Glucose is the preferred carbon source forEscherichia colibacteria. Researchers isolatedE. coli-M, a strain ofE. coliwith a single mutation that inactivates a key enzyme in one of the three glycolytic pathwaysE. colibacteria use to break down glucose. In an initial experiment to study the effect of the mutation on glucose utilization andATP

production by the bacteria, the researchers add210

4

E. coli-M bacteria to 400mL

of a growth medium that contains glucose. To measure the rate of bacterial growth, they monitor the optical density of the culture as a measure of the bacterial concentration over time.