ABSTRACTIn open environments such as water, enterohemorrhagicEscherichia coliO157:H7 responds to inorganic phosphate (Pi) starvation by inducing the Pho regulon controlled by PhoB. This activates the phosphate-specific transport (Pst) system that contains a high-affinity Pitransporter. In the Δpstmutant, PhoB is constitutively activated and regulates the expression of genes in the Pho regulon. Here, we show that Pistarvation and deletion of thepstsystem enhanceE. coliO157:H7 biofilm formation. Among differentially expressed genes of EDL933 grown under Pistarvation conditions and in the Δpstmutant, 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 ofE. coliO157:H7 during both Pistarvation and in the Δpstmutant. In the Δpstmutant, the presence ofwaaHwas associated with lipopolysaccharide (LPS) R3 core type modifications, whereas inE. coliO157:H7,waaHoverexpression had no effect on LPS structure during Pistarvation. Therefore,waaHparticipates inE. coliO157:H7 biofilm formation during Pistarvation, but its biochemical role remains to be clarified. This study highlights the importance of the Pistarvation stress response to biofilm formation, which may contribute to the persistence ofE. coliO157:H7 in the environment.IMPORTANCEEnterohemorrhagicEscherichia coliO157:H7 is a human pathogen that causes bloody diarrhea that can result in renal failure. Outside of mammalian hosts,E. coliO157:H7 survives for extended periods of time in nutrient-poor environments, likely as part of biofilms. InE. coliK-12, the levels of free extracellular Piaffect biofilm formation; however, it was unknown whether Piinfluences biofilm formation byE. coliO157:H7. Our results show that upon Pistarvation, PhoB activateswaaHexpression, which favors biofilm formation byE. coliO157:H7. These findings suggest that WaaH is a target for controlling biofilm formation. Altogether, our work demonstrates how adaptation to Pistarvation allowsE. coliO157:H7 to occupy different ecological niches.