5 - bacterial growth and cell division cycle Flashcards
4 stages of bacterial growth
lag
exponential
stationary
death
mechanisms of bacterial cell division
chromosome replication and segregation
septum formation and division
regulation of FtsZ ring formation
what happens in lag phase?
cells adjust to new conditions
what happens in exponential phase?
optimal growth with regular doubling in cell numbers
what happens in stationary phase?
growth limited by nutrient depletion or accumulation of toxic metabolites
what happens in death phase?
complex gradual loss of viability but with some cell turnover
what is total cell count?
total number of bacterial cells
what is viable cell count?
total number of living bacterial cells
plating methods for measuring bacterial growth
each colony assumed to represent the progeny of a single viable cell
but:
• only measures viable cells
• underestimates for cells in chains or clusters
• number of colonies dependent on growth conditions
• laborious and slow
turbidity methods for measuring bacterial growth
spectrophotometer
measures light scattering by cells
simple and convenient
not destructive and can be done continuously
but:
• measures all particles, including any dead cells
• low sensitivity
direct counting methods for measuring bacterial growth
most direct method - microscopic count of a know volume of culture
can accommodate clumping and chaining
but:
• doesn’t discriminate live/dead
• laborious - can be automated
steps in the cell division cycle
1) cell grows - cell structures duplicated and chromosomes replicated
2) daughter chromosomes segregate to different ends of the cell
3) septum forms at mid cell as Z ring constricts - new cell poles synthesised as ring constricts
4) cell division occurs at mid-cell - results in 2 identical daughter cells
chromosome replication
bidirectional replication from the oriC to the terC
regulated by 2 key proteins
• DnaA initiates replication
• SeqA blocks replication
1) oriC recruites DNA replication machinery - replisome
2) 2 replisomes bind to give 2 replication forks
3) these move in opposite directions
4) the forks meet at terC and the 2 chromosomes are separated
takes 40 mins in E.coli
bacterial cell division machinery
a complex contractile ring - divisome
FtsZ is a key player
assembles into a ring structure - the Z ring
recruits 10 other essential proteins
• for membrane and wall invagination
• construction of new cell poles
contraction of the Z-ring causes septum formation , new cell pole synthesis and ultimately division
bacterial cell division is guided by…
spatial cues
2 negative regulators govern division
• the Min system goes to the cell poles and inhibits division there
• the Nucleoid Occlusion (NO) system inhibits division in the vicinity of the nucleoid (chromosome)
division only occurs at mid-cell and only after the daughter chromosomes have segregated
