Oxacillin Minimum Inhibitory Concentration Research Articles

Short communication: N-Acetylcysteine–mediated augmentation of β-lactam antibacterial activity against methicillin-resistant Staphylococcus aureus isolated from bovine mastitis cases

The present study investigated the effects of N-acetylcysteine (NAC) on β-lactam antibacterial activity against 20 methicillin-resistant Staphylococcus aureus (MRSA) isolates from bovine mastitis. Minimum inhibitory concentrations (MIC) were determined by the E-test method. The presence of 10 mM NAC reduced the MIC of penicillin, ampicillin, oxacillin, cefoxitin, ceftazidime, and cefotaxime to MRSA. Importantly, the MIC of cefoxitin in MRSA in the presence of NAC was lower than the susceptible breakpoint of cefoxitin. The results provide a new way to use current β-lactam antibiotics combined with NAC against MRSA.

Bioguided isolation of antimicrobial polyphenols from Cuspidaria convoluta leaves and their synergistic effect with antibiotics

Objective: To identify and isolate phenolic compounds from Cuspidaria convoluta, and to evaluate their antibacterial activity and synergistic effect with antibiotics. Methods: The crude extract was prepared by maceration with methanol (5%). The dry extract was suspended in water and fractionated successively. The most active extract was selected by its antibacterial activity and its total phenol content was determined by spectrophotometry and by HPLC-MS/MS. Bioactive fractions of the most active extract were separated by column chromatography and evaluated by bioautography. Isolated compounds were identified. Minimum inhibitory concentration (MIC) of these compounds was determined by microdilution broth method, and synergism with antibiotics (ampicillin, gentamicin and oxacillin) was tested by checkerboard and time-kill assays. Results: Coumaric acid, catechin/epicatechin, and luteolin were purified and identified from the extract. There was an increase in the antibacterial activity of antibiotics when they were combined with these compounds. The combination of luteolin and ampicillin had the most potent antibacterial activities. The MICs of oxacillin for each of methicillin-resistant Staphylococcus aureus strains were reduced between 4 and 8-fold when these strains were coincubated with sub-MIC (≤ ½ MIC) levels of these compounds, demonstrating that the combination had synergistic effect for all cases. Conclusions: Cuspidaria convoluta contains important pharmacologically active substances that can be used to improve antibiotic efficacy.

Detection of β-lactamase and antibiotic susceptibility of clinical isolates of Staphylococcus aureus

Abstract Staphylococcus aureus (SA) is the pathogen of greatest concern in the clinical worldwide because of its intrinsic virulence and ability to cause the diverse array of life-threatening infections. In the last two decades or so, antimicrobial resistance has evolved into one of the most formidable problems of infectious diseases. Bacteria acquire resistance to β-lactam antibiotics, most commonly due to the production of enzymes called β-lactamase. This enzyme hydrolyzes the amide bond of β-lactam antibiotics to make them ineffective. In this study, screening of clinical isolates of SA showed β-lactamase activity in 11 clinical isolates (SA-1745, SA-2071, SA-2940, SA-3151, SA-4423, SA-4620, SA-4627, SA-4693, SA-4696, SA-10760, Methicillin-resistant Staphylococcus aureus (MRSA), and one wild- type strain (SA-96) by iodometric and nitrocefin disk methods. The antibiotic susceptibility profiles of 12 strains of SA were determined for four β-lactam antibiotics viz. penicillin, ampicillin, cefoxitin and oxacillin. All the clinical isolates showed resistant to these antibiotics. Further, minimum inhibitory concentration (MIC) of four β-lactam antibiotics was also determined by micro-broth dilution assay. The MIC of penicillin, ampicillin and oxacillin against wild-type strain of SA was 0.078 µg/ml, whereas the MIC of cefoxitin against wild-type strain was recorded as 1.25 µg/ml. The MIC of these antibiotics against 11 clinical isolates of SA including methicillin-resistant Staphylococcus aureus (MRSA) ranged between 1.95 µg/ml to 1000 µg/ml. All clinical isolates were highly resistant to penicillin and ampicillin followed by cefoxitin and oxacillin. The outcomes of the present study can be useful for the screening of other natural bioactive compounds against these virulent strains.

