The relationship between CFU count and OD during exponential growth in the four ancestral strains from the 200-day experiment.

The relationship between OD and CFU count can vary across bacterial strains. To test whether OD is a reliable indicator of CFU count in the strains in our 200-day experiment, we undertook an additional growth rate experiment following the same procedure as before. Every half hour for the first 5 hours of growth, then every hour up until 8 hours of growth, and at the end of the experiment (24 hours of growth), 100μl of the 300μl culture was removed from a single well. This was serially diluted and spot plated on three THY plates. Comparisons of OD and CFU count revealed a linear relationship in all four ancestral strains during the period of exponential growth (between 0.5 and 3.5 hours). For all four strains OD was found to be a good predictor of change in CFU count during exponential growth (Pearson’s correlation coefficient >0.9 for each strain). (XLSX)