Conclusions
While spectrophotometry is widely used to assess the growth and fitness of microbial cultures, it has important limitations. Flow cytometry is a more accurate method for measuring population size but it is not as widely available or as cost-effective as spectrophotometry. Our work shows that the relationship between cell density (from spectrophotometry) and cell count data (from flow cytometry) differs among Saccharomyces yeast species. Thus, when conducting research on non-model strains and species of yeast and flow cytometry is not available, we caution to calibrate cell density measurements carefully (e.g. by manual cell or colony counting) prior to comparative fitness analysis, and to take in account the diverse ecology, life history, and physical properties of yeast species to avoid biological misinterpretations.