Abstract
Escherichia coli O157:H7 is an important food-borne pathogen that can cause hemorrhagic colitis and hemolytic-uremic syndrome in humans. pO157_Sal, a novel conjugative plasmid is present in a Chinese O157:H7 outbreak strain Xuzhou21. Here we investigated the phenotypic and transcriptional differences between the wild type strain Xuzhou21 and the pO157_Sal cured mutant strain Xuzhou21m. RNA-Seq analysis found that all 52 ORFs encoded on pO157_Sal were transcribed. One hundred and sixty eight chromosomal and pO157 genes were differentially expressed (≥2 fold difference) between Xuzhou21 and Xuzhou21m. Sixty-seven and 101 genes were up-regulated and down-regulated respectively by pO157_Sal including genes related to stress response, adaption and virulence. The plasmid-cured mutant Xuzhou21m grew slower than wild type Xuzhou21 and pO157_Sal plasmid complemented strain Xuzhou21c in M9 medium under the condition of high NaCl or presence of sodium deoxycholate (NaDC), corroborating with the RNA-Seq data. Seven differentially expressed genes are associated with NaDC resistance, including the adenine-specific DNA-methyltransferase gene (dam), multidrug efflux system subunit gene mdtA, hyperosmotically inducible periplasmic protein gene osmY and oxidation-reduction related genes while two differentially expressed genes (osmY and pspD) are likely to be related to resistance to osmotic pressure. A number of differentially expressed genes were virulence associated including four genes encoding T3SS effectors from the chromosome and ehxD from pO157. Through complementation of Xuzhou21m with a plasmid construct carrying the pO157_Sal hha homolog we further showed that the pO157_Sal hha represses the expression of T3SS effectors. These findings demonstrated that the plasmid pO157_Sal affects the transcription of the chromosomal and pO157 plasmid genes and contributes to the enhanced ability to resist stress. We conclude that pO157_Sal plays an important role in regulating global gene expression and affects the virulence and adaptation of E. coli O157:H7.