HdeA (hns-dependent expression protein A) is a single domain alpha-helical protein localized in the periplasmic space. HdeA is involved in acid resistance essential for infectivity of enteric bacterial pathogens. Functional studies demonstrate that HdeA is activated by a dimer-to-monomer transition at acidic pH, leading to suppression of aggregation by acid-denatured proteins. The gene encoding HdeA was initially identified as part of an operon regulated by the nucleoid protein H-NS. This family also contains HdeB.