The relationship between optical density and cell number differs between species
To investigate the relationship between cell count (obtained from flow cytometry) and OD600 -value (from spectrophotometry) across all 23 strains, we used linear regression (Figure 1, all R2 > 0.87, p < 0.01). We found significant differences between species in the slope of this relationship (ANOVA: F7,22 = 5.63, p = 0.0025; Figure 2). The average slope extracted from S. cerevisiae regressions was significantly steeper than the slopes found in S. mikatae,S. jurei, S. arboricola , and S. uvarum (Tukey HSD, allp < 0.05). The random effect ‘strain’ was non-significant and only explained 4% of the variation in the data (Wald p = 0.81; Table S2).