According to previous studies, L. acidophilus possesses several mechanisms to counteract acid stress (Altermann et al., 2005; Azcarate-Peril, Altermann, Hoover-Fitzula, Cano, & Klaenhammer, 2004). Kullen and Klaenhammer (1999) reported that the F1F0- ATPase system maintaining the pHi was an important mechanism for promoting survival in the presence of accumulating organic acids.