Increased kanamycin-resistant Salmonella Schwarzengrund, serovar changing and antimicrobial susceptibility of Salmonella enterica from broiler chickens in Kagoshima, Japan between 2019 and 2023

This study investigated the prevalence of Salmonella, determined the minimum inhibitory concentrations (MICs) of antimicrobials, and examined antimicrobial resistance genes and plasmids in Salmonella strains from broiler chickens in Kagoshima Prefecture, Japan. A total of 3,774 cecal samples were collected from 236 broiler flocks between 2019 and 2023. Salmonella was prevalent in 202 (85.6%) and 753 (19.9%) broiler flocks and cecal content samples, respectively. Salmonella Manhattan, S. Schwarzengrund, and S. Infantis were the only serovars found in broiler chickens in Kagoshima Prefecture. Since 2021, the isolation rate of S. Schwarzengrund has been higher than that of other serovars. A total of 278/753 (36.9%) Salmonella strains were resistant to kanamycin, 99.3% of which were S. Schwarzengrund strains (MIC value ≥ 512 µg/mL). Furthermore, of the 268 Salmonella isolates that were tested for susceptibility to all antimicrobials used in this study, all were susceptible to cefoxitin and ofloxacin, and the majority were resistant to streptomycin-S (98.5%), sulfamethoxazole-Su (90.3%), tetracycline-T (79.5%), and kanamycin-Km (28.7%). In addition, three plasmid replicons (IncFIB, IncP1, and IncX4) were identified in Salmonella. IncFIB was present in all the Salmonella isolates examined and contained antimicrobial resistance genes. IncX4 and IncP1 were identified in streptomycin, sulfamethoxazole, and tetracycline (SSuT)-resistant S. Manhattan and streptomycin, sulfamethoxazole, tetracycline, and kanamycin (SSuTKm)-resistant S. Infantis, respectively, and contained none of the resistance genes. This study revealed an increased incidence of S. Schwarzengrund in broiler chickens, which exhibit high kanamycin resistance. Additionally, a significant percentage of the Salmonella isolates in this study were resistant to streptomycin, tetracycline, and sulfamethoxazole. This finding suggests that these antimicrobials are ineffective at controlling infections in broilers. In addition, we speculate that the spread of antibiotic resistance in Salmonella is enhanced by plasmids.