Evaluation of disc diffusion tests and agar screening for predicting mecA-mediated oxacillin resistance in Staphylococcus lugdunensis revealed a cefoxitin-susceptible, mecA-positive S. lugdunensis clonal complex 27 clone

J Glob Antimicrob Resist. 2020 Mar;20:260-265. doi: 10.1016/j.jgar.2019.08.021. Epub 2019 Sep 4.


Objectives: This study evaluated disc diffusion tests and agar screening for detecting mecA-mediated oxacillin resistance in Staphylococcus lugdunensis (S. lugdunensis).

Methods: Staphylococcus lugdunensis isolates (n = 179) from diverse sources in Hong Kong during 1998-2018 were investigated by disc diffusion tests (cefoxitin and oxacillin) and inoculation onto oxacillin (1 μg/mL and 2 μg/mL) and chromID methicillin-resistant Staphylococcus aureus (MRSA) agars. The results were compared with mecA PCR as the reference. Isolates with discordant results were further tested by MIC and penicillin-binding protein 2a (PBP2a) assays.

Results: Cefoxitin and oxacillin zone diameters were not distributed in ways that allowed reliable division of the mecA-positive (n = 52) and mecA-negative (n = 127) isolates. On applying the 2019 Clinical Laboratory Standards Institute (CLSI) M100 breakpoints for cefoxitin disc results, there was 88% categorical agreement (CA) and 40% very major error (VME). Screening using 2 μg/mL oxacillin agar reliably differentiated mecA-positive and mecA-negative isolates (100% CA) without any major error (ME) or VME results. The performance of screening using 1 μg/mL oxacillin agar or ChromID MRSA agar was variable (74-89% CA, 0-38% ME and 0-37% VME). The mecA-positive isolates (n = 21) that could not be detected by the cefoxitin disc test were further characterised. The cefoxitin MIC for all 21 isolates was ≤4 μg/mL. Twenty isolates had an oxacillin MIC of 1-2 μg/mL and one had an oxacillin MIC of 4 μg/mL. All had positive PBP2a results and were typed as clonal cluster 27/SCCmec V.

Conclusions: These findings highlight the need to evaluate phenotypic methods using mecA-positive S. lugdunensis with different oxacillin resistance phenotypes.

Keywords: Oxacillin resistance; SCCmec; Staphylococci.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Agar
  • Bacterial Load
  • Bacterial Proteins / genetics*
  • Carrier State / microbiology
  • Cefoxitin / pharmacology*
  • Cefoxitin / therapeutic use
  • Disk Diffusion Antimicrobial Tests
  • Humans
  • Methicillin Resistance*
  • Microbial Sensitivity Tests
  • Oxacillin / pharmacology
  • Phenotype
  • Staphylococcal Infections / drug therapy
  • Staphylococcal Infections / microbiology*
  • Staphylococcus lugdunensis / classification*
  • Staphylococcus lugdunensis / drug effects
  • Staphylococcus lugdunensis / growth & development


  • Bacterial Proteins
  • Cefoxitin
  • Agar
  • Oxacillin