Characteristics of the antimicrobial resistance of Staphylococcus aureus isolated from chicken meat produced by different integrated broiler operations in Korea

Vertical integration of the broiler industry allows producers to combine different biosecurity and sanitation practices, housing technologies, and feeding regimens to improve food safety. The purpose of this study was to investigate the prevalence of Staphylococcus aureus (S. aureus) and to characterize the antimicrobial-resistant isolates recovered from 7 different integrated broiler operation systems in Korea. Among 200 chicken meat samples, 94 were observed to be positive for S. aureus. However, the prevalence varied from 25.0 to 58.3% in chicken meats, indicating variation in S. aureus occurrence among the operations. Four methicillin-resistant S. aureus isolates (MRSA) were recovered from 3 different operations. A high proportion of the S. aureus isolates were resistant to penicillins (51.2%), tetracycline (38.8%), and ciprofloxacin (CIP; 33.9%). Especially, 3 different operations showed a high number of CIP resistance (45.5∼100%) and multidrug resistance (50.0∼100%). Among 41 CIP-resistant S. aureus isolates, 75.6% showed a double amino-acid exchange of both gyrA and parC, with CIP minimum inhibitory concentrations (MIC) of ≥32 μg/mL. Four MRSA isolates showed resistance to 5 or 7 classes of antimicrobial agents, exhibiting oxacillin, CIP, and enrofloxacin MIC ranges of 16 to 128, 32 to 64, and 8 to 128 μg/mL, respectively, and had double mutations of S84L/S80F in gyrA/parC. Our findings suggest that S. aureus with resistance to important antimicrobial compounds can now be found in association with integrated broiler operations, providing the data to support the development of a monitoring and prevention program in integrated operations.

Characterization of the Lytic Capability of a LysK-Like Endolysin, Lys-phiSA012, Derived from a Polyvalent Staphylococcus aureus Bacteriophage

Antibiotic-resistant bacteria (ARB) have spread widely and rapidly, with their increased occurrence corresponding with the increased use of antibiotics. Infections caused by Staphylococcus aureus have a considerable negative impact on human and livestock health. Bacteriophages and their peptidoglycan hydrolytic enzymes (endolysins) have received significant attention as novel approaches against ARB, including S. aureus. In the present study, we purified an endolysin, Lys-phiSA012, which harbors a cysteine/histidine-dependent amidohydrolase/peptidase (CHAP) domain, an amidase domain, and a SH3b cell wall binding domain, derived from a polyvalent S. aureus bacteriophage which we reported previously. We demonstrate that Lys-phiSA012 exhibits high lytic activity towards staphylococcal strains, including methicillin-resistant S. aureus (MRSA). Analysis of deletion mutants showed that only mutants possessing the CHAP and SH3b domains could lyse S. aureus, indicating that lytic activity of the CHAP domain depended on the SH3b domain. The presence of at least 1 mM Ca2+ and 100 µM Zn2+ enhanced the lytic activity of Lys-phiSA012 in a turbidity reduction assay. Furthermore, a minimum inhibitory concentration (MIC) assay showed that the addition of Lys-phiSA012 decreased the MIC of oxacillin. Our results suggest that endolysins are a promising approach for replacing current antimicrobial agents and may contribute to the proper use of antibiotics, leading to the reduction of ARB.

Prevalence of nasal colonization of methicillin-resistant Staphylococcus aureus among schoolchildren of Barabanki district, Uttar Pradesh, India.

Introduction:The study aimed to determine the prevalence of nasal colonization of methicillin-resistant Staphylococcus aureus (MRSA), the minimum inhibitory concentration (MIC) of oxacillin and vancomycin, inducible clindamycin resistance, and antimicrobial resistance pattern of S. aureus among children of Barabanki district, Uttar Pradesh, India.Materials and Methods:School-going children of age group of 5–15 years were identified and selected according to the inclusion and exclusion criteria. Two nasal swabs were collected from each child as per the Centers for Disease Control and Prevention guidelines and transported to laboratory. Swabs were cultured on mannitol salt agar and 5% blood agar and incubated for 18–24 h at 37°C. Identification was done as per routine laboratory protocol. Detection of MRSA was done through cefoxitin 30 μg discs and D-zone test. Antibiotic susceptibility pattern of S. aureus by Kirby–Bauer disc diffusion method along with MIC for oxacillin and vancomycin was performed simultaneously according to Clinical Laboratory Standards Institute guidelines.Results:Out of 300 children, 140 (46.67%) were found to be nasal carriage for S. aureus, among which MRSA was found to be 23 (7.67%). All S. aureus and MRSA isolates were sensitive to vancomycin with MIC <2 μg/ml, whereas 23 S. aureus were found resistant to oxacillin with MIC value >4 μg/ml. Resistance to penicillin and co-trimoxazole was highest, whereas all were sensitive to linezolid. MRSA showed 100% susceptibility to linezolid, followed by gentamicin (91.4%) and tetracycline (87%).Conclusion:With the risk involved in transmission of infection, steps for identifying the carriers and its eradication should be carried out. Rational use of antibiotics should be given preference too.

