Microbial growth Typically refers to an increase in population rather than in size.
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Transcript of Microbial growth Typically refers to an increase in population rather than in size.
![Page 1: Microbial growth Typically refers to an increase in population rather than in size.](https://reader035.fdocuments.in/reader035/viewer/2022062806/5697bfa51a28abf838c97a39/html5/thumbnails/1.jpg)
Microbial growth
Typically refers to an increase in population rather than in size
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Growth curves
Carried out using batch cultures or a closed system (no fresh media added)
Characterized by several phases
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Lag phase
Occurs when cells are placed into fresh media
Likely due to the cells’ need to synthesize new components before reproducing
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Lag phase
Can vary depending on:
1. Type of media
2. Condition of the cells
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Exponential phase
Cells are growing at the maximum rate possible under given conditions
Rate of growth is constant
Population most uniform
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Stationary phase
Bacteria in stationary phase are usually at a concentration of 109 cells per ml
Balance between cell division and cell death or cells cease to divide
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Stationary phase
Due to:
Nutrient depletion
Toxic waste accumulation
Critical cell density reached
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Stationary phase
Bacteria subjected to starvation may become resistant to killing
Some pathogens may become more virulent when starved
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Death phase
Decline in viable cells due to toxic wastes and nutrient depletion
Death may be at a constant rate (logarithmic)
Death rate may decrease after majority of population has died (resistant cells)
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Mathematics of growth
Cells dividing at a constant rate during exponential growth
Generation time/doubling time = time it takes for population to double
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Mathematics of growth
More convenient to graph as log10 of cell number vs. time
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Generation time
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Determining generation time
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Measurement of microbial growth
Measurement of cell number
Measurement of cell mass
Measurement of culture turbidity
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Measurement of cell number
Counting chambers
Coulter counters
Plating techniques
Membrane filter techniques
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Petroff-Hauser chamber
Used for counting prokaryotic cells
Use of stains or fluorescent or phase-contrast microscopes make counting easier
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Using a Petroff-Hauser chamber
Chamber is of known depth and has grid etched into bottom
25 squares cover an area of 1 mm2
Determining average number per square and multiplying by 25 gives total number of cells in chamber
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Using a Petroff-Hauser chamber
280 cells in 10 squares
280/10 = 28/square
28 x 25 = 700 cells/ mm2
Chamber is 0.02 mm deep
700/0.02 = 700 x 50
= 3.5 x 104 cells/mm3
= 3.5 x 107 cells/cm3
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Coulter counter
Cells forced through small opening with electrodes on either side
Passage of cell will cause resistance to increase and cell is counted
More useful for counting eukaryotes
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Counting chambers and Coulter counters
Neither can distinguish between living and dead cells
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Plating techniques
Diluted sample spread over the surface of agar plate
Number of cells can be calculated by multiplying colony number by dilution factor
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Membrane filter techniques
Useful for measuring number of cells in aquatic samples
Sample passed through filter with small pore size
Filters placed on agar plates to allow growth of colonies
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Membrane filter techniques
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Measurement of dry weight
Cells collected by centrifugation, washed and dried in an oven and weighed
Most useful for fungi
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Measurement of turbidity
Degree of light scattering induced by a culture is indirectly related to the cell number
Spectrophotometers measure amount of light scattering
Can measure transmittance or absorption of light
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Continuous culture of microorganisms
Two most common systems
Chemostat
Turbidostat
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Chemostat
Sterile media fed into vessel at same rate that media containing bacteria are removed
Final cell density is dependant on the conc. of a limiting nutrient
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Turbidostat
Makes use of a photocell to measure turbidity of culture
Flow rate of media is regulated to maintain a constant cell density
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Influence of environmental factors on growth
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Influence of environmental factors on growth
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Influence of environmental factors on growth
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Influence of environmental factors on growth
Acidophiles
Neutrophiles
Alkalophiles
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Influence of environmental factors on growth
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Influence of environmental factors on growth
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Quorum sensing
Bacteria can communicate via quorum sensing or autoinduction
Cell senses concentration of signal
When threshold is reached, cell begins expressing sets of certain genes
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Quorum sensing
Most common signal molecules in gram-negative bacteria are acyl homoserine lactones (HSLs)
Gram-positives often use an oligopeptide signal molecule
Important in pathogenicity and biofilm formation