MICRB 265 (October 18, 2013)
Microbial Nutrition and Growth (con’t)
This phase follows the exponential phase
Cells are still alive but they are not growing in count
Cells often shrink since there is a reduced need for ribosomes
Sporulation becomes more typical in older g+ve cultures when they approach this phase
o This is where we start seeing sporulation
Reasons for Stationary Phase
Substrate is limited in minimal and defined media
Oxygen is limited in complex media (high respiration and low solubility of oxygen)
o 100000x less soluble in liquid than in air
o Stationary phase is not due to cell crowding
Diffusion of oxygen is not limited in a colony
Presence of toxins
Growth rate = death rate
You can’t tell that the culture is still growing since the numbers don’t change
o Bacteria are waiting for conditions to become favourable again
Higher resistance to antibiotics is seen
Antibiotics mainly target growth… since the cell is no longer
growing, it is less affected by antibiotics
Clear decline in cell number
o Rate cannot be predicted or calculated
Rate may be linear or exponential but one thing is for sure… the death
phase is NOT a mirror image of the exponential phase!! Decreasing cell
count is reliant on different mechanisms as compared to those of
Viable, non-culturable phenomenon
o Under the microscope, you can see the living cells, however the # of viable
colonies you get when you transfer them to an agar plate is lower… o What does this tell you?
Not all cells are metabolically uniform when it comes to differing cultures
Limitations in substrate and nutrients may cause come cells to
become cannibalistic in nature… eat other cells in order to survive
a bit longer
o Some cells actually commit suicide (lyse themselves) to
provide nutrients to fellow cells…
When faced with starvation or growth arrest, cell may:
o Go through a death phase and there will still be a few survivors
o Go through genetically programmed death to provide survivors with nutrients to
survive until favourable conditions arrive (hopefully)
o Cells may become sterile and will not become active until passing through the
digestive system of an animal
Eg. Cholera becomes infection after passing through digestive system of
If a cell dies but doesn’t lyse itself, how do you know if it is dead?
o Stain them!
Cells are initially stained green.
Then stained with red stain that is able to permeate the damaged
membrane of dead cells but not the intact membrane of the live cells.
Red stain pushes out the green dye.
Dead cells appear red and the living cells appear green.
o Living definition here = intact cell membrane
Measurements of Microbial Growth
1. Direct count
a. Cells counted from microscopic slide
b. Fastest method and very accurate BUT it takes a plot of preparation