Biofilm production by Staphylococcal isolates from chronic osteomyelitis and orthopedic device-related infections in a tertiary care centre, Imphal

Background: Microorganisms growing in a biofilm are associated with chronic, recurrent infections, and health problems for patients with indwelling devices which are difficult to treat with antimicrobial agents. Staphylococcus species are the most common microorganisms isolated from these patients. This study was conducted to detect biofilm production in Staphylococcal isolates from orthopedic device-related infections (ODRI) and chronic osteomyelitis (CO) and correlation of antimicrobial susceptibility pattern between biofilm producers and nonbiofilm producers. Materials and Methods: A cross-sectional study was carried out at the Department of Microbiology in a tertiary care center after the Institutional Ethics Committee approval, from October 2014 to September 2016. Detection of biofilm was performed by Congo Red Agar (CRA), tube method (TM), and tissue culture plate (TCP) methods. Antimicrobial susceptibility test was performed by Kirby–Bauer disk diffusion method, and minimum inhibitory concentration for oxacillin and vancomycin was determined according to the Clinical Laboratory Standard Institute guidelines. Results: A total of 51 clinical isolates, consisting of 45 Staphylococcus aureus and 6 coagulase-negative Staphylococci (CoNS), were isolated from 47 CO and 4 ODRI patients of which 32 were methicillin-resistant S. aureus and 2 MRCONS. Biofilm production was detected in 22 isolates by TCP, 18 by TM, and 13 by CRA methods. Fifty percent ODRI isolates and 42.5% CO isolates were biofilm producers. The biofilm producers have lower antimicrobial sensitivity pattern in all except vancomycin. Conclusion: Biofilm producers were detected from 43.1% of total isolates with higher antimicrobial resistance pattern.

Nasal Carriage Rate of Methicillin Resistant Staphylococcus aureus among Health Care Workers at a Tertiary Care Hospital in Kathmandu, Nepal.

Methicillin-resistant Staphylococcus aureus is one of the most common causes of nosocomial infections. Due to its multidrug resistant nature; infections due to Methicillin-resistant Staphylococcus aureus are often very difficult to treat. Colonized health care workers are the important sources of Methicillin-resistant Staphylococcus aureus. The objectives of this study were to determine the nasal carriage rate of Methicillin-resistant Staphylococcus aureus among health care workers at Kathmandu Medical College and Teaching Hospital, Nepal and to assess their antimicrobial susceptibility patterns. A cross sectional study was conducted among 252 health care workers from July to November 2013. Mannitol salt agar was used to culture the nasal swabs. Antimicrobial susceptibility testing was performed by Kirby-Bauer disc diffusion technique following Clinical and Laboratory Standards Institute guidelines. Methicillin-resistant Staphylococcus aureus strains were confirmed by using cefoxitin disc and by determining the minimum inhibitory concentration of oxacillin by agar dilution method. Of 252 healthcare workers, 46(18.3%) were positive for Staphylococcus aureus among which 19(41.3%) were Methicillin-resistant Staphylococcus aureus carriers. Overall rate of nasal carriage of Methicillin-resistant Staphylococcus aureus was 7.5% (19/252).The higher percentages of lab personnel were nasal carriers of S. aureus (31.6%) and Methicillin-resistant Staphylococcus aureus (10.5%).The percentages of nasal carriage of S. aureus (35.7%) and Methicillin-resistant Staphylococcus aureus (14.3%) were highest in the health care workers from post operative department. Higher percentage of Methicillin-resistant Staphylococcus aureus were susceptible toward amikacin (100%) and vancomycin (100%) followed by cotrimoxazole (84.2%). High rates of nasal carriage of S. aureus and Methicillin-resistant Staphylococcus aureus were observed among the healthcare workers, which indicate the need of strict infection control measures to be followed to control the nosocomial infections.

