scholarly journals Comparison of Different Phenotypic Approaches To Screen and Detect mecC-Harboring Methicillin-Resistant Staphylococcus aureus

2017 ◽  
Vol 56 (1) ◽  
Author(s):  
André Kriegeskorte ◽  
Evgeny A. Idelevich ◽  
Andreas Schlattmann ◽  
Franziska Layer ◽  
Birgit Strommenger ◽  
...  
Keyword(s):  
Staphylococcus Aureus ◽  
Susceptibility Testing ◽  
Disk Diffusion ◽  
Broth Microdilution ◽  
Disk Diffusion Test ◽  
Methicillin Resistant ◽  
Content Type ◽  
Vitek 2 ◽  
Cefoxitin Disk ◽  
Cefoxitin Disk Diffusion

ABSTRACT Similar to mecA, mecC confers resistance against beta-lactams, leading to the phenotype of methicillin-resistant Staphylococcus aureus (MRSA). However, mecC-harboring MRSA strains pose special difficulties in their detection. The aim of this study was to assess and compare different phenotypic systems for screening, identification, and susceptibility testing of mecC-positive MRSA isolates. A well-characterized collection of mecC-positive S. aureus isolates (n = 111) was used for evaluation. Routinely used approaches were studied to determine their suitability to correctly identify mecC-harboring MRSA, including three (semi)automated antimicrobial susceptibility testing (AST) systems and five selective chromogenic agar plates. Additionally, a cefoxitin disk diffusion test and an oxacillin broth microdilution assay were examined. All mecC-harboring MRSA isolates were able to grow on all chromogenic MRSA screening plates tested. Detection of these isolates in AST systems based on cefoxitin and/or oxacillin testing yielded overall positive agreements with the mecC genotype of 97.3% (MicroScan WalkAway; Siemens), 91.9% (Vitek 2; bioMérieux), and 64.9% (Phoenix, BD). The phenotypic resistance pattern most frequently observed by AST devices was “cefoxitin resistance/oxacillin susceptibility,” ranging from 54.1% (Phoenix) and 83.8% (Vitek 2) to 92.8% (WalkAway). The cefoxitin disk diffusion and oxacillin broth microdilution assays categorized 100% and 61.3% of isolates to be MRSA, respectively. The chromogenic media tested confirmed their suitability to reliably screen for mecC-harboring MRSA. The AST systems showed false-negative results with varying numbers, misidentifying mecC-harboring MRSA as methicillin-susceptible S. aureus. This study underlines cefoxitin's status as the superior surrogate mecC-positive MRSA marker.

scholarly journals Synergy of β-Lactams with Vancomycin against Methicillin-Resistant Staphylococcus aureus: Correlation of Disk Diffusion and Checkerboard Methods

2015 ◽  
Vol 54 (3) ◽  
pp. 565-568 ◽  
Author(s):  
Cheng Len Sy ◽  
Tsi-Shu Huang ◽  
Chii Shiang Chen ◽  
Yao-Shen Chen ◽  
Hung-Chin Tsai ◽  
...  
Keyword(s):  
Staphylococcus Aureus ◽  
Screening Method ◽  
Automated System ◽  
Disk Diffusion ◽  
Methicillin Resistant ◽  
Content Type ◽  
Potential Synergy ◽  
Zones Of Inhibition ◽  
Vitek 2 ◽  
Mdd Method

Modified disk diffusion (MDD) and checkerboard tests were employed to assess the synergy of combinations of vancomycin and β-lactam antibiotics for 59 clinical isolates of methicillin-resistantStaphylococcus aureus(MRSA) and Mu50 (ATCC 700699). Bacterial inocula equivalent to 0.5 and 2.0 McFarland standard were inoculated on agar plates containing 0, 0.5, 1, and 2 μg/ml of vancomycin. Oxacillin-, cefazolin-, and cefoxitin-impregnated disks were applied to the surface, and the zones of inhibition were measured at 24 h. The CLSI-recommended checkerboard method was used as a reference to detect synergy. The MICs for vancomycin were determined using the Etest method, broth microdilution, and the Vitek 2 automated system. Synergy was observed with the checkerboard method in 51% to 60% of the isolates when vancomycin was combined with any β-lactam. The fractional inhibitory concentration indices were significantly lower in MRSA isolates with higher vancomycin MIC combinations (P< 0.05). The overall agreement between the MDD and checkerboard methods to detect synergy in MRSA isolates with bacterial inocula equivalent to McFarland standard 0.5 were 33.0% and 62.5% for oxacillin, 45.1% and 52.4% for cefazolin, and 43.1% and 52.4% for cefoxitin when combined with 0.5 and 2 μg/ml of vancomycin, respectively. Based on our study, the simple MDD method is not recommended as a replacement for the checkerboard method to detect synergy. However, it may serve as an initial screening method for the detection of potential synergy when it is not feasible to perform other labor-intensive synergy tests.

scholarly journals Multicenter Evaluation of a Modified Cefoxitin Disk Diffusion Method and PBP2a Testing To PredictmecA-Mediated Oxacillin Resistance in Atypical Staphylococcus aureus

