Antimicrobial resistance of Shigella sonnei isolates carrying class 2 integron in Northern Italy

Background: In the last decades, several reports on Shigella sonnei epidemiology from different countries have associated the epidemic circulation of this organism to a well defined strain, characterized by some proprierties, like a resistant phenotype streptomycin, sulfonamide, trimethoprim and tetracycline, a distinct XBaI pulsotype and the presence of class 2 integron. The objective of this study was to evaluate the genetic heterogeneity of molecular profiles, drug susceptibility pattern, and carriage of class 2 integron of S. sonnei collected in northern Italy. Materials: A total of twenty-five clinical isolates of S. sonnei identified in Lombardy region during the years 2009-2012 were studied. All isolates were subtyped by Pulsed-Field Gel Electropheresis (PFGE) according to the PulseNet protocol with XBaI enzyme to detect clusters and support epidemiological investigations. Furthermore, antimicrobial susceptibility testing was performed by the disk diffusion method as recommended by the National Committee of Clinical Laboratory Standards. Finally, polymerase chain reaction (PCR) was performed with the specific primer pair -hep74 and hep51- to detect the class 2 integron. Results: Eight XbaI pulsotypes with homology over 80% were recognized among the 25 clinical isolates. Twenty-three isolates were grouped in six clusters, while the remaining two isolates showed unique pulsotypes, one of which was reported in 2001 (Mammina et al., J. Clin. Microbiol., 2005). Two different class 2 integron structures were identified. Nine (36%) isolates contained a 2.22-kbp gene cassette (dhfr1, sat, and aad) were phenotypically resistant to trimethoprim and streptomycin, also ten (40%) isolate produced a smaller cassette structure of 1.37-kbp encoding only two genes within the cassette (dhfr1 and sat) were resistant to streptomycin. Finally, no class 2 integrons were detected in the remaining six (24%) isolates. Conclusion: The finding of our studies revealed the persistence of a pulsotype isolated in Lombardy in 2001. Moreover, ten S. sonnei isolates containing defective class 2 integron (1.37-kbp) were resistant to streptomycin. It is possible that in this case the streptomycin resistance may be due to aad gene present on a plasmid.