Antibiotic resistance and biofilm production among the strains of Staphylococcus aureus isolated from pus/wound swab samples in a tertiary care hospital in Nepal

BackgroundThe increasing drug resistance along with inducible clindamycin resistance, methicillin resistance and biofilm production among the strains of Staphylococcus aureus are present as the serious problems to the successful treatment of the infections caused by S. aureus. So, the main objectives of this study were to determine the antimicrobial susceptibility patterns along with the rates of inducible clindamycin resistance, methicillin resistance and biofilm production among the strains of S. aureus isolated from pus/wound swab samples.MethodsA total of 830 non-repeated pus/wound swab samples were processed using standard microbiological techniques. The colonies grown were identified on the basis of colony morphology, Gram’s stain and biochemical tests. Antimicrobial susceptibility testing was performed by Kirby–Bauer disc diffusion technique. Detection of inducible clindamycin resistance was performed by D test, while detection of methicillin resistant S. aureus (MRSA) was performed by determination of minimum inhibitory concentration of oxacillin by agar dilution method. Similarly, detection of biofilm formation was performed by microtiter plate method. Strains showing resistance to three or more than three different classes of antibiotics were considered multidrug resistant.ResultsTotal 76 samples showed the growth of S. aureus, among which 36 (47.4%) contained MRSA and 17 (22.4%) samples were found to have S. aureus showing inducible clindamycin resistance. Among the S. aureus isolated from outpatients, 41.9% were MRSA. Highest rates of susceptibility of S. aureus were seen toward linezolid (100%) and vancomycin (100%). Similarly, S. aureus isolated from 35 (46.1%) samples were found to be biofilm producers. Higher rate of inducible clindamycin resistance was seen among MRSA in comparison to methicillin susceptible S. aureus (MSSA). Similarly, higher rates of multidrug resistance and methicillin resistance were found among biofilm producing strains in comparison to biofilm non producing strains.ConclusionsThe rate of isolation of MRSA from community acquired infections was found to be high in Nepal. Increased rate of inducible clindamycin resistance as compared to previous studies in Nepal was noted. So for the proper management of the infections caused by S. aureus, D test for the detection of inducible clindamycin resistance should be included in the routine laboratory diagnosis. Further, detection of biofilm production should also be included in the routine tests. Linezolid and vancomycin can be used for the preliminary treatment of the serious infections caused by S. aureus.

Sphygmomanometers and thermometers as potential fomites of Staphylococcus haemolyticus: biofilm formation in the presence of antibiotics.

BACKGROUND The association between Staphylococcus haemolyticus and severe nosocomial infections is increasing. However, the extent to which fomites contribute to the dissemination of this pathogen through patients and hospital wards remains unknown.OBJECTIVES In the present study, sphygmomanometers and thermometers were evaluated as potential fomites of oxacillin-resistant S. haemolyticus (ORSH). The influence of oxacillin and vancomycin on biofilm formation by ORSH strains isolated from fomites was also investigated.METHODS The presence of ORSH on swabs taken from fomite surfaces in a Brazilian hospital was assessed using standard microbiological procedures. Antibiotic susceptibility profiles were determined by the disk diffusion method, and clonal distribution was assessed in pulsed-field gel electrophoresis (PFGE) assays. Minimum inhibitory concentrations (MICs) of oxacillin and vancomycin were evaluated via the broth microdilution method. Polymerase chain reaction (PCR) assays were performed to detect the mecA and icaAD genes. ORSH strains grown in media containing 1/4 MIC of vancomycin or oxacillin were investigated for slime production and biofilm formation on glass, polystyrene and polyurethane catheter surfaces.FINDINGS ORSH strains comprising five distinct PFGE types were isolated from sphygmomanometers (n = 5) and a thermometer (n = 1) used in intensive care units and surgical wards. ORSH strains isolated from fomites showed susceptibility to only linezolid and vancomycin and were characterised as multi-drug resistant (MDR). Slime production, biofilm formation and the survival of sessile bacteria differed and were independent of the presence of the icaAD and mecA genes, PFGE type and subtype. Vancomycin and oxacillin did not inhibit biofilm formation by vancomycin-susceptible ORSH strains on abiotic surfaces, including on the catheter surface. Enhanced biofilm formation was observed in some situations. Moreover, a sub-lethal dose of vancomycin induced biofilm formation by an ORSH strain on polystyrene.MAIN CONCLUSIONS Sphygmomanometers and thermometers are fomites for the transmission of ORSH. A sub-lethal dose of vancomycin may favor biofilm formation by ORSH on fomites and catheter surfaces.

