Fig 4. Cell damage by formaldehyde results in delayed colony appearance for the majority of cells, but not for the tolerant subpopulation.

Cells from a formaldehyde exposure experiment (liquid MPIPES medium with 4 mM formaldehyde) were sampled at 2- to 4-hour intervals, washed, and plated onto both permissive medium (no formaldehyde, allowing the growth of all cells) and selective medium (4 mM formaldehyde, allowing the growth of only the tolerant subpopulation). A) Images of colonies on plates. Colony size heterogeneity was evident only on permissive medium with cultures exposed to formaldehyde for 16 hours, consistent with a population containing both sensitive cells that formed colonies late due to formaldehyde-induced damage (small colonies) and tolerant cells that formed colonies early (large colonies). All images are shown at the same magnification level; dil = dilution factor prior to plating. B) Relationship between formaldehyde exposure and colony growth characteristics. Shading indicates abundance of colony-forming units in each population (see Fig 2); samples were diluted prior to plating for an average of 500 colonies per plate. Left panel: gray line shows linear regression of appearance time on exposure time for the first 12 hours. Every hour of exposure to formaldehyde led to a ~4.8-hour delay in colony appearance time among sensitive cells. At 16 hours, the population consisted of both damaged and tolerant cells; after 20 hours, all cells were tolerant due to the death of the damaged cells. Right panel: among tolerant cells, formaldehyde exposure had no effect on appearance time. Insets: exposure time affected only the variability among colony growth rates, but not their median.