Evaluation of penicillin G susceptibility testing methods for Staphylococcus lugdunensis

Abstract

Background: Staphylococcus lugdunensis belongs to the CoNS group, but is regarded to be more virulent than most other CoNS. It is also remarkably susceptible to antibiotics, including penicillin G.

Objectives: To evaluate different methods for penicillin susceptibility testing, to assess penicillin susceptibility rates among S. lugdunensis and to describe the clinical presentation including antibiotic treatment.

Methods: Clinical isolates of S. lugdunensis were tested for penicillin susceptibility using disc diffusion according to CLSI (10 U disc) and EUCAST (1 U disc), assessment of zone-edge appearance, nitrocefin test and Etest for MIC determination. PCR of the blaZ gene was used as a reference method.

Results: Of the 112 isolates included in the study, 67% were susceptible to penicillin G according to blaZ PCR. The EUCAST disc diffusion test had 100% sensitivity, whereas the CLSI method had one very major error with a false-susceptible isolate. When zone-edge appearance was included in the assessment, the false-susceptible isolate was correctly classified as resistant. Foreign-body infection was the most common focus of infection, affecting 49% of the participants. Only 4% of the patients were treated with penicillin G.

Conclusions: Penicillin susceptibility is common in S. lugdunensis and the disc diffusion method according to EUCAST had a higher sensitivity than that of CLSI. Assessment of zone-edge appearance could increase the sensitivity of the disc diffusion test. Penicillin susceptibility testing and treatment should be considered in S. lugdunensis infections.

Figure 1.

Sharp (a) and fuzzy (b) zone-edge appearance in S. lugdunensis. A sharp zone edge correlates with the presence of β-lactamase, whereas a fuzzy zone edge corresponds to the absence of β-lactamase.

Figure 2.

Histograms of zone diameters for S. lugdunensis isolates for the disc diffusion test using 1 U (a) and 10 U (b) penicillin G discs. Isolates are classified as resistant or susceptible based on the presence or absence of the blaZ gene. Six millimetres corresponds to the diameter of the disc and represents complete inhibition. Panel (c) shows how zone diameters for the 1 U penicillin G disc (x-axis) and the 10 U penicillin G disc (y-axis) are distributed among blaZ-positive and blaZ-negative isolates. The black arrow indicates a blaZ-positive isolate with a zone diameter ≥29 mm for the 10 U penicillin disc and the grey arrow indicates blaZ-negative strains with zone diameters <26 mm for the 1 U penicillin disc. Panel (d) shows the MIC distribution using Etests. The dotted lines in panels (a) to (d) indicate breakpoints for resistant isolates based on zone diameters (≥26 mm for the 1 U disc and ≥29 mm for the 10 U disc) or MIC value.