5. Bacterial growth and the cell division cycle Flashcards
The typical bacterial growth cycle
- Lag phase
- Exponential phase
- Stationary phase
- Death phase
Lag phase
Cells adjust to new conditions and synthesise required metabolic enzymes and metabolites
Exponential phase
Optimal growth with regular doubling in cell numbers
Stationary phase
Growth limited by nutrient depletion or accumulation of toxic metabolites. Rate of new cell production equals cell death
Death phase
Gradual loss of viability with some cell turnover
Measuring bacterial growth: Plating methods - Method
Culture plate onto solid nutrient medium after serial dilution.
Each colony assumed to represent progeny of single viable cell.
Colony forming units extrapolated giving cell numbers in original culture.
Measuring bacterial growth: Plating methods - Positives
Highly sensitive.
Growth conditions customizable to growth of just one species.
Measuring bacterial growth: Plating methods - Measurements
Only measures viable cells.
Underestimates cells in chains or clusters.
Number of colonies dependent on growth conditions
Inaccurate
Measuring bacterial growth: Plating methods - Uses
Food
Medicine
Aquatic microbiology
Measuring bacterial growth: Turbidity - Method
Measures light scattering by cells
Measuring bacterial growth: Turbidity - Positives
Simple and convenient
Non destructive
Can be done continuously
Measures all cells (including dead cells)
Measuring bacterial growth: Turbidity - Negatives
Low sensitivity
Culture turbidity has to be within a certain range for accuracy
Measuring bacterial growth: Direct counting - Method
Simplest method
Microscopic count of known volume
Measuring bacterial growth: Direct counting - Positives
Can accommodate clumping/chaining.
Doesn’t discriminate against live or dead cells - Via staining method
Measuring bacterial growth: Direct counting - Negatives
Laborious but can be automated
Measuring bacterial growth: Cytometry and FACs - Method
Flow cytometry: Measures particles in microfluidic flow
Measuring bacterial growth: Cytometry and FACs - Positives
Highly automated
Can measure fluorescence at multiple wavelengths
Call sorting possible via FACs (FLuorescent activated cell sorting)
Measuring bacterial growth: Cytometry and FACs - Negatives
Required right equipment, reagents and expetise
Cell division cycle
- New born cell
- Cell elongates and increases in size
- Cell structure duplicates
- Chromosomes duplicate
- Daughter chromosomes segregate to different ends of the cell
- Septum forms at mid cell as z-ring contracts
- New pole of cell synthesised as z-ring constricts
- Cell division at mid cell
Chromosome replication - Type of replication
Bidirectional replication.
Chromosome replication - Proteins used
Regulated by 2 proteins - dnaA initiates replication and seqA blocks replication.
Chromosome replication - Steps
- OriC recruits replisome
- 2 replisomes bind to give replication forks
- Replication forks move in opposite direction (bidirectional replication)
- Forks meet at TerC
- Chromosomes separate
Chromosome replication - Time taken for E.coli
40 minutes
Chromosome replication choreography
Important that each cell gets copy of chromosome.
OriC located at centre of cell and left/right arms of chromosome positioned.
During chromosome replication, daughter OriC’s localised to quarter cell.
After division, cells have one cope of chromosome.
Septum formation
Governed by divisome.
Assembles to form ring structure (z-ring) - Ftsz protein key role in formation.
Recruits 10 other essential proteins - Membrane and cell wall invagination, construct of new cell pole.
Z-ring contraction
Causes septum formation, cell pole synthesis and cell division
Cell division spatial cues
2 negative regulators govern division.
- Min system: inhibits division at cell pole
- Nucleoid occlusion system: Inhibits division in vicinity of nucleoid.
