Effects of deletion mutations of mleR (A), mleS (B), or mleP (C) on MLF protection against acid killing of Streptococcus mutans UA159. Cells were exposed to a constant pH of 3.0 with or without 25 mmol/L l-malate. N0, the initial number of colony-forming units per millilitre; N, the number of colony-forming units at time t of acid exposure. Error bars indicate SD, n = 3.