Microbial Growth Flashcards
(33 cards)
How do bacterial cells replicate?
- Bacterial cells replicate by a form of asexual reproduction called binary fission
What is generation time?
*time required to complete fission cycle from parent cell to two daughter cells. (Doubling time)
What is happening during the lag phase?
-Interval of time between when a culture is inoculated and when growth begins
-Nutrient assimilation
What is happening during the exponential phase?
Cells in this phase are typically in the healthiest state. Growth is at maximal rate.
What is happening during the stationary phase?
*Growth rate of population is zero.
*Number new divisions=number of cells dying
*Either an essential nutrient is used up or waste product of the organism accumulates in the medium
What is happening during the death phase?
Lack of nutrients and increasing accumulation of wastes lead to… number of cell deaths > number of new divisions
What is a continuous culture?
an open-system microbial culture of fixed volume
What are the most common types of continuous culture device
Chemostat and turbidostat
How is a chemostat used?
- Fresh medium continually supplied from a reservoir of sterile medium
- Volume maintained at constant level by overflow drain
- Bacteria grow at same rate as bacterial cells and spent medium are removed
- Rate of addition of fresh medium determines rate of growth
How is a turbidostat used?
- Turbidity of culture is held constant by manipulating rate at which medium is fed
- If turbidity increases, feed rate is increased to dilute turbidity back to set point
- If turbidity falls, feed rate is lowered so that growth can restore turbidity to its set point
- Turbidity is measured usually by spectrophotometer
State a way growth of bacteria can be counted
By using a hemocytometer
What are the advantages of calculating total cell count?
no incubation time is required
What are the disadvantages of calculating the total cell count?
*Alive and dead cells cannot easily be distinguished
*Motile bacteria are difficult to count
*requires a high concentration of bacteria
What methods can provide a viable count?
○ Spread-plate method
○ Pour-plate method
What are the advantages of calculating a viable count?
- Measures alive cells only
What are the disadvantages of calculating a viable count?
- Takes 24 hours or more for visible colonies to appear
- Only counts between 25 and 250 colonies are accurate (ballpark figure)
- Must perform serial dilutions to get appropriate numbers/plate
What is turbidity and how is it used?
*As bacteria multiply in media, it becomes turbid
Use a spectrophotometer to determine % transmission or absorbance
*Multiply by a factor to determine concentration
*Absorbance is related to the number of bacteria
What are the advantages of turbidity?
No incubation time is required
What are the disadvantages of turbidity?
*Cannot distinguish between live and dead bacteria.
*Requires a high concentration of bacteria (10 to 100 million cells/ml)
What factors affect growth?
*Temperature
*pH
*Osmotic pressure
How does temperature effect microbial growth?
Cardinal temperatures: the
-minimum
-optimum
-maximum
beyond the optimum the enzymes denature and bacterial growth is reduced
How does pH effect growth?
- Organisms sensitive to changes in acidity because H+ and OH- interfere with H bonding in proteins and nucleic acids
- Most bacteria and protozoa grow best in a narrow range around neutral pH (6.5-7.5) – these organisms are called neutrophiles
- Other bacteria and fungi are acidophiles – grow best in acidic habitats
○ Acidic waste products can help preserve foods by preventing further microbial growth - Alkalinophiles live in alkaline soils and water up to pH 11.5
What are the physical effects of water?
- Microbes require water to dissolve enzymes and nutrients required in metabolism
- Water is important reactant in many metabolic reactions
- Most cells die in absence of water
○ Some have cell walls that retain water
○ Endospores and cysts cease most metabolic activity in a dry environment for years - Two physical effects of water or salt
○ Osmotic pressure
○ Hydrostatic pressure
*Halophiles (salt lovers)
What is osmotic pressure?
- Is the pressure exerted on a semipermeable membrane by a solution containing solutes that cannot freely cross membrane; related to concentration of dissolved molecules and ions in a solution
- Hypotonic solutions have lower solute concentrations; cells placed in these solutions will swell and burst
- Hypertonic solutions have greater solute concentrations; cells placed in these solutions will undergo crenation (shriveling of cytoplasm)
