Chapter 5: Microbial Growth Flashcards
Growth
Measured as an increase in the number of cells
Binary fission
Cell division following enlargement of a cell to twice its minimum size
Generation time
Time required for microbial cells to double in number
Each daughter cell receives during cell division
A chromosome and sufficient copies of all other cell constituents to exist as an independent cell
What occurs simultaneously in bacteria and archaea?
Growth in cell size, chromosome replication, and septum formation
Is there mitosis in bacteria and archaea?
No
Generation time is dependent on
Growth medium and incubation conditions: carbon source, pH, temperature, etc.
Exponential growth
Growth of a microbial population in which cell number double at a constant and specific time interval
What kind of curve does exponential growth create?
One that has a slope that increases continuously
Growth rate (k)
Rate of increase in population number or biomass.
Expressed in bacteria and archaea as number of doublings per hour
Generation time
Time it takes for each to cell to become 2 cells
Specific growth rate
Fastest growth rate in the best medium and optimal temperatures
Batch culture
Closed-system microbial culture of fixed volume
Lag phase
Interval between inoculation of a culture and beginning of growth
Exponential phase
Cells in this phase are typically in the healthiest state
Stationary phase
Cells metabolically active, but growth rate of population is zero
Why is growth rate in stationary phase zero?
Either an essential nutrient is use up, or waste product of the organism accumulates in the medium
Death phase
If incubation continues after cells reach stationary phase, the cells will eventually die
Do all bacteria die in the death phase?
Some bacteria form spores/cysts or dormant stage that allow a significant proportion of cells to survive for a long time
Continuous culture
Open-system microbial culture of fixed volume
Chemostat
Most common type of continuous culture device
Both growth rate and population density of culture can be controlled independently and simultaneously
Dilution rate
Rate at which fresh medium is pumped in and spent medium is pumped out
Concentration of limiting nutrient controls
Population size and the growth rate
How are microbial cells counted by direct microscopic observations?
Petroff-Hausser counting chamber
Petroff-Hausser counting chamber
Each square corresponds to a calibrated volume
Limitations of microscopic counts
- Cannot distinguish between live and dead cells without special stains
- Small cells can be overlooked
- Precision is difficult to achieve
- Phase-contrast microscope required if a stain is not used
- Low density cell suspensions are hard to count
- Motile cells need to be immobilized
- Debris in sample can be mistaken for cells
- Brownian motion, some forms clumps
Flow cytometer
Second method for counting cells in liquid samples
Uses laser beams, fluorescent dyes, and electronics
Viable cell counts (plate counts)
Measurement of only living cells capable of growing to form a population
Methods of plate counts
Spread-plate method
Pour-plate method
Viable count issues
- Preparation and incubation time
- Unreliable on counts of natural samples
- Culture media and growth conditions can’t grow every microbe
The great plate anomaly
Direct microscope counts of natural sample reveal far more organisms than those recoverable on plates
How much microbial diversity is culturable?
1-10%
What supports the great plate anomaly?
Microscope methods count dead cells, whereas viable methods do not
Different organisms may have vastly different requirements for growth
Turbidity measurements
Indirect, rapid, and useful methods of measuring microbial growth
How is turbidity measured?
With a spectrophotometer
What is turbidity measured in?
Optical density
Why is turbidity measured in optical density?
Bacteria behave like small particles and absorb and scatter light
The larger the number of particles…
The greater the absorbance and the lower the light transmission to the photocell
Does absorbance distinguish cells?
No it can’t tell live cells from dead cell or if it’s chemical particles
Does turbidity require a stain?
No they do not disturb the sample
How is direct cell related to turbidity?
A standard curve must first be established to another counting method
What other counting methods are there?
Viable cell count
Weight of biomass produced
Measuring other parts of the cell which are proportional the the whole mass of cells
Issues with optical density
- Has a finite linear range of measurement
- Only works if the cells are evenly distributed throughout the medium
- Cuvette must not have scratches
- Culture may need to be diluted when the cells are at a very high density
Cardinal temperature
Minimum, optimum, and maximum temperatures at which an organism grows
Psychrophile
Low temperature
Mesophile
Midrange temperature
Thermophile
High temperature
Hyperthermophile
Very high temperature
Where are mesophiles found?
In warm-blooded animals, terrestrial and aquatic environments, temperate and tropical latitudes
Extremophiles
Organisms that grow under very hot or very cold conditions
Psychrophile temperature optima
Less than 20 degrees celsius
Psychophile environments
Permanently cold environments: deep oceans, Arctic and Antarctic environments
Psychotolerant
Organisms that can grow at 0 degrees celsius but have optima of 20-40 degrees celsius
Molecular adaptations that support psychrophily
Production of enzymes that function optimally in the cold
Modified cytoplasmic membranes - high unsaturated fatty acid content
What life forms exist at above 65 degrees celsius?
Prokaryotes - chemorganotrophic and chemolithotrophic
What is the upper limit temperature of phototrophy?
70 degrees celsius
Thermophile optimal temperature
45-80 degrees celsius
Hyperthermophile optimal temperature
Greater than 80 degrees celsius
Hyperthermophile environments
Boiling hot springs and seafloor hydrothermal vents
Methanopyrus kandleri
Archaeon that can grow at 122 degrees celsius
Molecular adaptations to thermophily
Specific modifications provide thermal stability to enzymes and proteins
Modifications in cytoplasmic membranes to ensure stability
Bacteria have what to survive high temperatures
Lipids rich in saturated fatty acids
Archaea have what to survive high temperatures
Lipid monolayer rather than bilayer
What produces enzymes widely used in industrial microbiology?
Hyperthermophiles
Taq polymerase
Used to automate the repetitive steps in the polymerase chain reaction technique
Hydrolytic enzymes
Proteases, cellulases, lipases
True/false: enzymes of thermophiles are more stable and tend to have higher activity than mesophilic counterparts
True
What are the upper temperature limits for life?
Suggested at 140-150 degrees celsius
What are the closest descendants of ancient microbes?
Hyperthermophiles
What supports the theory that hyperthermophiles are the closest descendants?
- Are found on the deepest, shortest branches of phylogenetic tree
- Oxidation of H2 is common to many hyperthermophiles and may have been the first energy-yielding metabolism
