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Microbial growth
Typically refers to an increase in population rather than in size
Growth curves
Carried out using batch cultures or a closed system (no fresh media added)
Characterized by several phases
Lag phase
Occurs when cells are placed into fresh media
Likely due to the cells’ need to synthesize new components before reproducing
Lag phase
Can vary depending on:
1. Type of media
2. Condition of the cells
Exponential phase
Cells are growing at the maximum rate possible under given conditions
Rate of growth is constant
Population most uniform
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
Stationary phase
Due to:
Nutrient depletion
Toxic waste accumulation
Critical cell density reached
Stationary phase
Bacteria subjected to starvation may become resistant to killing
Some pathogens may become more virulent when starved
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)
Mathematics of growth
Cells dividing at a constant rate during exponential growth
Generation time/doubling time = time it takes for population to double
Mathematics of growth
More convenient to graph as log10 of cell number vs. time
Generation time
Determining generation time
Measurement of microbial growth
Measurement of cell number
Measurement of cell mass
Measurement of culture turbidity
Measurement of cell number
Counting chambers
Coulter counters
Plating techniques
Membrane filter techniques
Petroff-Hauser chamber
Used for counting prokaryotic cells
Use of stains or fluorescent or phase-contrast microscopes make counting easier
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
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
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
Counting chambers and Coulter counters
Neither can distinguish between living and dead cells
Plating techniques
Diluted sample spread over the surface of agar plate
Number of cells can be calculated by multiplying colony number by dilution factor
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
Membrane filter techniques
Measurement of dry weight
Cells collected by centrifugation, washed and dried in an oven and weighed
Most useful for fungi
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
Continuous culture of microorganisms
Two most common systems
Chemostat
Turbidostat
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
Turbidostat
Makes use of a photocell to measure turbidity of culture
Flow rate of media is regulated to maintain a constant cell density
Influence of environmental factors on growth
Influence of environmental factors on growth
Influence of environmental factors on growth
Influence of environmental factors on growth
Acidophiles
Neutrophiles
Alkalophiles
Influence of environmental factors on growth
Influence of environmental factors on growth
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
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