Bacteriological Profile and Antimicrobial Susceptibility Patterns of Bacteria Isolated from Pus/Wound Swab Samples from Children Attending a Tertiary Care Hospital in Kathmandu, Nepal.

In Nepal, little is known about the microbiological profile of wound infections in children and their antimicrobial susceptibility patterns. Total of 450 pus/wound swab samples collected were cultured using standard microbiological techniques and the colonies grown were identified with the help of biochemical tests. The antimicrobial susceptibility testing was performed by Kirby-Bauer disc diffusion technique. Methicillin-resistant Staphylococcus aureus isolates were detected by using cefoxitin disc and confirmed by determining minimum inhibitory concentrations (MIC) of oxacillin. 264 (59%) samples were culture positive. The highest incidence of bacterial infections was noted in the age group of less than 1 year (76%). Out of 264 growth positive samples, Gram-positive bacteria were isolated from 162 (61%) samples and Gram-negative bacteria were found in 102 (39%) samples. Staphylococcus aureus (99%) was the predominant Gram-positive bacteria isolated and Pseudomonas aeruginosa (44%) was predominant Gram-negative bacteria. About 19% of S. aureus isolates were found to be methicillin-resistant MIC of oxacillin ranging from 4 μg/mL to 128 μg/mL. Among the children of Nepal, those of age less than 1 year were at higher risk of wound infections by bacteria. Staphylococcus aureus followed by Pseudomonas aeruginosa were the most common bacteria causing wound infections in children.

Pterostilbene Enhanced Anti-Methicillin Resistant &lt;i&gt;Staphylococcus aureus&lt;/i&gt; (MRSA) Activity of Oxacillin

Methicillin-resistant Staphylococcus aureus (MRSA) is a deadly pathogen that initially was limited to hospital and healthcare facilities but has gradually became a growing problem in healthy children and adults. Pterostilbene belongs to the phenylpropanoid phytoalexin which is involved in plant response to various pathogen and herbivores attack. The aim of this study is to evaluate the anti-MRSA action of pterostilbene in combination with selected antibiotics; vancomycin, linezolid and oxacillin against ATCC 43300 and ATCC 33591. The minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and fractional inhibitory concentration (FIC) index values were determined. Microbroth dilution technique and microdilution checkerboard (MDC) assay were employed. The MIC and MBC of pterostilbene against ATCC 33591 was 31.25 and 62.50 µg mL-1, respectively. While for ATCC 43300, the MBC value was also twice (62.50 µg mL-1) its MIC value of 31.25 µg mL-1. This indicated that pterostilbene was bacteriostatic against both MRSA strains. Our MIC/MBC study also showed that linezolid exhibited bacteriostatic action but, oxacillin and vancomycin were bactericidal. MDC study showed that pterostilbene-oxacillin combination exhibited lowest FIC value (0.56) against both MRSA strains which indicated partial synergistic interaction. On the other hand, pterostilbene was additive (FIC 1.00) in combination with vancomycin whereas pterostilbene-linezolid combination displayed indifference effect with FIC of 1.25 against both MRSA strains. Pterostilbene in combination with oxacillin partially enhanced anti-MRSA activity with twofold reduction in MIC of oxacillin by acting at different site at the bacterial cell wall from that of oxacillin but more specific to the site of action of vancomycin.

Presence of mecA-positive multidrug-resistant Staphylococcus epidermidis in bovine milk samples in Brazil

