EUCAST disc diffusion criteria for the detection of mecA-Mediated β-lactam resistance in Staphylococcus pseudintermedius: oxacillin versus cefoxitin

Abstract

Objectives: Until recently, the European Committee on Antimicrobial Susceptibility Testing (EUCAST) recommended the cefoxitin disc to screen for mecA-mediated b-lactam resistance in Staphylococcus pseudintermedius. A recent study indicated that cefoxitin was inferior to oxacillin in this respect. We have re-evaluated cefoxitin and oxacillin discs for screening for methicillin resistance in S. pseudintermedius. Methods: We included 224 animal and human S. pseudintermedius isolates from Europe (n ¼ 108) and North America (n ¼ 116), of which 109 were mecA-positive. Disc diffusion was performed per EUCAST recommendations using 30-mg cefoxitin and 1-mg oxacillin discs from three manufacturers and Mueller eHinton agar from two manufacturers. Results: Cefoxitin inhibition zones ranged from 6 to 33 mm for mecA-positive S. pseudintermedius (MRSP) and from 29 to 41 mm for mecA-negative S. pseudintermedius (MSSP). The corresponding oxacillin zone intervals were 6e20 mm and 19e30 mm. For cefoxitin 16% (95% CI 14.8e18.0%) of the isolates were in the area where positive and negative results overlapped. For oxacillin the corresponding number was 2% (1.6 e2.9%). For oxacillin a breakpoint of susceptible (S) ! 20 mm and resistant (R) <20 mm resulted in only 0.4% and 1.1% very major error and major error rates respectively. Conclusions: This investigation confirms that the 1-mg oxacillin disc predicts mecA-mediated methicillin resistance in S. pseudintermedius better than the 30-mg cefoxitin disc. For a 1-mg oxacillin disc we propose that 20 mm should be used as cut off for resistance, i.e. isolates with a zone diameter <20 mm are resistant to all b-lactam antibiotics except those with activity against methicillin-resistant staphylococci.

Tetrahedral Framework Nucleic Acids Loading Ampicillin Improve the Drug Susceptibility against Methicillin-Resistant Staphylococcus aureus

“…The MIC of ssDNA-ampicillin was the same as that of ampicillin, which showed that ssDNA did not make ampicillin more effective against MRSA 18908 . The MIC of tFNAs-ampicillin was 0.5 × MIC Amp , which was below the dose for drug resistance suggested by the European Committee on Antimicrobial Susceptibility Testing . The OD600 value (representing the density of bacteria) of different groups as the concentration of the ampicillin changes from 1 × MIC Amp to 0.25 × MIC Amp as shown in Figure B.…”

“…The MIC of tFNAs-ampicillin was 0.5 × MIC Amp , which was below the dose for drug resistance suggested by the European Committee on Antimicrobial Susceptibility Testing. 47 The OD600 value (representing the density of bacteria) of different groups as the concentration of the ampicillin changes from 1 × MIC Amp to 0.25 × MIC Amp as shown in Figure 3B. The OD600 value of the tFNAs-ampicillin group was significantly less than that of the ampicillin group and the ssDNA-ampicillin group at 0.5 × MIC Amp and 0.25 × MIC Amp .…”

Driving Laboratory Standardization of Bacterial Culture and Antimicrobial Susceptibility Testing in Veterinary Clinical Microbiology in Europe and Beyond

“…Veterinary laboratories, which actively perform AMR surveillance, often follow either the CLSI ( 10 ) or EUCAST procedures ( https://www.eucast.org/fileadmin/src/media/PDFs/EUCAST_files/Resistance_mechanisms/EUCAST_detection_of_resistance_mechanisms_170711.pdf ) for specific detection of resistance mechanisms; however, these are not entirely applicable for veterinary clinical isolates. For instance, consensus on detection methods for methicillin resistance in important animal pathogens, such as methicillin-resistant Staphylococcus pseudintermedius (MRSP) or S. schleiferi (MRSS), is still lacking ( 10 , 11 ). In addition, detection of these and other multidrug-resistant (MDR) organisms emerging in companion animals (e.g., carbapenem-resistant Escherichia coli and Acinetobacter baumannii [ 12 , 13 ]) is often restricted to specialized research laboratories, raising the question of whether many AMR issues remain undetected.…”

Staphylococcus pseudintermedius’s PBP4 Is Directly Associated with the Dissociated Oxacillin and Cefoxitin Phenotype

“…However, FOX it is not an accurate method of screening for methicillin resistance in S. pseudintermedius [ 13 , 14 ]. Given this limitation, in 2016 the Clinical and Laboratory Standard Institute (CLSI) recommended the use of the oxacillin (OXA) disk instead of FOX for the detection of methicillin resistance in S. pseudintermedius ; a similar recommendation was recently issued by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) [ 15 ]. However, the mechanistic bases linked to FOX failure in detecting methicillin resistance in MRSP is still unknown.…”