Multicenter prospective study on the prevalence of colistin resistance in : relevance of -positive clinical isolates in Lombardy, Northern Italy.

Background: The emergence of the plasmid-mediated colistin resistance mechanism in 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 isolates circulating in the Lombardy region, Northern Italy. The presence of -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 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 -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 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=12) and inpatients (n=6). Colistin MIC values ranged from 4 to 8 mg/L. The gene was detected in 10/18 isolates (7 from outpatients, 3 from inpatients). Rep-PCR and MLST analysis revealed the presence of nine different clusters. Further -type genes were not detected.

Conclusion: Resistance to colistin in clinical isolates appears low in our geographic area. With regard to -positive isolates, they accounted for approximately 50% of colistin-resistant isolates, thus representing a relevant resistance mechanism in this context. Although overall limited, the presence of determinant in our region should not be ignored and great concern should be given to the continuous surveillance.