Improved Detection of mecA-Mediated β-Lactam Resistance in Staphylococcus lugdunensis Using a New Oxacillin Salt Agar Screen.

Pak-Leung Ho,Vincent Chi-Chung Cheng,Cindy Wing-Sze Tse,Kin-Hung Chow,Melissa Chun-Jiao Liu,Ying-Hang Law,Andes Lau,Alan Ka-Lun Wu,Man-Ki Tong,Tak-Lun Que

doi:10.3389/fmicb.2021.704552

Abstract

Oxacillin resistance mediated by mecA in Staphylococcus lugdunensis is emerging in some geographic areas. We evaluated cefoxitin disk diffusion (DD) and a new oxacillin agar (supplemented with 2 μg/ml oxacillin and 2% sodium chloride) screen for the detection of mecA-mediated resistance in S. lugdunensis. A total of 300 consecutive, non-duplicated clinical S. lugdunensis isolates from diverse sources in Hong Kong in 2019 were tested. The categorical agreement and errors obtained between cefoxitin DD test, oxacillin agar screen and mecA PCR were analyzed. Isolates with discordant results were further tested by MIC, penicillin binding protein 2a (PBP2a) assays, population analysis and molecular typing. PCR showed that 62 isolates were mecA-positive and 238 isolates were mecA-negative. For cefoxitin DD results interpreted using S. aureus/S. lugdunensis breakpoints, the categorical agreement (CA) for two brands of Muller-Hinton agars, MH-II (Becton Dickinson) and MH-E (bioMérieux) were both 96.0%; MEs were both 0%; and VMEs were 19.4 and 12.9%, respectively. The new oxacillin agar reliably differentiated mecA-positive and mecA-negative isolates (100% CA) without any ME or VME results. The 8 isolates with false susceptibility in the cefoxitin DD testing had cefoxitin and oxacillin MICs in the susceptible range. The isolates showed heterogeneous oxacillin resistance with resistant subpopulations at low frequencies. All had positive PBP2a results and were typed as sequence type 27/SCCmec V. The findings highlight the inability of cefoxitin DD and MIC tests for reliable detection of some mecA-positive S. lugdunensis isolates.

Highlights

Staphylococcus lugdunensis is an unusual coagulase-negative Staphylococcus
An additional four isolates were cefoxitinresistant in both agars but the zone sizes in Muller-Hinton E (MH-E) agar were smaller than in Muller-Hinton II (MH-II) agar
We presented cefoxitin disc diffusion (DD) data for detection of mecA-mediated oxacillin resistance in 300 clinical isolates of S. lugdunensis

Summary

Introduction

Staphylococcus lugdunensis is an unusual coagulase-negative Staphylococcus. Coagulase-negative staphylococci are relatively avirulent in immunocompetent host, S. lugdunensis is an exception (Heilbronner and Foster, 2021). It has been reported to cause other invasive infections such as brain abscess, osteomyelitis, septic arthritis and peritonitis (Heilbronner and Foster, 2021). Antibiotic treatment of staphylococcal infection can be impeded by the acquisition of resistance to multiple classes of antibiotics, especially βlactams. Β-lactam resistance is mostly mediated by mecA, encoding PBP2a, which catalyzes cell wall synthesis in the presence of otherwise inhibitory concentrations of betalactam. Since oxacillin or cefoxitin is used as a surrogate for phenotypic detection mecA-mediated resistance, isolates that contain mecA are called oxacillin (or cefoxitin)-resistant, while isolates lacking mecA are designated oxacillin (or cefoxitin)susceptible (CLSI, 2020)

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