RESUMO
Salmonella enterica serovar Derby (S. Derby) ranks fifth among nontyphoidal Salmonella serovars causing human infections in the European Union. S. Derby isolates (36) collected between 2006 and 2018 in a Spanish region (Asturias) from human clinical samples (20) as well as from pig carcasses, pork- or pork and beef-derived products, or wild boar (16) were phenotypically characterized with regard to resistance, and 22 (12 derived from humans and 10 from food-related samples) were also subjected to whole genome sequence analysis. The sequenced isolates belonged to ST40, a common S. Derby sequence type, and were positive for SPI-23, a Salmonella pathogenicity island involved in adherence and invasion of the porcine jejune enterocytes. Isolates were either susceptible (30.6%), or resistant to one or more of the 19 antibiotics tested for (69.4%). Resistances to tetracycline [tet(A), tet(B) and tet(C)], streptomycin (aadA2), sulfonamides (sul1), nalidixic acid [gyrA (Asp87 to Asn)] and ampicillin (blaTEM-1-like) were detected, with frequencies ranging from 8.3% to 66.7%, and were higher in clinical than in food-borne isolates. The fosA7.3 gene was present in all sequenced isolates. The most common phenotype was that conferred by the tet(A), aadA2 and sul1 genes, located within identical or closely related variants of Salmonella Genomic Island 1 (SGI1), where mercury resistance genes were also present. Diverse IncI1-I(α) plasmids belonging to distinct STs provided antibiotic [blaTEM-1, tet(A) and/or tet(B)] and heavy metal resistance genes (copper and silver), while small pSC101-like plasmids carried tet(C). Regardless of their location, most resistance genes were associated with genetic elements involved in DNA mobility, including a class one integron, multiple insertion sequences and several intact or truncated transposons. By phylogenetic analysis, the isolates were distributed into two distinct clades, both including food-borne and clinical isolates. One of these clades included all SGI1-like positive isolates, which were found in both kinds of samples throughout the entire period of study. Although the frequency of S. Derby in Asturias was very low (0.5% and 3.1% of the total clinical and food isolates of S. enterica recovered along the period of study), it still represents a burden to human health linked to transmission across the food chain. The information generated in the present study can support further epidemiological surveillance aimed to control this zoonotic pathogen.
RESUMO
Objective:To analyze the antimicrobial resistance and genomic characteristics of Salmonella enterica serovar Derby strains isolated from human and food sources in Hangzhou. Methods:A total of 60 Salmonella enterica serovar Derby strains isolated in Hangzhou during the period from 2015 to 2020 were subjected to antimicrobial susceptibility testing, pulsed field gel electrophoresis (PFGE) typing and whole-genome sequencing. Multilocus sequence typing (MLST), core genome multilocus sequence typing (cgMLST) and the identification of antimicrobial resistance genes were performed using the sequencing data. Phylogenetic tree based on the single nucleotide polymorphism (SNP) sites in the 60 genomes from Hangzhou and 379 genomes from public databases was constructed. Results:No significant difference was observed in the drug resistance rates between the clinical strains and food strains in Hangzhou. The multidrug resistance (MDR) rate was 76.7% (46/60). All of the 60 Salmonella Derby strains were positive for the antimicrobial resistance genes aac(6′)- Iaa and fosA7. The 60 strains were subtyped into 46 molecular types by PFGE and 53 molecular types by cgMLST(HC2). Except for one strain belonging to ST3220, the other Salmonella Derby strains were ST40. The phylogenetic analysis showed that some strains isolated in Hangzhou were close to the strains in Southeast Asia, suggesting the possibility of cross-border transmission of ST40 strains, with the main food sources being pork and fish; other strains were close to those circulating in Beijing, Guangzhou, Hubei, Chongqing and other provinces, suggesting the possibility of cross-province transmission of the strains, with the main food sources being pork, beef and chicken. Conclusions:The epidemic of Salmonella Derby in Hangzhou was mainly caused by the spread of ST40 strains and MDR was common. Clinical infections might be closely related to the consumption of pork, beef, chicken and fish. There was the possibility of cross-border transmission of Salmonella Derby between Hangzhou and Southeast Asia and cross-province transmission in China.
