Antimicrobial Resistance and Extended-Spectrum β-Lactamases of Salmonella enterica Serotypes Isolated from Livestock and Processed Food in Portugal: An Update

As Salmonella is a common foodborne pathogen, the present study aimed to determine the distribution of Salmonella enterica serotypes isolated during 2011–2012 from poultry, swine, cattle, and processed food in Portugal, and to characterize the antimicrobial susceptibility and the extended-spectrum β-lactamases (ESBLs). Results were also compared with data obtained before the implementation of the National Control Program in Poultry and the ban of antimicrobial agents in animal feed in the European Union (EU). A total of 14 serotypes were identified, from 258 isolates recovered, with Salmonella Typhimurium (32.6%, n=84) and Salmonella Enteritidis (10.1%, n=26) being the most common. Salmonella Enteritidis in poultry was less frequent than in previous studies, which might be associated with the implementation of the National Control Program for Salmonella in poultry. Nevertheless, other serotypes seem to occupy this biological niche, and may be more common in human salmonellosis in the future. The majority of isolates (70.2%, n=181) were resistant to at least one class of antimicrobial agent and exhibited higher frequency of resistance to tetracycline (47.7%, n=123) and ampicillin (36.0%, n=93), with Salmonella Typhimurium being the more resistant serotype. Resistance to fluoroquinolones was shown in 8% (n=21) of isolates, a lower value compared to data obtained before 2004. ESBLs producers Salmonella Typhimurium blaCTX₋M₋₁ and Salmonella Enteritidis blaSHV₋₁₂ were isolated from swine and poultry, respectively. The blaCTX₋M₋₁ and blaSHV₋₁₂ genes were carried on conjugative plasmids of IncHI2replicon types and IncI1, respectively. This was the first report of a blaCTX₋M₋₁ in Salmonella Typhimurium in Portugal. Overall, the results revealed changes in animal origin Salmonella serotypes, mainly emerging serotypes, in frequency of resistance, and in occurrence of ESBLs-producing Salmonella. The control measures taken by the EU seem to have some impact on the resistance rate of some antibiotics such as quinolones. The emergence of ESBLs and its potential spread among animal reservoirs and the food chain highlight the continuous antimicrobial surveillance at the animal level.