Background. Urinary tract infections (UTIs) and asymptomatic bacteriurias (ABU) represent a large field of interest for antimicrobial stewardship programmes especially after 2020 EUCAST update in antimicrobial susceptibility testing interpretation and the possible related increase in carbapenems’ prescription rate. The aim of this study was to evaluate the impact of the 2020 EUCAST update on antibiotic prescription in UTI due to Pseudomonas aeruginosa organism and their characteristics. Methods. A retrospective observational study. We enrolled all the patients with P. aeruginosa isolation from urine, admitted to our hospital from 2018 to 2021. We compared demographic, clinical, and microbiological characteristics and treatment between cases before 2020 EUCAST update (period A, 2018–2019) and after it (period B, 2020–2021). Results. A total of 643 cases was analysed, 278 in period A and 365 in period B; 65% were ABU. Carbapenems’ prescription rate significantly increased in period B when considering ABU alone (21.4% vs. 41%, p = 0.016) and all the treated cases (treated ABU and UTI; 27.8% vs. 41.4%, p = 0.013); anti-Pseudomonas cephalosporins prescription significantly decreased in period B when considering ABU alone (15.7% vs. 3.6%, p = 0.021), UTI alone (20.7% vs. 5.9%, p = 0.009) and all the treated cases (18.5% vs. 5.9%, p = 0.001). Conclusions. The 2020 EUCAST update could have contributed to an increase in carbapenem prescriptions. UTI and ABU represent a large field of interest for stewardship interventions both from a diagnostic and therapeutic point of view.

Serratia marcescens complex bacteria are recognized causes of outbreaks in Neonatal Intensive Care Units (NICU). We investigated a cluster of VIM-producing Serratia spp. in the NICU of Santa Maria della Misericordia Hospital, Perugia, through a retrospective epidemiological and genomic analysis integrating clinical and surveillance data, antimicrobial susceptibility testing, environmental screening, and whole-genome sequencing (WGS). In November 2024, VIM-producing Serratia was isolated from eight neonates, of whom five developed clinical infections, including two bloodstream infections. Nine VIM-producing Serratia isolates recovered from these eight neonates and one non-VIM-producing isolate from a bloodstream infection (used for comparison) were analysed. Routine MALDI-TOF MS identified all isolates as S. marcescens, whereas WGS resolved them into two genomic lineages: seven S. bockelmannii ST388 isolates and three S. sarumanii ST382-like isolates. All VIM-producing Serratia carried bla_VIM-1 and bla_SHV-12 and showed multidrug-resistant phenotypes. Long-reads sequencing reconstructed complete, and almost identical, IncA-ST12 plasmids in all nine VIM-producing Serratia isolates; the plasmids carried bla_VIM-1 and bla_SHV-12 together with additional resistance determinants. Environmental, water, and healthcare-worker screening did not identify a Serratia reservoir. No new VIM-producing Serratia infections occurred after the November cluster. These findings show how WGS complements routine MALDI-TOF MS and rapid carbapenemase testing by providing the taxonomic, epidemiological, and plasmid-level resolution needed to investigate complex healthcare-associated outbreaks.

Serratia marcescens complex bacteria are recognized causes of outbreaks in neonatal intensive care units. We investigated a cluster of VIM-producing Serratia spp. in the Neonatal Unit of Azienda Ospedaliera di Perugia, Italy, using epidemiological surveillance, antimicrobial susceptibility testing, environmental sampling, and whole-genome sequencing (WGS). In November 2024, ten neonates were colonized and/or infected with Serratia. Eight carried VIM-producing isolates, including two patients with bloodstream infections. Nine VIM-producing Serratia isolates were recovered because one patient yielded both a rectal and a blood culture isolate; one non-VIM-producing bloodstream isolate was also available and included for comparison. WGS showed that the ten isolates, initially identified as S. marcescens by routine diagnostic methods, belonged to two species within the S. marcescens complex: seven Serratia bockelmannii ST388 isolates and three ST382-like Serratia sarumanii isolates. All VIM-producing Serratia spp. isolates carried bla_VIM-1 and bla_SHV-12 and showed multidrug-resistant phenotypes. bla_VIM-1 was also detected in Escherichia coli, Citrobacter freundii, and Pseudomonas putida isolates from the same ward. A shared IncA-ST12 replicon suggested possible horizontal dissemination of a related mobile resistance element. Environmental and healthcare-worker screening did not detect Serratia. These findings highlight the value of WGS for distinguishing clonal transmission from dissemination of mobile resistance determinants during complex healthcare-associated outbreaks.