Required for optimal acid stress protection, which is important for survival of enteric bacteria in the acidic environment of the host stomach. Exhibits a chaperone-like activity at acidic pH by preventing the aggregation of many different periplasmic proteins. Homodimer at neutral pH. Dissociates into monomers at acidic pH. Induced by the EvgS/EvgA two-component regulatory system. Negatively regulated by H-NS and the TorS/TorR two-component regulatory system. Mutants display increased sensitivity to acid stress at pH 2 and pH 3. In vitro, HdeA is more efficient than HdeB at pH 2 and HdeB is more efficient than HdeA at pH 3. In vivo, both are required for optimal protection against acid stress at either pH 3 or pH 2 (PubMed:17085547). Belongs to the HdeB family. Extended N-terminus. Extended N-terminus.