Fig. 1

The design of Experiment 1. In short-term treatment, cells were treated with low pH (initial pH 6.6–5.2) for 3 h in the absence or presence of S9 mix, followed by recovery culturing for 21 h using normal media (approximately pH 7.7). In continuous treatment, cells were treated with low pH (initial pH 6.8–6.1) for 24 h without S9 mix. Then, the pH of the medium was measured at the start and end of treatment (3 and 24 h after treatment in the short-term and continuous treatments, respectively). RICC and MN frequencies were determined 24 h post-treatment initiation