Bacteria of the genus Staphylococcus are one of the major pathogens causing bovine mastitis. In recent decades, resistance of this genus to oxacillin (methicillin) has been a matter of concern due to the possibility of reducing the effectiveness of mastitis treatments and the transfer of resistance determinants to other bacteria. Oxacillin resistance was studied in 170 staphylococci from bovine milk samples, including 79 Staphylococcus aureus and 91 coagulase-negative staphylococci (CNS). The susceptibility profile of 10 antimicrobial agents used in veterinary practice was determined by the Etest method. In addition to the Etest, the phenotypic characterization of oxacillin resistance was tested using the cefoxitin disk diffusion test. All isolates were screened by PCR to detect the mecA gene in 2 different regions of the gene. The isolates with an oxacillin minimum inhibitory concentration ≥0.5µg/mL or resistant to cefoxitin were identified by sequencing a 536-bp fragment of the 16S rRNA gene. This group of isolates was also evaluated for the presence of blaZ and mecC genes. Molecular analysis of the mecA gene was carried out by typing of the staphylococcal cassette chromosome mec (SCCmec). The relatedness of the mecA-positive isolates was evaluated by macrorestriction of chromosomal DNA followed by pulsed-field gel electrophoresis. With the exception of penicillin and oxacillin, 86% of the isolates showed susceptibility to cephalothin, gentamicin, erythromycin, sulfonamide, trimethoprim-sulfamethoxazole, and tetracycline. All S. aureus isolates were susceptible to oxacillin, whereas 47% (n=43) of the CNS isolates were resistant. The CNS isolates showed a higher resistance to cephalothin, erythromycin, tetracycline, and gentamicin in comparison with S. aureus. The mecA gene was only detected in 10 CNS isolates, identified as Staphylococcus epidermidis, and classified into 3 pulsotypes (A, B, and C) and 4 subtypes (A1, B1, B2, and B3). Among the isolates with an oxacillin resistance phenotype, 12 were positive for the blaZ gene, and 9 of them were mecA-positive. Two of the oxacillin-resistant isolates amplified the mecA homolog gene of Staphylococcus sciuri and none amplified mecC. Three SCCmec types, I, IV, and V, were found. Our results suggest that Staphylococcus epidermidis can be a reservoir for mecA for other Staphylococcus species. Studies investigating the molecular and phenotypic profile of antimicrobial resistance in staphylococcal species should be performed for controlling the spread of resistance and the selection of appropriate therapeutic measures.

Molecular study on methicillin-resistant Staphylococcus aureus strains isolated from dogs and associated personnel in Jordan

ObjectiveTo determine the prevalence, genetic relatedness, and pattern of antimicrobial susceptibility in methicillin-resistant Staphylococcus aureus (S. aureus) (MRSA) isolated from household dogs, farm dogs, and stray dogs, compared to isolates from their associated personnel. MethodsMRSA was isolated from 250 nasal swabs (150 swabs from dogs and 100 swabs from humans). PCR assays were used to detect the presence of both the nuc and mecA genes, which confirmed the identity of S. aureus isolates and the presence of methicillin resistance, respectively. Disk diffusion was used to determine the antibiotic susceptibility against 15 antimicrobial agents along with an E-test that determined the minimum inhibitory concentration for oxacillin. Pulsed field gel electrophoresis was conducted to determine the genetic relatedness of MRSA isolates from dogs to those from associated and unassociated personnel. ResultsThe prevalence of S. aureus in dogs and humans was 12.7% and 10.0% respectively, while the prevalence of MRSA isolates in dogs and humans was 5.3% and 5.0%, respectively. The prevalence of MRSA isolates in household dogs, farm dogs, and stray dogs was 7.8%, 4.7%, and 0.0%, respectively. MRSA isolates demonstrated a significantly higher rate of multi-resistance against three or more antimicrobial agents than methicillin-susceptible S. aureus (MSSA). Trimethoprim-sulphamethoxazole and chloramphenicol were the most effective antibiotics against all MRSA isolates. Pulsed field gel electrophoresis revealed a strong association between dog MRSA isolates and MRSA isolates from strongly associated personnel. ConclusionsMRSA is prevalent in house dogs, as well as in dog rearing centers and among their strongly associated personnel. A strong association was found between the MRSA isolates from dogs and those from humans who are in close contact. In addition, MRSA isolates showed a high rate of multi-resistance compared to MSSA isolates.

Driving Forces of Mechanisms Regulating Oxacillin-Resistance Phenotypes of MRSA: Truly Oxacillin-Susceptible mecA-Positive Staphylococcus aureus Clinical Isolates also Exist.

