Regulation of waaH by PhoB during Pi Starvation Promotes Biofilm Formation by Escherichia coli O157:H7

Abstract

In open environments such as water, enterohemorrhagic Escherichia coli O157:H7 responds to inorganic phosphate (P_i) starvation by inducing the Pho regulon controlled by PhoB. This activates the phosphate-specific transport (Pst) system that contains a high-affinity P_i transporter. In the Δpst mutant, PhoB is constitutively activated and regulates the expression of genes in the Pho regulon. Here, we show that P_i starvation and deletion of the pst system enhance E. coli O157:H7 biofilm formation. Among differentially expressed genes of EDL933 grown under P_i starvation conditions and in the Δpst mutant, we have found that a member of the PhoB regulon, waaH, predicted to encode a glycosyltransferase, was highly expressed. Interestingly, WaaH contributed to biofilm formation of E. coli O157:H7 during both P_i starvation and in the Δpst mutant. In the Δpst mutant, the presence of waaH was associated with lipopolysaccharide (LPS) R3 core type modifications, whereas in E. coli O157:H7, waaH overexpression had no effect on LPS structure during P_i starvation. Therefore, waaH participates in E. coli O157:H7 biofilm formation during P_i starvation, but its biochemical role remains to be clarified. This study highlights the importance of the P_i starvation stress response to biofilm formation, which may contribute to the persistence of E. coli O157:H7 in the environment.IMPORTANCE Enterohemorrhagic Escherichia coli O157:H7 is a human pathogen that causes bloody diarrhea that can result in renal failure. Outside of mammalian hosts, E. coli O157:H7 survives for extended periods of time in nutrient-poor environments, likely as part of biofilms. In E. coli K-12, the levels of free extracellular P_i affect biofilm formation; however, it was unknown whether P_i influences biofilm formation by E. coli O157:H7. Our results show that upon P_i starvation, PhoB activates waaH expression, which favors biofilm formation by E. coli O157:H7. These findings suggest that WaaH is a target for controlling biofilm formation. Altogether, our work demonstrates how adaptation to P_i starvation allows E. coli O157:H7 to occupy different ecological niches.

Keywords: EHEC; Pho regulon; biofilms; lipopolysaccharide; phosphate.