# Performance of disc diffusion and four commercially available MIC tests to determine mecillinam susceptibility on carbapenemase-producing Enterobacterales

## TL;DR

This study compares different methods to test susceptibility to mecillinam in bacteria resistant to last-resort antibiotics, finding that disc diffusion and VITEK 2 are reliable for some cases.

## Contribution

The study evaluates and compares the performance of mecillinam susceptibility testing methods on carbapenemase-producing Enterobacterales isolates.

## Key findings

- Disc diffusion and VITEK 2 showed high accuracy for mecillinam susceptibility in E. coli isolates.
- Sensititre and VITEK 2 overestimated MICs but had low very major error rates.
- Agar dilution remains the reference method for mecillinam MIC determination.

## Abstract

Urinary tract infections (UTIs) are predominantly caused by Enterobacterales, and escalating resistance poses significant challenges due to carbapenemase production, which can compromise the efficacy of last-resort antibiotics. Pivmecillinam, as the prodrug of mecillinam, may serve as a treatment option for UTIs caused by carbapenemase-producing Enterobacterales (CPE). However, accurate methods are required to accurately assess susceptibility to this antibiotic. This study aimed to evaluate the in vitro susceptibility of mecillinam on a collection of 161 well-characterized CPE isolates collected from July to November 2023 in France and assess the performances of disc diffusion, Liofilchem MTS gradient strip, bioMérieux E-test, Sensititre broth microdilution, and VITEK 2 compared to agar dilution (reference method) in this specific population of CPE. None of the commercially available tests met the International Organization for Standardization standards with this entire collection. Using Clinical and Laboratory Standards Institute (CLSI) or European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines on the complete collection of CPE, disc diffusion possessed the best categorical agreement [77.0 (124/161) or 84.5% (131/161), respectively]. Sensititre broth microdilution and VITEK 2 overestimated MICs (by 1.8 and one doubling-dilution, respectively) but had low very major error (VME) rates (0% (0/53) for Sensititre and 4.2% (6/53) for VITEK 2 with CLSI and 0% (0/53) for Sensititre and 11.3% (6/53) for VITEK 2 with EUCAST). Notably, when focusing solely on E. coli isolates (n = 45), performance improved considerably. Both disc diffusion and VITEK 2 demonstrated high categorical agreements [95.6 (43/45) and 93.3% (42/45), respectively] with no VME observed for either method using CLSI breakpoints. In conclusion, while agar dilution remains the reference method for mecillinam MIC determination, disc diffusion and VITEK2 are accurate alternatives to determine mecillinam susceptibility in CPE isolates, especially for Escherichia coli.

The rising prevalence of carbapenemase-producing Enterobacterales (CPE) poses significant challenges to effective antibiotic therapy, particularly for urinary tract infections. Pivmecillinam, the prodrug of mecillinam, offers a promising treatment option due to its activity against certain carbapenemase-producing strains. However, selecting accurate susceptibility testing methods is crucial for ensuring appropriate clinical use. This study rigorously evaluates the performances of various susceptibility testing methods against a challenging collection of CPE isolates, identifying the strengths and limitations of each. By providing critical insights into their reliability, particularly for routine clinical settings, this research supports improved diagnostic accuracy and optimal use of mecillinam in combating multidrug-resistant infections. These findings are especially relevant as pivmecillinam gains international approval, underscoring its potential as a valuable alternative in the fight against antibiotic resistance.