Antimicrobial Susceptibility Testing for Staphylococcus lugdunensis

Abstract

Evaluation of penicillin and oxacillin susceptibility testing was conducted on two hundred Staphylococcus lugdunensis isolates. Disc diffusion with penicillin 1 IU (P1, EUCAST) and penicillin 10 IU (P10, CLSI) was compared with nitrocefin discs (Cefinase®) and automated broth microdilution (Vitek2®). Oxacillin susceptibility was extrapolated from cefoxitin 30μg disc diffusion (FOX) and compared with Vitek2®. Reference methods were blaZ and mecA PCR. Penicillin zone diameter and zone edge correlated with blaZ in all except two P10 susceptible isolates (VME; very major error) and one P1 resistant isolate (ME). One hundred and forty-eight isolates were blaZ -negative of which one hundred and forty-six and one hundred and forty-nine isolates were susceptible by P1 and P10 respectively. One hundred and twenty-seven isolates were penicillin susceptible by Vitek2®. Vitek2® overcalled resistance in twenty-one blaZ -negative, twenty P1 and twenty-two P10 susceptible isolates (Vitek2® ME rate, 14.2%). Two mecA -positive isolates were oxacillin resistant by FOX and Vitek2® (categorical agreement). However, eighteen FOX susceptible, mecA -negative isolates tested resistant by Vitek2®. In conclusion, Vitek2® over-estimated penicillin and oxacillin resistance compared with disc diffusion and PCR. Disc diffusion with zone edge interpretation was more accurate and specific than automated broth microdilution for S. lugdunensis in our study.

Antimicrobial Resistance and Antimicrobial Activity of Staphylococcus lugdunensis Obtained from Two Spanish Hospitals

“…This resistance was mediated in 100% of the penicillin resistant isolates by the expression of the bla Z gene. However, other studies have noted a phenotype-genotype discrepancy in relation to penicillin resistance detection when other commercial microdilution methods were used [ 26 ].…”

Clinical and microbiological characteristics of Staphylococcus lugdunensis

“…The primary mechanism of resistance encountered in S. lugdunensis is penicillinase encoded by the blaZ gene resulting in resistance to penicillin, however organisms remain susceptible to penicillinase-resistant penicillins such as oxacillin and methicillin. The prevalence of blaZ positive, penicillin resistant S. lugdunensis varies in the literature from 7% to 48%, however most recent studies suggest the prevalence is approximately 25% [41–45,46 ▪▪ ]. There are multiple methods used to detect beta-lactamase in staphylococci, most commonly penicillin disk diffusion (DD), penicillin minimum inhibitory concentration (MIC) and chromogenic cephalosporin-based tests (nitrocefin).…”

“…EUCAST also provide the caveat that no currently available method can reliably detect penicillinase production in all species of staphylococci. A recent study by Teh et al [46 ▪▪ ] compared penicillin disk diffusion at two concentrations (P1 and P10), automated broth microdilution (BMD) (Vitek2) and nitocefin disk method compared to blaZ PCR as the reference method. Penicillin resistance and blaZ positivity were recorded in 25% of the 200 isolates tested.…”