Prevalence of Chromosomally Located bla(CTX-M-55) in Salmonella Typhimurium ST34 Isolates Recovered from a Tertiary Hospital in Guangzhou, China

Nontyphoidal Salmonella (NTS) is one of the most prevalent bacterial causes of gastrointestinal infections worldwide. Meanwhile, the detection rate of CTX-M-55 ESBL-positive has increased gradually in China. To identify the molecular epidemiological and genomic characteristics of bla(CTX-M-55)-carrying nontyphoidal Salmonella (NTS) clinical isolates, a total of 105 NTS isolates were collected from a Chinese tertiary hospital. Antimicrobial susceptibility testing was performed to determine the resistance phenotype. Whole-genome sequencing and bioinformatics analysis were used to determine the antimicrobial resistance genes, serotypes, phylogenetic relationships, and the genetic environment of the bla(CTX-M-55) gene. The results showed that among the 22 ceftriaxone resistant isolates, the bla(CTX-M-55) was the most common β-Lactamase gene carried by 14 isolates, including serotypes S. Typhimurium (10/14), S. Muenster (2/14), S. Rissen (1/14), and S. Saintpaul (1/14). Phylogenetic analysis shows that 10 bla(CTX-M-55)-positive S. Typhimurium ST34 isolates were divided into two clusters. The genetic relationship of isolates in each cluster was very close (≤10 cgMLST loci). The bla(CTX-M-55) gene was located on the chromosome in 10 isolates, on IncI1 plasmid in three isolates, and IncHI2 plasmid in one isolate. In conclusion, the bla(CTX-M-55) gene, mainly located on the chromosome of S. Typhimurium ST34 isolates, was the main driving force associated with the resistance of NTS to cephalosporins. Therefore, close attention to the clonal dissemination of bla(CTX-M-55)-carrying S. Typhimurium ST34 in clinical settings must be monitored carefully. IMPORTANCE ESCs are the first choice for treating NTS infections. However, ESBLs and AmpC β-lactamases are the most typical cause for ESCs resistance. The CTX-M-55 ESBL-positive rate has gradually increased in the clinic in recent years. At present, the research about bla(CTX-M-55)-positive Salmonella mainly focuses on the foodborne animals or the environment while less on clinical patients. Thus, this study was carried out for identifying molecular epidemiological and genomic characteristics of bla(CTX-M-55)-carrying NTS clinical isolates. The results showed that the bla(CTX-M-55) gene, mainly located on the chromosome of S. Typhimurium ST34 isolates from Conghua District, was the main driving force associated with the resistance of NTS to cephalosporins. Therefore, our work highlights the importance of monitoring the clonal dissemination of bla(CTX-M-55)-carrying S. Typhimurium ST34 in clinical settings.