Module 3: Bacterial Reproduction and Growth Curves Flashcards
Generation time
Time for bacterial population to double
Factors affecting generation time
Genetic control
Available nutrients
Environmental conditions
In vivo
In the human body
Longer generation times due to host defences
In vitro
In the laboratory
Shorter generation times
Viable counts
Counts only live bacteria
Total counts
Both viable and dead bacteria
Colony count
Only accurate way to determine viable count
Measured amount of bacterial suspension added to molten agar, poured into petri plate, solidified, incubated
Each colony = one viable bacterium
Total count under microscope
Ruled chamber with measured volume of bacterial suspension
Difficult due to small size of bacteria
Total count by electronic particle counters
Similar to blood cell counters
Difficult due to small size of bacteria
McFarland standards
Used to determine total counts by visual turbidity
Various amounts of 1% sulfuric acid and 1.175% aqueous barium chloride
White precipitate of barium sulfate
0.5 McFarland standard = 1.5 x 10^8 bacterial/mL
Spectrophotometer
Measures turbidity as %T (transmission) or optical density
More bacteria = less light measured
Nephelometry
Measures the amount of light scattered
Lag phase
Little or no increase in the number of cells
Actively metabolizing enzymes and other macromolecules
Logarithmic/exponential phase
Cells dividing at constant rate
Doubling in direct proportion to time
Benefits: good morphology, motility, most susceptible to antimicrobials
Stationary phase
Total number of viable cells constant
All cells stop dividing or growth rate = death rate
Not good to demonstrate motility, Gram + may stain Gram -
Benefits: easy to find spores
