Multicenter prospective study on the prevalence of colistin resistance in Escherichia coli: relevance of mcr-1-positive clinical isolates in Lombardy, Northern Italy Luigi Principe,1 Aurora Piazza,2,3 Carola Mauri,1 Adriano Anesi,4 Silvia Bracco,5 Gioconda Brigante,6 Erminia Casari,7 Carlo Agrappi,8 Mariasofia Caltagirone,2 Federica Novazzi,2 Roberta Migliavacca,2 Laura Pagani,2 Francesco Luzzaro1 1Microbiology and Virology Unit, A. Manzoni Hospital, Lecco, Italy; 2Clinical-Surgical, Diagnostic and Pediatric Sciences Department, Unit of Microbiology and Clinical Microbiology, University of Pavia, Pavia, Italy; 3Romeo and Enrica Invernizzi Pediatric Research Center, Department of Biomedical and Clinical Sciences, University of Milan, Milan, Italy; 4Clinical Pathology Laboratory, ASST Lodi, Lodi, Italy; 5Clinical Pathology Laboratory, ASST Vimercate, Vimercate, Italy; 6Clinical Pathology Laboratory, ASST Valle Olona, Busto Arsizio, Italy; 7Clinical Pathology Laboratory, IRCCS “Humanitas,” Rozzano, Italy; 8Microbiology and Virology Unit, ASST Ovest Milanese, Legnano, Italy Background: The emergence of the plasmid-mediated colistin resistance mechanism in Escherichia coli has raised concern among public health experts as colistin is a last-line antimicrobial resort. The primary aim of the study was to investigate the prevalence of this resistance trait in E. coli isolates circulating in the Lombardy region, Northern Italy. The presence of mcr-type genes and their genetic relationship were also studied.Materials and methods: A prospective study was performed during a 4-month period (May to August, 2016) in six acute care Hospitals. Consecutive nonduplicate clinical isolates of E. coli from any type of clinical specimen, with the exception of rectal swabs, were included in the study. Isolates that exhibited MIC values for colistin >2 mg/L were further investigated. Bacterial identification was obtained by matrix-assisted laser desorption ionization-time of flight mass spectrometry. Amplification of mcr-type genes (−1 to −5 variants) and microarray analysis were accomplished. Repetitive sequence-based PCR (Rep-PCR) and multilocus sequence typing (MLST) analysis were used for genotyping.Results: Overall, 3,902 consecutive E. coli isolates (2,342 from outpatients, 1,560 from inpatients) were evaluated during the study period. Of them, 18/3,902 (0.5%), collected from 4/6 centers, showed resistance to colistin. These isolates were mostly obtained from urine of both outpatients (n