Bacterial Reproduction and Life Cycle

Bacterial Growth Cycle

Lag phase ­: organisms are becoming acclimated to the media and new conditions; precursor molecules are being accumulated and synthesized; growth is arithmetic rather than exponential with the number of new organisms roughly equal to the number dying.

Log or Exponential Phase ­: organisms have acclimated to conditions; growth rate is maximal with number of new organisms far exceeding number dying; metabolic wastes beginning to accumulate in the media

Plateau or Stationary Phase ­: metabolic wastes accumulating rapidly; carbon dioxide increasing causing pH change in media; oxygen tension decreasing; space and media being used up; results in roughly equal amounts of new and dying organisms

Decline and Death Phase ­: exacerbation of previous conditions leading to increased number of deaths versus births; metabolic wastes becoming toxic and cumulative; short term plateaus, representing mutant forms, may appear occasionally

Information about Bacterial Population Growth, Dr. R. G. Steane

Conditions described above seldom exist in nature; they are more likely to be found in a closed system such as a Petri dish.

Total Cell Counts ­: direct microscopic examination or the Petroff Hausser grid slide method; normally used with liquid samples Limitations include imprecision, need for a high density suspension, both living and dead cells are counted

Electronic counter such as photoelectric colorimeters or spectrophotometers are indirect methods which measure turbidity or cloudiness of suspension; gives a relative estimate of cell weight

Viable or Living Counts ­: include Plate Count method employing a special agar; usually requires a series of dilutions ; the Spread Plate or Streak Plate requires a dry surface for the liquid sample of a known volume to soak in; or the Pour Plate which uses a known volume of sample dissolved within the media

In each case the final plate must have between 30 and 300 colonies to be considered reliable; this assumes that each viable cell in the sample will produce a visible colony; the lower limit of 30 represents the number which is considered to be statisti- cally significant in terms of sample size; an upper limit of 300 may actually be less but usually represents how many colonies can exist separately on the plate without fusing into other colonies and thus obscuring the count