Serotype Distribution and Antimicrobial Resistance of Salmonella Isolates from Poultry Sources in China

Background: Salmonella is an important zoonotic pathogen, of which poultry products are important reservoirs. This study analyzed the prevalence, antimicrobial resistance, and characterization of Salmonella from broiler and laying hen sources in China. Methods: A total of 138 (12.27%) strains of Salmonella were isolated from 1125 samples from broiler slaughterhouses (20.66%, 44/213), broiler farms (18.21%, 55/302), and laying hen farms (6.39%, 39/610). Multiplex PCR was used to identify the serotypes. Antibiotic susceptibility testing to a set of 21 antibiotics was performed and all strains were screened by PCR for 24 selected antimicrobial resistance genes (ARGs). In addition, 24 strains of Salmonella were screened out by whole-genome sequencing together with 65 released Salmonella genomes to evaluate phylogenetic characteristics, multilocus sequence typing (MLST), and plasmid carriage percentages. Results: A total of 11 different serotypes were identified, with the dominance of S. Enteritidis (43/138, 31.16%), S. Newport (30/138, 21.74%), and S. Indiana (19/138, 13.77%). The results showed that S. Enteritidis (34.34%, 34/99) and S. Newport (51.28%, 20/39) were the dominant serotypes of isolates from broilers and laying hens, respectively. The 138 isolates showed the highest resistance to sulfisoxazole (SXZ, 100%), nalidixic acid (NAL, 54.35%), tetracycline (TET, 47.83%), streptomycin (STR, 39.86%), ampicillin (AMP, 39.13%), and chloramphenicol (CHL, 30.43%), while all the strains were sensitive to both tigacycline (TIG) and colistin (COL). A total of 45.65% (63/138) of the isolates were multidrug-resistant (MDR) strains, and most of them (61/63, 96.83%) were from broiler sources. The results of PCR assays revealed that 63.77% of the isolates were carrying the quinolone resistance gene qnrD, followed by gyrB (58.70%) and the trimethoprim resistance gene dfrA12 (52.17%). Moreover, a total of thirty-four ARGs, eighty-nine virulence genes, and eight plasmid replicons were detected in the twenty-four screened Salmonella strains, among which S. Indiana was detected to carry the most ARGs and the fewest plasmid replicons and virulence genes compared to the other serotypes. Conclusions: This study revealed a high percentage of multidrug-resistant Salmonella from poultry sources, stressing the importance of continuous monitoring of Salmonella serotypes and antimicrobial resistance in the poultry chain, and emergency strategies should be implemented to address this problem.