Six strains of Propionibacterium (three strains of P. acidipropionici, one strain of P. jensenii, and two strains of P. thoenii) were identified as potential acid- and low-pH-tolerant strains from a screen of the culture at Iowa State University collection for acid production and final pH achieved on various carbon sources. The lowest pH at which the strains could initiate growth and remain viable were determined with lactic, propionic, and hydrochloric acid as acidulants. The ability to produce large amounts of acid or achieve low final pH values at the end of culture growth was not directly related to a strain's ability to initiate growth or survive in low-pH conditions;When grown in batch culture at pH 7, the proton motive force ([delta]p) of P. thoenii P127 was comprised of only the electrical potential ([delta][psi]); the internal pH remained constant at pH 7. During growth at pH 5, the [delta]p consisted of both [delta][psi] and pH gradient ([delta]pH) and the internal pH decreased slowly throughout the fermentation. Growth at low pH was characterized by low specific growth rate, low internal ATP concentrations, and low propionic and acetic acid yields. The overall [delta]p values were about the same for cultures grown at low or neutral pH, but the individual components of [delta]p ([delta][psi] and [delta]pH) were affected by external pH;Logarithmically growing cells of P127 exposed to mild acid conditions (pH 5.5) for one doubling were more resistant to lethal pH conditions (pH 3.5) than were cells shifted directly from neutral pH to pH 3.5. This acid tolerance response required protein synthesis and allowed the cells to maintain a proton motive force during exposure to pH 3.5. In contrast, the [delta]p of the unadapted cells collapsed. Exposure of P127 to acidic pH values caused the cells to manipulate the components of the [delta]p to maintain a large transmembrane potential. Strain P127 could not initiate growth at pH values below 5.0; in addition, the [delta]pH dramatically decreased when cells were exposed to pH values <5.0. This may suggest that the cells' inability to maintain a large enough [delta]pH is responsible for growth inhibition.