Diauxic Growth
Although microorganisms could encounter a wide variety of nutrients, some are common
while others are unusual. For example, glucose is by far the most common sugar.
Any microorganisms that wasted its energy and metabolic resources making enzymes
for the metabolism of an uncommon nutrient would be at a disadvantage when common
nutrients are present. When a common nutrient such at glucose is present at some
appreciable concentration, it halts the synthesis of enzymes for an alternate carbon-energy
source such as lactose. The next figure illustrates growth on a mixture of these sugars:
Note that when the glucose has fallen to low concentration, there is a plateau in the
curve for cell mass. After a brief delay while the enzymes for lactose are synthesized
and activated, growth resumes to reach a new peak as the lactose is exhausted. The
curve for cell mass declines if the cells die and lyse. In many experiments, death may
take quite a while and the decline may not be noticed.
Cartoon that develops the concept of diauxie