As MRSA are considered Staphylococcus aureus isolates with oxacillin minimum inhibitory concentration (MIC) of &#8805;4 mg/L or harboring the mecA gene. However, the presence of mecA does not necessarily lead to oxacillin resistance and mecA gene-carrying isolates may have oxacillin MIC within the susceptible range (&#8805;2 mg/L). During the last few years it has become apparent that oxacillin-susceptible (OS) mecA-positive S. aureus isolates (commonly called OS-MRSA) are rather commonly detected worldwide and may remain undiagnosed using phenotypic susceptibility testing methods. This review will summarize the current reports on OS-MRSA isolations and the underlying mechanisms regulating the expression of oxacillin resistance and also oxacillin susceptibility in mecA-positive S. aureus isolates. As MRSA commonly cause severe infections against which effective therapies are limited, understanding of these mechanisms could enable the identification of new targets for the treatment or reversion of the MRSA phenotype.

Prevalence and antimicrobial susceptibility pattern of methicillin-resistant, vancomycin-resistant, and Panton-Valentine leukocidin positive Staphylococcus aureus in a tertiary care hospital Dhaka, Bangladesh

ObjectivesTo observe the prevalence of methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant S. aureus (VRSA), and Panton-Valentine leukocidin (PVL)-positive S. aureus, this study was carried out in a tertiary care hospital in Dhaka, Bangladesh. Materials and methodsS. aureus strains were recovered from 200 postoperative wound swab samples from patients hospitalized in Dhaka Medical College Hospital between July 2011 and June 2012. Methicillin resistance was determined by the oxacillin and cefoxitin disc diffusion method, the minimum inhibitory concentration (MIC) of oxacillin, and mecA gene detection. VRSA resistance was determined by the disc diffusion method, the MIC of vancomycin, and screening for the vanA and vanB genes. The PVL gene was also detected in MRSA strains. ResultsFifteen of the 44 isolated strains of S. aureus were MRSA (2 of them were VRSA) and 29 were methicillin-sensitive S. aureus. All MRSA isolates were highly resistant to oxacillin (MIC ≥ 256 μg/mL). When compared with polymerase chain reaction (PCR), the sensitivity and specificity of the oxacillin disc diffusion method were 93.33% and 100% respectively; for the cefoxitin disc diffusion method and MIC of oxacillin both the sensitivity and specificity were 100%. Four (26.67%) MRSA isolates were positive for PVL genes which were also mecA positive. The MRSA strains were highly resistant to ciprofloxacin (93.33%), ceftriaxone (86.63%), azithromycin (73.33%), gentamycin (73.33%), and amoxiclav (66.67%). All (100%) MRSA strains were sensitive to linezolid and 86.67% were sensitive to vancomycin. The VRSA strains had an MIC ≥256 μg/mL for vancomycin and were positive for the vanB gene but negative for the vanA gene. ConclusionThe results of this study provide insight into the high proportion of MRSA and presence of VRSA in Bangladesh.

Impact of prior healthcare-associated exposure on clinical and molecular characterization of methicillin-susceptible Staphylococcus aureus bacteremia: results from a retrospective cohort study.

By virtue of medical advances and an aging society, people have increased opportunities for healthcare exposure. Little is known about the impact of healthcare exposure on the clinical features and molecular typing of methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia. We classified the onset of MSSA bacteremia into 3 mutually exclusive categories according to the Centers for Disease Control definition, and conducted a retrospective cohort study to investigate the differences among patients with community-associated (CA), healthcare-associated community onset (HACO), and hospital onset (HO) MSSA bacteremia at a medical center from January 1, 2002 through December 31, 2011. Antibiotic susceptibilities and multilocus sequence typing of MSSA isolates were also determined. A total of 290 patients with MSSA bacteremia, including of 165 (56.9%), 91 (31.4%), and 34 (11.7%) of HACO, HO, and CA, respectively, were studied. ST188 (29.3%) was the most common sequence type regardless of classification. Patients with HACO bacteremia were significantly older, had more solid tumors, higher Charlson scores, and more catheter-related bloodstream infections than those with CA bacteremia. The proportions of osteoarticular infections among patients with both HACO and CA bacteremia were higher than that of patients with HO bacteremia. By univariate analysis, patients with HO bacteremia had significantly higher in-hospital mortality compared to those with CA or HACO bacteremia (31.9% vs 18.8% and 20.4%). Multivariate analysis showed that Charlson score (odds ratio [OR], 1.29; 95% confidence interval [CI], 1.10-1.52), septic shock (OR, 5.28; 95% CI, 2.37-11.78), liver cirrhosis (OR, 3.57; 95% CI, 1.14-11.24), receipt of β-lactams other than oxacillin and cefazolin as definitive therapy (OR, 9.27; 95% CI, 4.25-20.23), and higher oxacillin minimum inhibitory concentration (MIC) (≥0.5 mg/L) (OR, 2.35; 95% CI, 1.05-5.25) of the causative pathogen were independently associated with in-hospital mortality. In conclusion, patients with HACO bacteremia had different host factors compared with those with CA bacteremia. Infection foci varied with different onset settings. Overall, ST188 was the most predominant sequence type. Onset settings were not independently associated with outcomes.

