Assessing mecillinam resistance in E. coli strains causing UTI: A comparative evaluation of phenotypic testing methods

Question

Mecillinam is recommended by German guidelines as a first-line treatment for uncomplicated urinary tract infections (uUTI). However, comprehensive data on antibiotic susceptibility are lacking due to the rare demand for microbiological diagnostics for uUTIs as well as the challenge of performing susceptibility testing with agar dilution as the reference method. We aim to evaluate various phenotypic methods for reliably predicting mecillinam resistance in E. coli, the most common UTI pathogen.

Methods

40 E. coli strains from routine urine cultures were selected, including 20 randomly chosen and 20 with elevated or resistant MIC to mecillinam based on Vitek testing with Ast-N371 (BioMérieux). 5 methods were compared in triplicates: agar dilution, Vitek Ast-N371, agar gradient diffusion (AGD), disk diffusion, and broth microdilution conducted in two different media—Müller-Hinton Broth (MHB) and artificial urine (AU). Categorial agreement was assessed based on the median, using EUCAST susceptibility breakpoints (MIC ≤8 mg/L, zone diameter ≥15 mm). Major errors (ME) were defined as false resistance and very major errors (VME) as false susceptibility compared to the reference method.

Results

Agar dilution categorized 28 strains as susceptible and 12 as resistant, with 6 near the breakpoint (8 mg/L or 16 mg/L). Categorial agreement to the reference method was 90% for disk diffusion, 83% for Vitek, 78% for AGD, 63% for MHB and 80% for AU. AGD had the most VME (n = 10), generally underestimating MIC values, while disk diffusion was the most reliable method with no ME and 4 VME. Notably, 3 of the 4 VME resulted in strains with an MIC of 16 mg/L. Microdilution methods, including Vitek, generally demonstrated a low reliability, with decreased reproducibility and more ME/ VME compared to disk diffusion. Only 2/12 resistant strains and 13/28 susceptible strains showed categorial agreement in all employed methods.

Conclusion

Mecillinam resistance testing remains challenging, with routine methods yielding unreliable results. The next phase of the study aims to elucidate the underlying genes associated with mecillinam resistance.