Antimicrobial resistance in zoonotic nontyphoidal Salmonella: an alarming trend?

Zoonotic bacteria of the genus Salmonella have acquired various antimicrobial resistance properties over the years. The corresponding resistance genes are commonly located on either plasmids, transposons, gene cassettes or variants of the Salmonella Genomic Islands SGI1 and SGI2. Human infections due to nontyphoidal Salmonella isolates are mainly due to ingestion of contaminated food. The two predominantly found Salmonella enterica subsp. enterica serovars in the USA and in Europe are S. Enteritidis and S. Typhimurium. In addition, many other nontyphoidal Salmonella serovars have been implicated in foodborne Salmonella outbreaks. Summary reports of the antimicrobial susceptibility patterns of nontyphoidal Salmonella isolates over time suggest a moderate to low level of antimicrobial resistance and multiresistance. However, serovar-specific analyses showed in part either a steady state, a continuous decline or a recent increase in resistance to certain antimicrobial agents. Resistance to critically important antimicrobial agents, e.g. 3rd generation cephalosporins and (fluoro)quinolones is part of many monitoring programs and the corresponding results confirmed that extended-spectrum β-lactamases are still rarely found in nontyphoidal Salmonella serovars whereas resistance to (fluoro)quinolones is prevalent at variable frequencies among different serovars from humans and animals in different countries. Although it is likely that nontyphoidal Salmonella isolates from animals represent a reservoir for resistance determinants, it is mostly unknown where and when Salmonella isolates acquired resistance properties and which exchange processes have happened since then.