Molecular characterisation of multidrug-resistant Salmonella enterica serotype Infantis from humans, animals and the environment in Italy

During 2005-2006, Salmonella enterica serotype Infantis strains isolated from human and non-human sources and resistant to ampicillin (A), chloramphenicol (C), streptomycin (S), sulphonamide (Su), tetracycline (T), kanamycin (K) and trimethoprim/sulfamethoxazole (Sxt) emerged in Italy. The aim of this study was to analyse the molecular basis of antibiotic resistance and to evaluate the clonal origin of multiresistant S. Infantis strains isolated from different sources. Seventy S. Infantis strains, susceptible or resistant to antimicrobial drugs, were chosen for this study. Polymerase chain reaction (PCR) and conjugation experiments were performed to identify and localise the resistance genes in multidrug-resistant strains. PCR-based replicon typing was carried out for characterisation of conjugative plasmids. All strains were tested by pulsed-field gel electrophoresis (PFGE) according to the PulseNet protocol, and cluster analysis was performed using BioNumerics software. Strains with resistance (R)-type ACSSuTKSxt harboured blaTEM-1, strA-B, sul2, tet(B), catA1 and aphA-1 resistance genes as well as a 2.2-kb class 1 integron containing folA, catB3, aadA4 and sul1 gene cassettes. A unique plasmid, belonging to the HI1 incompatibility group, harboured all the resistance genes. Cluster analysis showed that all ACSSuTKSxt-resistant strains belonged to a large cluster (A) with >90% genetic similarity. The presence of a plasmid harbouring all the resistance gene cassettes as well as molecular typing by PFGE demonstrated the circulation of a cluster of S. Infantis R-type ACSSuTKSxt during 2005-2006 in Italy. The presence of a plasmid conferring multidrug resistance could have facilitated the spread of a group of similar isolates through a variety of sources.