Emergence of a clonal lineage of multidrug-resistant ESBL-producing Salmonella Infantis transmitted from broilers and broiler meat to humans in Italy during the period of 2011-2014.

We report the spread in the Italian broiler chicken industry of a clone of multidrug-resistant (MDR), ESBL-producing (blaCTX-M-1) Salmonella enterica subsp. enterica serovar Infantis, firstly detected in Italy 2011 and causing clinical cases in 2013-2014. A set of n=49 of extended-spectrum cephalosporin (ESC)-resistant (R) isolates S. Infantis (2011-2014) from humans, food-producing animals and meat thereof were studied along a selected set of n=42 ESC-susceptible (ESC-S) isolates (2001-2014): they were characterized by macrorestriction-PFGE analysis and genetic environment of ESC-resistance. Isolates representative of PFGE-patterns and origin were submitted to Whole Genome Sequencing. The emerging ESC-R clone, detected mainly in broiler chickens, broiler meat and humans also showed ciprofloxacin microbiological resistance (MIC 0.25 mg/L) and harboured a conjugative megaplasmid (approx. 280-320 Kb, pESI-like), similar to that described in ESC-S S. Infantis in Israel in 2014. This megaplasmid carried the ESBL gene blaCTX-M-1 and further genes (tet(A), sul1, dfrA1 and dfrA14) mediating a minimum resistance pattern of cefotaxime, tetracycline, sulfonamides, and trimethoprim clinical resistance, beside genes conferring enhanced attitudes to colonization, virulence (fimbriae, yersiniabactin), resistance and fitness (qacEΔ1, mer operon) in the intensive-farming environment. Further spread of this emerging MDR, ESBL-producing S. Infantis clone in primary productions deserves quick and thorough risk-management strategies.