Antimicrobial resistance of Listeria monocytogenes strains isolated from humans in Northern Italy

Background: Listeria monocytogenes rarely develops acquired resistance to antibiotics. However, some studies have recently reported an increased rate of resistance to one or several clinically relevant antibiotics in environmental isolates and less frequently in clinical strains. This probably remains a marginal phenomenon for clinical strains, although only a limited number of studies have focused on the evaluation of antimicrobial resistance in Listeria. The objective of this study was to investigate the antimicrobial susceptibility of L. monocytogenes strains isolated from humans in Northern Italy. Materials: A total of 61 clinical strains of L. monocytogenes identified in Lombardy region during the years 2012-2013 were studied. All strains were tested with the disk diffusion method as recommended by European Committee on Antimicrobial Susceptibility Testing (EUCAST). Molecular subtyping were performed by Pulsed-Field Gel Electropheresis (PFGE) according to the PulseNet protocol with AscI and ApaI enzymes to detect clusters and support epidemiological investigations. Results: All 61 human L. monocytogenes strains were found to be susceptible to penicillin G (1 unit), ampicillin (2 µg) and meropenem (10 µg), while resistance to trimethoprim-sulfamethoxazole (1.25-23.75 µg) and erythromycin (15 µg) were found in 11 (18%) and 6 (9.8%) clinical strains according to EUCAST breakpoint. Twenty-one PFGE pulsotypes and seven clusters with homology over 80% were recognized, but no correlation was observed between L. monocytogenes molecular type and resistance to trimethoprim-sulfamethoxazole and/or erythromycin. Finally, only one of the seven clusters shows a PFGE homology of 100% and a complete overlap with the drug susceptibility pattern. Conclusion: Our findings reveal an increased rate of resistance compared to the previous study in the same geographical area (Madeo et al., 2014). Despite the low prevalence of antimicrobial resistance presently observed in human strains of L. monocytogenes isolated in Northern Italy, the emergence of resistant strains worldwide underlines the need for a continuous surveillance for this pathogen. Moreover, new susceptibility data for human strains of L. monocytogenes can provide useful information for epidemiological and public health purposes.