Fig 7. Subpopulations of formaldehyde-tolerant cells are distributed within a wild-type population with continuous, exponentially-decreasing frequency.

M. extorquens cells not previously exposed to formaldehyde were plated onto methanol agar medium containing a range of formaldehyde concentrations at 1-mM intervals. The frequency of tolerant cells is expressed as the ratio of the colony-forming units (CFU) on formaldehyde medium at the specified concentration to the CFU on formaldehyde-free (0 mM) medium. Error bars denote the standard deviation of replicate experiments from 5 different dates (shown individually in S4 Fig). Detection limit is indicated by the dashed horizontal line.