MICRB 265 (10/18/13) Microbial Nutrition and Growth (con't)

4 Pages
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Department
Microbiology (Biological Sciences)
Course Code
MICRB265
Professor
Brian Lanoil

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MICRB 265 (October 18, 2013) Microbial Nutrition and Growth (con’t) Stationary Phase  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 Cryptic Growth  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 Death Phase  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 exponential growth  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 animals Live/Dead Staining  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 i
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