Phenotypic Resistance Profiles, Biofilm Formation, and In Vitro Carbapenem-Sparing Antimicrobial Activity in Enterobacterales Causing Acute Pyelonephritis.

Empirical management of acute pyelonephritis in Eastern Europe is increasingly constrained by extended-spectrum β-lactamase (ESBL)-producing Enterobacterales and by uropathogen phenotypes-such as strong biofilm formation-which may further blunt antimicrobial activity. We aimed to characterise resistance mechanisms, minimum inhibitory concentration (MIC) distributions, biofilm-forming capacity, and the in vitro performance of carbapenem-sparing agents and to test whether these microbiological features improve prediction of clinical failure beyond standard bedside risk scores. We retrospectively analysed 102 Enterobacterales isolates recovered from 129 consecutive culture-confirmed adult pyelonephritis admissions at "Victor Babeș" University Hospital, Timișoara (March 2022-March 2025). MIC values were determined by Vitek 2 and interpreted using EUCAST v13 breakpoints; ESBL, AmpC, and carbapenemase phenotypes were confirmed by combination disk and modified carbapenem inactivation methods. Biofilm formation was quantified by the microtiter-plate crystal-violet assay. Mediation, Restricted Mean Survival Time (RMST), and decision-curve analyses were used to assess added clinical value. ESBL was confirmed in 30/102 (29.4%) isolates, AmpC in 9 (8.8%), and carbapenemase in 4 (3.9%). ESBL+ isolates were more often strong biofilm formers (33.3% vs. 12.5%;