Lactobacillus acidophilus was suspended in broth and buffer at pH 2, 3 and 4 and incubated at 37°C for 2 hr. In both broth and buffer at pH 2, viable cell numbers decreased rapidly, and none were recovered. after 45 min. At pH 4 in broth and buffer, the number of cells was not significantly reduced in 2 hr. In potassium phthalate buffer at pH 3, no viable cells were recovered after 30 min, while in KC1 buffer and broth at pH 3, no significant reduction was seen. Cells subjected to low pH for up to 5 h were able to adhere to human intestinal cells in vitro. Exposure to low pH did not appear to disrupt the ruthenium red staining layer exterior to the cell wall.
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