Different activated methyl cycle pathways affect the pathogenicity of avian pathogenic Escherichia coli

The activated methyl cycle (AMC) regulates the cellular levels of S-adenosyl-L-homocysteine (SAH) in bacteria, which plays a crucial role in bacterial pathogenicity. There are two AMC pathways in bacteria: one is a two-step reaction pathway (named the LuxS/Pfs pathway) in which LuxS and Pfs catalyze the conversion of SAH to l-homocysteine and autoinducer-2 (AI-2), and the other is a one-step reaction (named the SahH pathway) mediated by S-adenosyl-L-homocysteine hydrolase (SahH), which completes this cycle without producing AI-2. In this study, we evaluated the effects of different AMC pathways on the pathogenicity of avian pathogenic Escherichia coli (APEC). The plasmid pSTV-sahH (containing the sahH gene of Pseudomonas aeruginosa) was transformed into the wild-type APEC strain DEI7 (containing the LuxS/Pfs pathway) and the pfs mutant strain DE17 Delta pfs, which lacks the LuxS/Pfs pathway, to create the strains SahH-DE17 Delta pfs (containing the SahH pathway) and SahH-DE17 (containing the LuxS/Pfs and SahH pathways). The results showed that the different AMC pathways had different effects on the growth rate, AI-2 activity, and motility in APEC. Furthermore, we showed that the 50% lethal doses of the DE17 Delta pfs and SahH-DE17 Delta pfs strains were reduced by 650-fold and 52-fold, respectively, in ducklings, compared with that of the DEI7 strain. The DE17 Delta pfs strain exhibited significantly reduced adherence and invasion (p < 0.01). In addition, the DE17 Delta pfs and SahH-DE17 Delta pfs strains also showed reduced survival in vivo, as evidenced by significant (p < 0.01) reductions in their bacterial loads in infected liver, spleen, kidney, and blood. This study suggests that different AMC pathways affect the pathogenesis of APEC.