3.3: Bacterial Cell division and Population Growth

Question 1

how is growth measured

Answer 1

as an increase in the number of cells

(growth means multiplying count numbers not size)

Question 2

binary fission

Answer 2

cell division (splits in 2) following enlargement of a cell to twice its minimum size

Question 3

generation time

Answer 3

time required for the number of cells to double

Question 4

what is balanced growth

Answer 4

cell constituents increase proportionally

(like 55% protein and 20% RNA would still be true for each 2 cells that divided)

Question 5

do growth in cell size, chromosome replication and septum formation typically occur in steps or simultaneously?

Answer 5

simultaneously

Question 6

binary fission leads to what pattern of growth

Answer 6

exponential growth -cell numers double at a constant and specific time interval (generation time).

(this causes them to appear to increase slowly at first but then increase a lot on regular arithmetic plot)

Question 7

how can the relationship between initial number of cells present in a culture and number present after a period of exponential growth be expressed mathematically? (ie. what’s the formula for final cell number using initial)

Answer 7

Nt =N0 x 2^n
* Nt is final cell number
* N0 is the initial cell number
* n is the number of generations during the period of exponential growth

Question 8

how does microbial growth appear on a normal arithmetic scaled graph? what’s the reality?

Answer 8

because they grow exponentially, the increase in cell number is initially slow but increases at an ever faster rate, following an exponential curve (but in reality they have a even pace of growth)

Question 9

why is a logrithmic scale used for plotting microbial growth

Answer 9

The constant rate of growth is only noticable when plotted on a logarithmic scale (they grow exponentially).

Question 10

what is growth rate (k), what is it expressed as? why?

Answer 10

• the rate of increase in population number or biomass.
• expressed as number of doublings or generations per hour, since bacteria and archaea grow by binary fission.

Question 11

growth can also be looked at with generation time (g) what is this? and what are the units?

Answer 11

• time it takes for each cell to divide (or for the number of cells in a population to double)
• the generation time (g) and is expressed in minutes or hours per generation. (inverse of growth rate)

Question 12

what is the formula for specific growth rate (k)?

Answer 12

• k = (Log Nt - Log N0)/ 0.301 (change in time)
• N0 =number of cells at time1
• Nt = number of cells at time2
• change in time = time2 -time1

Question 13

whats the lagphase is it included in growth rate calculation?

Answer 13

• period where numbers are not increasing
• not included in calculation

Question 14

what’s the formula for generation time (g)

Answer 14

• g= 1/k
• where k= the calculated growth rate (you must calculate this first)

(could use inverse button of k, so you don’t have to retype k value)

Question 15

for each organism there is a** specific growth rate**, what is this

Answer 15

an organisms fastest growth rate in the best medium and optimal temperature.

Question 16

what is E. coli’s specific growth rate

Answer 16

doubles every 20 mins under optimal conditions

Question 17

what is Clostridium perfringen’s specific growth rate

Answer 17

doubles every 10 mins

Question 18

what is mycobacterium tuberculosis’ specific growth rate

Answer 18

doubles once every 24 hours (slow grower)

Question 19

why doesn’t penicillin work great against Mycobacterium turberculosis?

Answer 19

it grows slow, so penicillin inhibiting crosslinking of peptidoglycan doesn’t cause osmotic lysis so quick.

Question 20

what’s batch culture?

Answer 20

a closed-system microbial culture of fixed volume.
-after inoculation, no new nutrients are added, and no wastes are removed.

Question 21

what are the four phases a growth curve of bacteria grown in batch culture typically show?

Answer 21

1. lag phase
2. Exponential (log) phase
3. stationary phase
4. death phase

Question 22

describe when the lag phase is in a batch culture growth curve, and what happens in this phase? How can this vary?

Answer 22

• period of time between inoculation of a culture and beginning of growth (i.e. increase in cell numbers)
• Cells adapt to new medium and conditions
  -synthesize building blocks (if its minimal medium), make new enzymes etc (they need to grow)

(time can vary depending on species and how big change in cultrue conditions is, going from unhealthy to healthy medium leg could be much longer)

Question 23

describe what happens in the exponential (log) phase of a batch culture growth curve

Answer 23

• population exhibits exponential growth
• cells are healthy, actively growing and of uniform composition (balanced growth)
  -bacteria from this phase are often chosen for further experiments
  -ex: mid-log phase cultures.

Question 24

describe what happens in the stationary phase

Answer 24

• cells are metabolically active, but cell numbers stop increasing
• either an essential nutrient is used up or a waste product has accumulated in the medium
• cells may undergo secondaru metabolism - production of antibiotics, endospores etc.

Question 25

what happens in the death phase

Answer 25

* if incubation continues after cells reach stationary phase, the cells will eventually die * rate at which cells die varies greatly between species * not all bacteria die, some bacteria form endospores or other dormant cell types (akinetes, cysts etc. -not as good as endospores, don't last as long.)

Question 26

continous culture

Answer 26

* a open system microbial culture of fixed volume * nutrient is continously added * wastes removed * supports indefinite growth

Question 27

chemostat

Answer 27

* most common type of continous culture device * growth rate and population density of the culture can be controlled independently

Question 28

dilution rate

Answer 28

rate at which fresh medium is pumped in and spent medium is pumped out. -including cells ## Footnote (for chemostat rate of running in is same as out)

Question 29

what controls population density in a chemostat

Answer 29

concentration of a limiting nutrient

Question 30

when dilution rate is **low** specific growth rate is _ meaning nutrients are _ and cells grow _

Answer 30

specific growth rate is low, meaning nutrients are limiting, and cells grow slowly.

Question 31

when dilution rate **increases**, growth rate _, because....

Answer 31

increases, because of a faster inflow of nutrients, so cells grow faster

Question 32

if dilution rate is increased beyond a certain point...

Answer 32

cell density decreases and **washout** occurs (they've hit their maximum growth rate, rate levels off, the limiting nutrients are no longer limiting. and cells are pumped out faster than they're multiplying so eventually they washout.)

Question 33

what are biofilms

Answer 33

* biofilm -cluster of cells (pure culture or mixed community) that adhere to one another (doesn't have to be adhering to surface) * capsules, slime layers, pili, and fimbriae can all be involved

Question 34

planktonic

Answer 34

* cells free-floating in liquid (opposite of biofilm)

Question 35

list and describe the steps of how biofilms are formed

Answer 35

1. Attachment -a few cells collide with a surface and attach 2. Colonization -expression of biofilm specific genes: -intracellular signaling molecules (released to influence behaviour of neighbors) -extracellular polymeric substances (EPS) -complex of polysaccharide, DNA and protein 3. development -a complex network of columns and channels form (so not just surface gets nutrients) -allows nutrients to reach all cells in the biofilm 4. dispersal -cells are freed from mature biofilms to return to planktonic state and colonize new surfaces (usually triggered by something like nutrient availability)