Question

When measuring the growth curve of E. coli by spectrophotometer, the death phase is not normally seen as a change in absorbance because

	a.	the cells are dividing most rapidly at that stage
	b.	the cells are still “gearing up” to start dividing before log phase
	c.	the spectrophotometer inhibits the death phase, so cells stay in stationary phase
	d.	dead cells absorb and diffuse light to nearly the same degree as live cells do
	e.	live cells are black, but dead cells are transparent

Answer 1

Answer: d

Phases of bacterial growth:

1. Lag phase:

• Preparation and adjustment phase for cell division phase.
• Increase in cell volume, not in cell number.

2. Log phase or Exponential phase:

• Growth phase with cell division.
• There is logarithmic exponential increase in cell number as 2n.

3. Stationary phase:

• Growth ceases, due to depletion of nutrients and accumulatio of wastes.
• Death of cell initiates ad death rate balances with growth rate, net number of cells remain constant.

4. Death phase:

• Rate of cell death exceeds the rate of cells generated.
• There is ultimately logarithmic decrease in cell numbers.

• The cell density are estimated as visual changes recorded through spectrophotometric changes.
• If the dead cells remain within the tube, the spectrophotometric changes may not be able to distinguish the living and dead cells.
• Thus, the dead cells in death phase will give similar values as live cells and death phase may not be observed by spectrophotometric changes.