Evaluation of several routine methods for fosfomycin and mecillinam susceptibility testing of Enterobacterales urine isolates.

OBJECTIVES: Performance evaluation of routine laboratory methods to determine the susceptibility of Enterobacterales urinary isolates to fosfomycin (oral administration) and mecillinam. METHODS: We collected 347 Enterobacterales isolates from monomicrobial midstream urine samples from women with significant bacteriuria and leukocyturia. Mostly non-Escherichia coli isolates (i.e. Klebsiella spp., Citrobacter koseri, Enterobacter cloacae complex and Proteus mirabilis) were included (n = 298). Performance of VITEK®2, ETEST®, and disc diffusion to determine fosfomycin and mecillinam susceptibility was evaluated following International Organization for Standardization (ISO) 20776-2:2021 (or 20776-2:2007 for disc diffusion) in comparison with the agar dilution reference method. RESULTS: For fosfomycin testing, VITEK®2 and ETEST® were close to reaching ISO requirements (essential agreement  ≥ 90%; bias  ±30%) for C. koseri, E. coli and P. mirabilis. Categorical agreement (CA) and major error rates were acceptable for disc diffusion. Fosfomycin displayed lower activity against E. cloacae complex and Klebsiella spp., with MIC50 (minimum inhibitory concentration required to inhibit the growth of 50% of tested isolates) equal to the E. coli EUCAST breakpoint (8 mg/L). For these species, the three alternative techniques overestimated MICs and resistance, and did not meet performance criteria. For mecillinam testing of Enterobacterales isolates, apart from P. mirabilis, ETEST® nearly fulfilled ISO requirements, and CA rates were acceptable for disc diffusion. ISO criteria were reached for C. koseri and E. coli testing with VITEK®2, apart from too high rates of very major errors. For P. mirabilis, performances were unacceptable, whatever the routine method used. CONCLUSIONS: Commercially available tests may serve as alternatives to agar dilution to assess fosfomycin (oral) and mecillinam susceptibility of Enterobacterales urinary isolates, with important interspecies variabilities. Additional studies comprising more fosfomycin- and mecillinam-resistant isolates are needed to strengthen our conclusions.

[Involvement of inorganic phosphate starvation in Pseudomonas aeruginosa bacterial virulence]. / Implication de la carence en phosphates inorganiques dans la virulence bactérienne de Pseudomonas aeruginosa.

Pseudomonas aeruginosa is a bacterium causing a wide spectrum of nosocomial and opportunistic respiratory infections. As an element essential for bacterial metabolism , phosphorus is incorporated as an inorganic phosphate and regulated by a two-component PhoB-PhoR system. Recently, it has been shown that as a result of overexpression of virulence factors, including the PhoB transcription factor, P. aeruginosa exhibited increased virulence in phosphate-deficient conditions. Exploration of the relationship between phosphate homeostasis and P. aeruginosa virulence could effectively contribute to the development of new, simple and innovative therapeutic strategies.

Fosfomycin, from susceptibility to resistance: Impact of the new guidelines on breakpoints.

The fosfomycin breakpoint using the disc diffusion method (DDM) changed in the 2019 CA-SFM/EUCAST guidelines v2 (24mm versus 19mm). We assessed its impact on categorization of Enterobacterales recovered from urine samples in emergency departments. A total of 7749 and 2348 strains were tested using the DDM and the broth microdilution method (BMD), respectively. The DDM with the 19-mm breakpoint was in accordance with the BMD. Using the 24-mm breakpoint, the overall rate of fosfomycin resistance in Enterobacterales increased by three-fold (5.6% vs 18.1%, P<0.01) and reached 2.8% and 86.5% in E. coli and K. pneumoniae, respectively. French guidelines for the management of community-acquired UTI remain appropriate. The accuracy of the methods for routine fosfomycin susceptibility testing should be assessed. The role of fosfomycin in the treatment of documented CA-UTI due to Enterobacterales other than E. coli should be evaluated considering its rate of resistance and recent data reporting low accuracy.