The ability of a lactic acid bacterium to survive passage through the gastrointestinal tract is a key
point in its function as a probiotic. In this study, protein synthesis by the probiotic bacterium,
Lactobacillus reuteri, was analyzed under transiently decreased pH conditions. L. reuteri cells
grown to the midexponential growth phase at 377C were exposed to transient (1 h) low-pH
stresses from pH 6.8 to pH 5.0, 4.5, or 4.0. 2-DE allowed us to identify 40 common proteins that
were consistently and significantly altered under all three low-pH conditions. PMF was used to
identify these 40 proteins, and functional annotation allowed them to be distributed to six major
classes: (i) transport and binding proteins; (ii) transcription–translation; (iii) nucleotide metabo-
lism and amino acid biosynthesis; (iv) carbon energy metabolism; (v) pH homeostasis and stress;
and (vi) unassigned. These findings provide new insight into the inducible mechanisms under-
lying the capacity of gastrointestinal L. reuteri to tolerate acid stress.