Emergence of a Clonal Lineage of Multidrug-Resistant ESBL-Producing <i>Salmonella</i> Infantis Transmitted from Broilers and Broiler Meat to Humans in Italy between 2011 and 2014

<div><p>We report the spread of a clone of multidrug-resistant (MDR), ESBL-producing (<i>bla</i>_CTX-M-1) <i>Salmonella enterica</i> subsp. <i>enterica</i> serovar Infantis, in the Italian broiler chicken industry and along the food-chain. This was first detected in Italy in 2011 and led to human infection in Italy in 2013–2014.A set (n = 49) of extended-spectrum cephalosporin (ESC)-resistant (R) isolates of <i>S</i>. Infantis (2011–2014) from humans, food-producing animals and meat thereof, were studied along with a selected set of earlier and more recent ESC-susceptible (ESC-S) isolates (n = 42, 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 from broiler chickens, broiler meat and humans, showed a minimum pattern of clinical resistance to cefotaxime, tetracycline, sulfonamides, and trimethoprim, beside ciprofloxacin microbiological resistance (MIC 0.25 mg/L). All isolates of this clone harbored a conjugative megaplasmid (~ 280–320 Kb), similar to that described in ESC-susceptible <i>S</i>. Infantis in Israel (pESI-like) in 2014. This megaplasmid carried the ESBL gene <i>bla</i>_CTX-M-1, and additional genes [<i>tet</i>(A), <i>sul1</i>, <i>dfrA1</i> and <i>dfrA14</i>] mediating cefotaxime, tetracycline, sulfonamide, and trimethoprim resistance. It also contained genes conferring enhanced colonization capability, virulence (fimbriae, yersiniabactin), resistance and fitness (<i>qacE1</i>, <i>mer</i>) in the intensive-farming environment. This emerging clone of <i>S</i>. Infantis has been causing infections in humans, most likely through the broiler industry. Since <i>S</i>. Infantis is among major serovars causing human infections in Europe and is an emerging non-typhoidal <i>Salmonella</i> globally, further spread of this lineage in primary productions deserves quick and thorough risk-management strategies.</p>