Diverse Mobile Genetic Elements and Conjugal Transferability of Sulfonamide Resistance Genes (sul1, sul2, and sul3) in Escherichia coli Isolates From Penaeus vannamei and Pork From Large Markets in Zhejiang, China

Abstract

High prevalence rates of sulfonamide resistance genes <i>sul1</i>, <i>sul2</i>, and <i>sul3</i> have been observed in Gram-negative bacteria isolated from humans, domestic animals, and aquaculture species worldwide. We investigated the distribution characteristics, location, conjugative transferability, and genetic environments of <i>sul</i> genes from <i>Escherichia coli</i> isolates collected from <i>Penaeus vannamei</i> and pork samples from three large markets in Zhejiang, China. The prevalence rates of <i>sul</i> genes in sulfonamide-resistant <i>E. coli</i> isolates from <i>P. vannamei</i> and pork samples were 90.0 and 88.6%, respectively, and the prevalence of <i>sul1</i> and <i>sul2</i> was significantly higher than that of <i>sul3</i> (<i>p</i> < 0.05). Twenty-four representative <i>sul</i>-positive <i>E. coli</i> isolates were analyzed in detail. Southern blot hybridization confirmed that <i>sul</i> genes of <i>E. coli</i> isolates were located on plasmids and/or chromosomes. Transfer of resistance through conjugation was observed in all 18 <i>E. coli</i> isolates harboring <i>sul</i> genes on plasmids. Replicon typing identified seven different incompatibility groups and IncF was the dominant replicon type among <i>sul</i> gene-containing plasmids from both sources. PCR walking analysis indicated that 87.5% (35/40) of <i>sul</i> gene-related fragments carried insertion sequences (ISs) belonging to a variety of families in diverse sites, with IS<i>26</i> occurring most frequently. In addition, the <i>sul1</i> gene was detected mainly in fragments carrying class 1 integrons. Co-location on the same fragment with resistance genes that may contribute to the persistence and dissemination of <i>sul1</i> and/or <i>sul2</i> genes. The diversity of mobile genetic elements and resistance genes adjacent to <i>sul3</i> was much lower than those adjacent to <i>sul1</i> and <i>sul2</i>, especially those located in chromosomes, which reduced the transmission potential of the <i>sul3</i> gene. In conclusion, combined with the results of clonal relatedness analysis by PFGE and MLST of 24 representative <i>E. coli</i> isolates from <i>P. vannamei</i> and pork samples, it showed that a small number of <i>sul</i> genes were vertically transmitted among <i>E. coli</i> from <i>P. vannamei</i> and that horizontal gene transfer was likely the main transmission mechanism of <i>sul</i> genes from both sources. Our results provide important information to better understand the risk of transmission of <i>sul</i> genes from seafood and meat to humans.