Comparison of four commercial methods for the determination of linezolid susceptibility in Gram-positive cocci and practical algorithm proposal

Introduction Antimicrobial resistance represents a growing global concern, with methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci representing major contributors. Linezolid (LNZ) is one of the few available treatments for such infections, yet resistance has already been documented. It is therefore essential to accurately detect resistant strains to provide adequate treatment to patients and prevent nosocomial dissemination. This study evaluates the performance of four commercial methods employed by laboratories for determining Gram-positive cocci susceptibility to LNZ. Methods A panel of 99 stains from our routine and National Reference Centre (NRC) collections were studied, including 26 S. aureus (15 LNZ-resistant), 26 S. epidermidis (14 LNZ-resistant), 27 E. faecium (23 LNZ-resistant) and 20 E. faecalis (6 LNZ-resistant). Susceptibility to LNZ was determined by home-made broth microdilution method (BMD), considered as gold standard, and by four commercial methods namely disk diffusion (LNZ10, Bio-Rad, USA), gradient strip diffusion (E-TEST®, bioMérieux, France), broth microdilution assay (UMIC, Biocentric, France) and automated antimicrobial susceptibilty testing (AST; VITEK 2, bioMérieux). For the diffusion methods and UMIC, two incubation times were evaluated (18h and 48h). EUCAST guidelines and breakpoints were used. Categorical agreement (CA), major errors (ME's) and very major errors (VME's) were defined as recommended by the FDA: commercially available antibiotics methods should have a CA ≥ 90%, and a percentage of ME's ≤ 3% and of VME's ≤ 1.5%. Results All commercial methods exhibited similar performance, with accurate classification of LNZ- susceptible strains. However, all failed in detecting LNZ-resistance, except for S. epidermidis. The overall percentage of VME among S. aureus was 66.7%. Among E. faecium, it was 78.3% and 82.6% when using Vitek2 or another method, respectively. Among E. faecalis, it was 21.4% and 14.3% when using UMIC or another method, respectively. The percentage of VME's decreased with additional incubation times. The use of E-test exhibited the lowest percentage of VME's. However, none of the methods achieved the expected 90% CA. Discussion Even though discrepancies in LNZ AST methods have been reported, no practical guidelines are available for routine laboratories. According to this study, the following algorithm is proposed: if the strain is classified as LNZ-resistant, it should be considered as such and a PCR could be performed to reveal LNZ resistance genes, if the strain is classified as susceptible and if the patient must be treated with LNZ, an E-test read at 48h is recommended. Given the potential for false-susceptible results, particularly with S. aureus and E. faecium, it is crucial to monitor patients undergoing LNZ treatment. Should further analysis be required, the strains can be submitted to the NRC. Conclusion There is currently no optimal commercial method for the detection of LNZ-resistance. We therefore propose an algorithm that should assist laboratories in a routine practice context.