Monitoring of Antimicrobial Resistance of Salmonella Serotypes Isolated from Humans in Northwest Italy, 2012-2021

Abstract

Salmonella enterica is among the most common causes of foodborne outbreaks in humans in Europe. The global emergence of resistance to antimicrobials calls for close monitoring of the spread and prevalence of resistant Salmonella strains. In this study, we investigated the occurrence of antimicrobial resistance of Salmonella serotypes isolated from humans between 2012 and 2021 in Piedmont, northwest Italy. A total of 4814 Salmonella strains (168 serotypes) were tested against six classes of antimicrobials. Many strains (83.3%) showed resistance to at least one antibiotic: tetracycline (85.1%), ampicillin (79.2%), quinolones (47.4%), and gentamicin (28.4%). Between the first (2012-2016) and the second study period (2017-2021), a decrease in antimicrobial resistance was noted for tetracycline (from 92.4% to 75.3%), ampicillin (from 85.3% to 71.3%), quinolones (from 49.4% to 44.6%), and cefotaxime (from 34.8% to 4.0%). Many multidrug resistant Salmonella strains (43.6%) belonged to S. ser. Typhimurium, S. ser. Infantis, and S. ser. Typhimurium 1,4,[5],12:i:-. Overall, multidrug resistance decreased from 60.7% to 26.4%, indicating a reduction in the antimicrobial resistance of Salmonella strains in Piedmont and in Europe and demonstrating the effectiveness of the measures that were put in place to reduce antimicrobial resistance.

Keywords: Salmonella spp.; disease surveillance; drug-resistant bacteria; foodborne infections.

Figure 1

Prevalence of Salmonella serotypes involved in human infections in Piedmont during 2012–2021.

Figure 2

Prevalence and 95% CI of AMR in the most common serotypes recovered in the two halves of the study period. Prevalence is expressed as the number of isolates that showed resistant phenotypes against at least one drug. Asterisks denote significant differences in AMR between the two time periods (Pearson’s chi-squared test, * p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 3

Percentage of susceptibility and resistance of Salmonella strains causing infections in humans in the two halves of the study period. Asterisks denote significant differences in AMR against each antibiotic between the two time periods (Pearson’s chi-squared test, ** p < 0.01; *** p < 0.001).

Figure 4

Number of strains resistant against the six classes of antimicrobials in each Salmonella serotype. Serotypes with fewer than 10 strains are summed and denoted as “Others”.

Figure 5

Prevalence and 95% CI of MDR in the most frequent serotypes recorded in the two halves of the study period. Asterisks denote significant differences in the prevalence of MDR between the two study periods (Pearson’s chi-squared test, * p < 0.05; ** p < 0.01; *** p < 0.001).