5.6 The Growth Cycle (Continued…)
Interval between inoculation of a culture and beginning of growth (i.e. increase in
Cells in this phase are typically in the healthiest state.
Cells metabolically active, but growth rate of population is zero
Either an essential nutrient is used up, or waste product of the organism accumulates
in the medium.
If incubation continues after cells reach stationary phase, the cells will eventually die
72 Microbiology 1010 – Lecture Notes
Not all bacteria die, some bacteria form spores/cysts or dormant stages that allow a
significant proportion of cells to survive for a long time.
Continuous culture – An open-system microbial
culture of fixed volume
Most common type of continuous culture
Both growth rate and population density of
culture can be controlled independently
Continuous Culture (Continued…)
Dilution rate: rate at which fresh medium is
pumped in and spent medium is pumped
Concentration of a limiting nutrient controls the population size and the growth rate.
Microbial cells can be enumerated by direct microscopic observations using a Petroff-
Hausser counting chamber
Each square corresponds to a calibrated volume.
o But results can be unreliable…
73 Microbiology 1010 – Lecture Notes
Microscopic Counts (Continued…)
Limitations of microscopic counts
Cannot distinguish between live and dead cells without special stains
Small cells can be overlooked
Precision is difficult to achieve (need a lot of counts)
Phase-contrast microscope required if a stain is not used
Cell suspensions of low density (<10 cells/ml) hard to count
Motile cells need to immobilized
Debris in sample can be mistaken for cells
Cells may move (Brownian motion), some form clumps Based on random
distribution and dispersal of the cells
Microscopic Counts (Continued…)
A second method for enumerating cells in liquid samples is use of a flow cytometer
Uses laser beams, fluorescent dyes, and electronics
Viable cell counts (plate counts): measurement of only living cells capable of growing to form a
Two main ways to perform plate counts:
o Pour-plate method
74 Microbiology 1010 – Lecture Notes
Viable Count Issues
Plate counts require significant amounts of preparation and manipulations (dilution tubes, agar
plates), and incubation time (overnight or more) to get the measurements for a single culture.
Plate counts can be highly unreliable when used to assess total cell numbers of natural samples
(e.g., soil and water)
Selective culture media and growth conditions target only particular species
There is no medium that will grow every microbe!
Can only count the types of bacteria that can grow in the medium you selected to use!
They are inaccurate because sometimes colonies grow on top of each other...there is
different anomalies that can cause numbers that are inaccurate.
If you looking to culture bacteria…. You’re not going to have all microorganisms (there isn’t a media
that grows all types of bacteria).
The great plate anomaly: direct microscopic counts of natural samples reveal far
more organisms than those recoverable on plates
Modern genomic techniques suggest that only 1-10% of microbial diversity is culturable from
most environmental samples (including the diver