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
http://web.ukonline.co.uk/webwise/spinneret/copyrt.htm, 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