This study investigated features of the acid tolerance response (ATR) inLactobacillus casei is an aciduric, rod-shaped, facultatively heterofermentative lactic acid bacterium (LAB) that can be isolated from a variety of environments including raw and fermented milk and meat or plant products, as well as the oral, intestinal, and reproductive tracts of humans and animals (24). Like other LAB species, L. casei produces lactic acid as a major end product of carbohydrate fermentation during growth, and strains of L. casei are used as acid-producing starter cultures in the preparation of fermented foods, as health-enhancing probiotic cultures, and for the production of L(ϩ)-lactic acid (18,47,52). In each of these applications, the industrial performance of L. casei strains is dependent, in one way or another, on their acidurance.During fermentation, L. casei transports lactic acid outside the cell as lactate ion via an electrogenic proton-lactate symporter. As the pH of the medium (pH o ) decreases or the concentration of lactate increases, the concentration of protonated (undissociated) lactic acid in the medium also increases. The undissociated form of lactic acid is membrane soluble and thus can enter the cytoplasm by simple diffusion (27). Metabolically active bacteria maintain a pH gradient (⌬pH) where the intracellular pH (pH i ) is more alkaline than the pH o (4, 25), so diffusion of acid into the cytoplasm results in rapid dissociation and release of protons and anions inside the cell.