2016 ◽  
Vol 55 (2) ◽  
pp. 485-494 ◽  
Author(s):  
Shelley A. Miller ◽  
James Karichu ◽  
Peggy Kohner ◽  
Nicolynn Cole ◽  
Janet A. Hindler ◽  
...  
Keyword(s):  
Staphylococcus Aureus ◽  
Clinical Laboratory ◽  
Disk Diffusion ◽  
Diffusion Method ◽  
Disk Diffusion Method ◽  
Content Type ◽  
Oxacillin Resistance ◽  
Mueller Hinton ◽  
Cefoxitin Disk ◽  
Cefoxitin Disk Diffusion

ABSTRACTPhenotypic variants ofStaphylococcus aureusthat display small colonies, reduced pigmentation, and decreased hemolysis and/or coagulase activity are periodically isolated by the clinical laboratory. Antimicrobial susceptibility testing (AST) of these isolates is complicated, because many do not grow on routine AST media, including Mueller-Hinton agar (MHA) and cation-adjusted Mueller-Hinton broth. This multicenter study evaluated cefoxitin disk diffusion for 37 atypicalS. aureusisolates (156 readings) with MHA supplemented with 5% sheep's blood (BMHA), usingmecAPCR as the reference standard. The correlation of two commercial PBP2a assays withmecAPCR was also assessed. Ten isolates were negative and 27 positive formecA. No major errors for cefoxitin were observed, but 19.5% very major errors (VMEs) were observed at 24 h of incubation, and 17.2% VMEs were observed at 48 h. The proportions of VMEs ranged from 14.7 to 23.0% at 24 h, and from 13.3 to 17.6% at 48 h, across three testing laboratories. PBP2a tests were performed from growth on BMHA and blood agar plates (BAP), with and without cefoxitin disk induction. The Alere PBP2a SA culture colony test sensitivities formecAwere 90.0% with uninduced growth and 97.4% with induced growth from BMHA. On BAP, sensitivity was 96.0% with induced growth. The sensitivities of the Oxoid PBP2′ latex agglutination test were 85.7% with uninduced growth and 93.9% with induced growth from BMHA and 95.9% with induced growth on BAP. On the basis of these data, we recommend that laboratories perform onlymecAPCR and/or PBP2a tests when requested to perform AST on atypical isolates ofS. aureus.

scholarly journals Comparative Evaluation of the VITEK 2, Disk Diffusion, Etest, Broth Microdilution, and Agar Dilution Susceptibility Testing Methods for Colistin in Clinical Isolates, Including Heteroresistant Enterobacter cloacae and Acinetobacter baumannii Strains

2007 ◽  
Vol 51 (10) ◽  
pp. 3726-3730 ◽  
Author(s):  
Jerome R. Lo-Ten-Foe ◽  
Anne Marie G. A. de Smet ◽  
Bram M. W. Diederen ◽  
Jan A. J. W. Kluytmans ◽  
Peter H. J. van Keulen
Keyword(s):  
Acinetobacter Baumannii ◽  
Susceptibility Testing ◽  
Enterobacter Cloacae ◽  
Clinical Isolates ◽  
Disk Diffusion ◽  
Broth Microdilution ◽  
Diffusion Test ◽  
Disk Diffusion Test ◽  
Agar Dilution ◽  
Vitek 2

ABSTRACT Increasing antibiotic resistance in gram-negative bacteria has recently renewed interest in colistin as a therapeutic option. The increasing use of colistin necessitates the availability of rapid and reliable methods for colistin susceptibility testing. We compared seven methods of colistin susceptibility testing (disk diffusion, agar dilution on Mueller-Hinton [MH] and Isosensitest agar, Etest on MH and Isosensitest agar, broth microdilution, and VITEK 2) on 102 clinical isolates collected from patient materials during a selective digestive decontamination or selective oral decontamination trial in an intensive-care unit. Disk diffusion is an unreliable method to measure susceptibility to colistin. High error rates and low levels of reproducibility were observed in the disk diffusion test. The colistin Etest, agar dilution, and the VITEK 2 showed a high level of agreement with the broth microdilution reference method. Heteroresistance for colistin was observed in six Enterobacter cloacae isolates and in one Acinetobacter baumannii isolate. This is the first report of heteroresistance to colistin in E. cloacae isolates. Resistance to colistin in these isolates seemed to be induced upon exposure to colistin rather than being caused by stable mutations. Heteroresistant isolates could be detected in the broth microdilution, agar dilution, Etest, or disk diffusion test. The VITEK 2 displayed low sensitivity in the detection of heteroresistant subpopulations of E. cloacae. The VITEK 2 colistin susceptibility test can therefore be considered to be a reliable tool to determine susceptibility to colistin in isolates of genera that are known not to exhibit resistant subpopulations. In isolates of genera known to (occasionally) exhibit heteroresistance, an alternative susceptibility testing method capable of detecting heteroresistance should be used.

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