Diauxic Growth of Bacteria

  Diauxic Growth is observed much often that when bacteria are allowed to grow in the medium contalning two different types of carbon/energy sources, they show a characteristic diauxic growth.

The diauxic growth is characterized by the occurrence of two lag phases which separate the log phase in two parts.

Diauxic growth of bacteria, when grown on medium, containing two different carbon sources.

Biochemical reasoning of diauxy

The phenomenon was first observed by Monod during his studies of E.coli grown on medium containing glucose and lactose as the sources of carbon and energy.
  The first lag phase was observed due to the adaptation of organisms to the new medium conditions. This was followed by utilization of glucose.
  Second lag phase was observed when glucose got exhausted from the medium and organisms become prepared to utilize lactose.

  Initially organisms start utilizing glucose, a most readily utiltzable source of carbon and energy and show first lag and log phase. But when, all glucose is consumed by the organisms, the culture again enters a second lag phase during which it induces formation of enzymes required for lactose utilization, after which It again enters second log phase.

  The ability of organisms to use only glucose and not lactose initially is due to the phenomenon of catabolite repression.
  Catabolite repression is the phenomenon where the catabolite of a pathway represses synthesis of an enzyme.

E.coli possesses ability to degrade both glucose and lactose and obtain ATP from them. However, enzymes for glucose utilization are constitutive, whereas those for lactose degradation (β galactosidase) are inducible and are produced only in presence of lactose.
  Synthesis of enzymes for lactose degradation is controlled by phenomenon of catabolite repression.

Basis of catabolite repression

  ATP is a catabolite, produced as a result of degradation of sugars. High concentration of ATP represses synthesis of enzymes required for lactose degradation. Thus, when glucose is already present in the medium, organisms will start degrading It and produce ATP.
  This ATP in turn will repress formation of enzymes for lactose degradation. Hence organisms will not be able to degrade lactose Initially.
  However, when all glucose is consumed by organisms again ATP level in the cell will drop. This will make condition favorable for   β galactosidase synthesis and organisms will become capable of utilizing lactose.
   Thus, during second lag phase, organisms will become prepared to utilize lactose and hence will be able to enter another log phase.