Factors Affecting Bacterial Growth Constants

Abstract

Bacterial population growth can largely, if not completely, be characterized by three fundamental growth constants: (1) Maximum Growth Rate, (2) Lag Time, and (3) Final Optical Density (OD). Due to their ability to capture growth dynamics, these fundamental growth constants have traditionally also been used as measurements of fitness. However, these metrics can be incredibly sensitive to the experimental choices in seemingly unintuitive ways. We quantified these fundamental growth constants in different growth conditions for B. subtilis and E. coli and extract the maximum growth rate, lag time, and final OD. Growing E. coli from different dilutions, we observe that the growth rate trajectory taken is dilution dependent. Moreover, the maximum growth rate is also initial OD or dilution dependent. Inoculating at a lower initial density allows the bacterial culture to have a higher maximum growth rate. In our model, simple nutrient depletion is able to recapitulate these experimental findings qualitatively. In addition to the initial inoculum size affecting the maximum growth rate, other factors such as the variable presence of glycerol can affect the robust measurement of lag. We observed that the presence of glycerol in day 1 of growth affects the lag times in day 2 for B. subtilis 168 dramatically. Traditionally this might not have been a problem in B. subtilis because most labs streak out cultures and start liquid culture from a colony, or inoculate into large volumes. However, for an experimental method that is tractable to working with libraries and high throughput measurements these solutions aren’t possible.