Lecture 7 (Linda Stewart) - Microbial Growth

Question 1

Describe binary fission as observed in bacteria and archaea

Answer 1

• A parent cell prepares for division by enlarging its cell wall, cell membrane, and overall volume.
• DNA replication starts.
• The septum begins to grow inward at the chromosomes move toward opposite ends of the cell. Other cytoplasmic components are distributed to the two developing cells.
• The septum is synthesised completely through the centre, creating two separate cell chambers.
• The daughter cells become divided.
• Some species remain attached forming chains or doublets.

Question 2

Summarise the two major events in a typical bacterial cell cycle

Answer 2

Chromosome replication and partitioning:

• The initiation mass for chromosome replication is reached.
• DNA replication is initiated by the replisome in both directions.
• The origins of replication separate and the cell elongates as replication continues.
• When the threshold cell length is reached the chromosomes separate.

Cytokinesis (Septation):

• The site for septum formation is selected.
• The Z ring is assembled by the tubulin homologue FtsZ.
• The Z ring is linked to the plasma membrane (cell wall).
• The machinery for cell wall synthesis is assembled.
• The cell contracts and the septum is formed.

Question 3

Describe the four phases of a microbial growth curve observed when microbes are grown in a batch culture

Answer 3

Lag Phase:

• Happens at the beginning of batch culture growth.
• During this phase the cell is synthesising new components to replenish spent materials and to adapt to new conditions.

Exponential (Log) Phase:

• Rate of growth and division is constant and maximal.
• Cells exhibit balanced growth.
• Population is most uniform in terms of chemical and physical properties.

Stationary Phase:

• Active cells stop reproducing or reproductive rate is balanced by death rate.
• The total number of viable cells remains constant.
• Could be due to nutrient limitation, limited oxygen availability, toxic waste accumulation, or the critical population density has been reached.

Senescence and Death Phase:
- Death rate exceeds rate of reproduction and the population rapidly declines.

Question 4

Describe three hypotheses proposed to account for the decline in cell numbers during the death phase of a growth curve

Answer 4

• Detrimental environmental changes such as nutrient deprivation and buildup of toxic waste causes irreparable harm to cells.
• Some cells are viable but not culturable (VBNC). This is thought to be the result of a genetic response triggered in starving, stationary phase cells. Once the appropriate conditions are available, VBCN microbes resume growth.
• Programmed cell death where a fraction of the population is genetically programmed to die after growth ceases. The cells that die leak their nutrients and enable growth of cells in the population that did not initiate cell death.

Question 5

Predict how the presence of viable but nonculturable cells in food or water systems might impact public health

Answer 5

• Pose a public health threat as many assays that test for food and drinking water safety are culture based.
• Therefore assays will not reveal the presence of viable but nonculturable cells.

Question 6

What is the difference between exponential growth rate and specific growth rate

Answer 6

• Exponential growth rate is when the population is doubling every generation.
• Specific growth rate is the increase in the number of cells during a certain time period.

Question 7

Distinguish between batch culture and continuous culture

Answer 7

• A batch culture is when microorganisms are cultivated in a closed system.
• a continuous culture is when microorganism are cultivated in an open system where there is a continual provision of nutrients and continual removal of waste.

Question 8

Discuss the relationship between the dilution rate of a chemostat and population size and growth rate

Answer 8

• Growth rate can increase when the total available energy provided by the nutrient supply exceeds the maintenance energy.
• Slightly higher dilution rates make more nutrients available to the microbes.
• The cell density will begin to rise when the dilution rate provides enough nutrients for both maintenance and reproduction.

Question 9

Differentiate chemostats and turbidostats

Answer 9

• In a chemostat the rate of incoming medium is equal to the rate of removal of medium from the vessel. In a turbidostat the flow rate of media through the vessel is automatically regulated to maintain a predetermined turbidity.
• In a chemostat the quantity of an essential nutrient is limited whereas the culture medium in a turbidostat contains all nutrients in excess.
• A chemostat is most stable and effective at low dilution rates but a turbidostat operates best at high dilution rates.