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Comparative Study
. 2018 Jan 24;56(2):e01653-17.
doi: 10.1128/JCM.01653-17. Print 2018 Feb.

Evaluation of Oxacillin and Cefoxitin Disk Diffusion and MIC Breakpoints Established by the Clinical and Laboratory Standards Institute for Detection of mecA-Mediated Oxacillin Resistance in Staphylococcus schleiferi

H K Huse  1 , S A Miller  1 , S Chandrasekaran  1 , J A Hindler  1 , S D Lawhon  2 , D A Bemis  3 , L F Westblade  4 , R M Humphries  5
Affiliations
Comparative Study

Evaluation of Oxacillin and Cefoxitin Disk Diffusion and MIC Breakpoints Established by the Clinical and Laboratory Standards Institute for Detection of mecA-Mediated Oxacillin Resistance in Staphylococcus schleiferi

H K Huse et al. J Clin Microbiol. .

Abstract

Staphylococcus schleiferi is a beta-hemolytic, coagulase-variable colonizer of small animals that can cause opportunistic infections in humans. In veterinary isolates, the rate of mecA-mediated oxacillin resistance is significant, with reported resistance rates of >39%. The goal of this study was to evaluate oxacillin and cefoxitin disk diffusion (DD) and MIC breakpoints for detection of mecA-mediated oxacillin resistance in 52 human and 38 veterinary isolates of S. schleiferi Isolates were tested on multiple brands of commercial media and according to Clinical and Laboratory Standards Institute (CLSI) methods. Zone diameters and MIC values were interpreted using CLSI breakpoints (CLSI, Performance Standards for Antimicrobial Susceptibility Testing. M100-S27, 2017) for Staphylococcus aureus/Staphylococcus lugdunensis, coagulase-negative staphylococci (CoNS), and Staphylococcus pseudintermedius Results were compared to those of mecA PCR. Twenty-nine of 90 (32%) isolates were mecA positive. Oxacillin inhibition zone sizes and MICs interpreted by S. pseudintermedius breakpoints reliably differentiated mecA-positive and mecA-negative isolates, with a categorical agreement (CA) of 100% and no very major errors (VMEs) or major errors (MEs) for all media. For cefoxitin DD results interpreted using S. aureus/S. lugdunensis and CoNS breakpoints, CA values were 85% and 75%, respectively, and there were 72% and 64% VMEs, respectively, and 0 MEs. For cefoxitin MICs interpreted using S. aureus/S. lugdunensis breakpoints, CA was 81%, and there were 60% VMEs and no MEs. Our data demonstrate that oxacillin DD or MIC testing methods using the current S. pseudintermedius breakpoints reliably identify mecA-mediated oxacillin resistance in S. schleiferi, while cefoxitin DD and MIC testing methods perform poorly.

Keywords: PBP2a; Staphylococcus schleiferi; breakpoints; cefoxitin; mecA; oxacillin.

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Figures

FIG 1
FIG 1
Distributions of cefoxitin and oxacillin growth inhibition zone diameters and MICs as determined by DD and BMD testing on all media tested. M100-S27 DD and MIC breakpoints are shown for S. aureus/S. lugdunensis (Sa/Sl), coagulase-negative staphylococci (excluding S. lugdunensis and S. pseudintermedius) (CoNS), and S. pseudintermedius. R, resistant; S, susceptible. (A) Cefoxitin DD testing (for BD, Hardy, and Remel media, n = 253). (B) Oxacillin DD testing (for BD, Hardy, and Remel media, n = 253). (C) Cefoxitin MICs (for Difco, BD, and Oxoid media, n = 270). (D) Oxacillin MICs (for Difco, BD, and Oxoid media, n = 270).
FIG 2
FIG 2
Growth of a single strain of S. schleiferi on BD (left), Remel (middle), and Hardy (right) MHA plates with oxacillin (OX) and cefoxitin (FOX) disks.
See this image and copyright information in PMC

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