%) followed by seg and seq (33.3%, each), hlg (26.7%), seb and hlgv (20.0%, each), sec , seh , sel , sep , and tst (13.3%, each), and sei , sem , sen , and seo (6.7%, each). None of isolates carried other tested virulence genes. The most frequently detected classical SEs were SEB and SEC (26.7%, each), followed by SEA and SEE (20.0%, each), and SED (6.7%). Resistance was most frequently observed in ampicillin, penicillin, and cefoxitin (100%, each), followed by trimethoprim/sulfamethoxazole (93.3%), erythromycin (73.3%), amoxicillin/clavulanic acid (46.7%), tetracyclines (26.7%), and ciprofloxacin (6.7%). All isolates were susceptible to other tested antibiotics. A dominant molecular type belonged to ST398-IVa-t034 (26.7%), followed by ST59-IVa-t437 (20.0%), ST88-III-t14340 and ST1-IVa-t114 (13.3%, each), and ST5-II-t002, ST630-t4549, ST5-II, and ST4495-t10738 (6.7%, each). Our findings indicated that OS-MRSA strains had a low prevalence rate among outbreak strains and foodborne strains, which frequently harbored SCCmec IVa, and carried a variety of toxin genes, and also expressed numerous classical SEs. In addition, all OS-MRSA isolates were susceptible to the majority of antibacterial agents except β -lactam. Our study is the first to report that OS-MRSA isolates are associated with food poisoning outbreaks worldwide.

Background: In a recent decade, the occurrence of S. aureus isolates with low-level oxacillin resistance, have been reported increasingly. The aim of this study was to estimate the prevalence of S. aureus with low-level of oxacillin resistance and to determine their molecular characteristics, including spa types, SCC mec types and presence of toxin genes., Methods: A total of 249 S. aureus strains were analyzed. Antimicrobial susceptibility was preliminarily tested by the disk diffusion method, and further was verified with the E-test and agar dilution methods. All borderline oxacillin-resistant strains (BORSA) were screened for the mec A gene and virulence factors, including Panton-Valentine leukocidin (PVL). Staphylococcal cassette chromosome mec (SCC mec ) typing and spa typing were also carried out., Results: Twelve (4.8%) borderline oxacillin-resistant strains with MIC ≤4 µg/mL were identified. Almost all strains (11/12) were oxacillin-susceptible methicillin resistant S. aureus carrying mecA gene (OS-MRSA). Among the 12 bordeline strains, five spa types (t437, t037, t015, t216, t267) and two SCC mec types (III, IV) were identified, with the most prevalent being t437-SCC mec IV pvl -positive. The second most frequent spa type, t037-SCC mec III, was sea -positive and did not produce coagulase. The majority of borderline strains originated from skin infections and diabetic foot ulcers and were multidrug-resistant (macrolides, lincosamides and chloramphenicol)., Conclusion: This study demonstrated that S. aureus with borderline resistance to oxacillin represented primarily SCC mec IV spa type t437 and coagulase-negative SCC mec III spa type t037 and were isolated from skin infections and diabetic foot ulcers., Competing Interests: The authors report no conflicts of interest in this work., (© 2019 Stańkowska et al.)