Amino acid decarboxylation-antiporter reactions are one of themost important systems for maintaining intracellular pH betweenphysiological limits under acid stress. We analyzed the Lactobacillusacidophilus NCFM complete genome sequence and selected fouropen reading frames with similarities to genes involved withdecarboxylation reactions involved in acid tolerance in severalmicroorganisms. Putative genes encoding an ornithine decarboxylase,an amino acid permease, a glutamate -aminobutyrate antiporter,and a transcriptional regulator were disrupted by insertionalinactivation. The ability of L. acidophilus to survive low-pHconditions, such as those encountered in the stomach or fermenteddairy foods, was investigated and compared to the abilitiesof early- and late-stationary-phase cells of the mutants bychallenging them with a variety of acidic conditions. All ofthe integrants were more sensitive to low pH than the parentalstrain. Interestingly, each integrant also exhibited an adaptiveacid response during logarithmic growth, indicating that multiplemechanisms are present and orchestrated in L. acidophilus inresponse to acid challenge.



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