Addition of thymidine to culture media for accurate examination of thymidine-dependent small-colony variants of methicillin-resistant Staphylococcus aureus: A pilot study

Small-colony variants (SCVs) are slow-growing subpopulations of various auxotrophic bacterial strains. Thymidine-dependent SCVs (TD-SCVs) are unable to synthesize thymidine; hence, these variants fail to grow in a medium without thymidine. In this study, we used 10 TD-SCVs of Staphylococcus aureus, of which four strains possessed mecA. We compared the efficacy of a newly modified medium containing thymidine for the detection of TD-SCVs of methicillin-resistant S. aureus (MRSA) to the efficacy of routinely used laboratory media. We observed that none of the 10 TD-SCVs of S. aureus grew in Mueller–Hinton agar, and four TD-SCVs of MRSA failed to grow on all MRSA screening media, except for the ChromID™ MRSA medium. Laboratory tests conducted using medium with thymidine incorporated showed that thymidine did not affect the minimum inhibitory concentrations of oxacillin and cefoxitin for clinical isolates of S. aureus, and was able to detect MRSA, including TD-SCVs. These findings showed that thymidine-incorporated media are able to detect TD-SCVs of MRSA without altering the properties of other clinically isolated MRSA strains.

Genotypes and oxacillin resistance of Staphylococcus aureus from chicken and chicken meat in Poland

The genotypes and oxacillin resistance of 263 Staphylococcus aureus isolates cultured from chicken cloacae (n = 138) and chicken meat (n = 125) was analyzed. Fifteen spa types were determined in the studied S. aureus population. Among 5 staphylococcal protein A gene (spa) types detected in S. aureus from chicken, t002, t3478, and t13620 were the most frequent. Staphylococcus aureus isolates from meat were assigned to 14 spa types. Among them, the genotypes t002, t056, t091, t3478, and t13620 were dominant. Except for 4 chicken S. aureus isolates belonging to CC398, the remaining 134 isolates were clustered into multilocus sequence clonal complex (CC) 5. Most of meat-derived isolates were assigned to CC5, CC7, and CC15, and to the newly described spa-CC12954 complex belonging to CC1. Except for t011 (CC398), all other spa types found among chicken isolates were also present in isolates from meat. Four S. aureus isolated from chicken and one from meat were identified as methicillin-resistant S. aureus (MRSA) with oxacillin minimum inhibitory concentrations from 16 to 64 μg/mL. All MRSA were assigned to spa types belonging to ST398, and included 4 animal spa t011 SCCmec V isolates and 1 meat-derived spa t899, SCCmec IV isolate. Borderline oxacillin-resistant S. aureus (BORSA) isolates, shown to grow on plates containing 2 to 3 μg/mL of oxacillin, were found within S. aureus isolates from chicken (3 isolates) and from meat (19 isolates). The spa t091 and t084 dominated among BORSA from chicken meat, whereas t548 and t002 were found within animal BORSA. We report for the first time the presence of MRSA in chicken in Poland. We demonstrate that MRSA CC398 could be found in chicken meat indicating potential of introduction of animal-associated genotypes into the food chain. We also report for the first time the possibility of transmission of BORSA isolates from chicken to meat.

Detection of methicillin-resistant Staphylococcus aureus carrying the mecC gene in human samples in Slovenia

SUMMARY Following the recognition of a mecC MRSA isolate from a patient hospitalized in the northeastern region of Slovenia, a national collection of 395 community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) isolates from 2006 to 2013 was screened. An additional six mecC MRSA strains were found and characterized as spa types t843, t9397 and t10009, and multilocus sequence type ST130. The low oxacillin minimum inhibitory concentrations and absence of the mecA gene make recognition of these MRSA strains problematical for diagnostic laboratories. In such strains the presence of mecC should be determined.