Research note: Study on the characteristics of Salmonella typhimurium derived from Larus vegae

Samples of four dead Larus vegae along the coast of Beidaihe, Qinhuangdao City, were identified by PCR amplification, multi-sequence site typing (MLST), and serum agglutination test. The drug resistance phenotype, drug resistance gene, and virulence gene were detected by the Kirby-Bauer (K-B) method and PCR method, and the pathogenicity of mice and pigeons was further tested. The phylogenetic tree was constructed by whole genome sequencing and bioinformatics analysis. The results showed that the isolated bacteria were identified as Salmonella typhimurium(S. typhimurium) by multiple tests, named ST-G, ST subtype ST19, serotype 1,4,[5],12:i:1,2. The results showed that the isolates were resistant to erythromycin, co-trimoxazole, and clindamycin. The results of drug resistance genes showed qnrS and gyrA, and no other drug resistance genes were detected. Virulence genes carried a high rate, 13 virulence genes except stn had corresponding fragment size detected. The pathogenicity test of mice showed that the incidence of mice in the injection group was 60%, and all the mice in the high-dose group died. Pathological sections showed multifocal bleeding, inflammatory cell infiltration, and intestinal villi rupture. The pathogenicity test of rock pigeons showed that liver swelling, gallbladder filling, and intestinal bleeding swelling of infected pigeons were consistent with the symptoms of salmonella infection in clinical pigeons. The morbidity and mortality of the St-G test group were higher than that of the SL1344 quality control group, indicating that the St-G isolated strain was more virulent. Phylogenetic tree results showed that this isolate was highly homologous to the 2023 Russian isolate. Salmonella typhimurium was isolated from Larus vegae for the first time, which provides a basis for the study and prevention of salmonellosis transmitted by wild birds.