Fluorescent protein expression as a proxy of bacterial fitness in a high throughput assay

Bacterial growth is classically assessed by measuring the increase in optical density of pure cultures in shaken liquid media. Measuring growth using optical density has severe limitations when studying multistrain interactions as it is not possible to measure the growth of individual strains within mixed cultures. Here we demonstrated that constitutively expressed fluorescent proteins can be used to track the growth of individual strains in different liquid media. Fluorescence measurements were highly correlated with optical density measurements and cell counts. This allowed us to assess bacterial growth not only in pure cultures, but also in mixed bacterial cultures and determine the impact of competitors on a focal strain, thereby assessing relative fitness. Furthermore, we were able to track the growth of two different strains simultaneously by using fluorescent proteins with differential excitation and emission wavelengths. Bacterial densities measured by fluorescence yielded more consistent data between technical replicates than optical density measurements. Our setup employs fluorescent microplate readers that allow for high throughput and replication. ImportanceWe expand on an important limitation of the concept of measuring bacterial growth which is classically limited to one strain at a time. By adopting this approach, it is possible to measure growth of several bacterial strains simultaneously in high temporal resolution and in a high throughput manner. This is important to investigate bacterial interactions such as competition and facilitation.