Bacterial growth and death Flashcards
How would you carry out a total cell count using direct counting?
Use counting chambers and liquid samples
Describe the cell cycle in bacteria
Bacterial cell elongates and the chromosome is replicated. The two chromosomes separate and the septum forms in mid-cell region and the parent cell divides into two identical daughter cells.
How would you use a petroff-hausser chamber?
The sample to be counted is added by pipette to the edge of the coverslip that covers the grid area. Then the number of cells in several of the larger squares of the grid is determined. Calculations are then based on the average number of cells in 25 of these squares.
What are viable counts used for?
Enumeration of living cells only and determines number of cells in a sample capable of forming colonies on a suitable agar medium
What is the formula to work out CFU/mL?
Average CFUs x 1/dilution factor x 1/volume
How do you measure culture turbidity?
Use a spectrophotometer to give absorbance measurement (units: optical density OD) and plot this data over time
Describe a typical growth curve
When lower number of viable cells there is the lag phase. As time goes on the number of viable cells increases which is called the exponential (log) phase. The curve then plateaus which is the stationary phase and then the number of viable cells decreases which is called the death phase.
What happens in the lag phase and the reasons for it?
Lag phase happens before growth as cells adapt to new conditions.the reasons for this May be because of a change in nutrient medium. Cells may be old or depleted of ATP or ribosomes. Or in an inoculate state due to injury or spore form
What happens in the exponential phase (log phase) and the reasons for it?
Cells are growing and dividing at maximal rate possible given their genetic potential and conditions under which they’re growing. The rate of growth is constant due to cells dividing and doubling at regular intervals. In this phase the population is most uniform in terms of chemical and physiological properties.
What happens in the stationary phase and the reasons for it?
Population growth has ceased so the total number of viable cells remains constant. Resins for this could be nutrient limitation, oxygen availability or toxic waste products may accumulate
What happens in the death phase and the resins for it?
Decline in viable cells. Reasons could be due to nutrient deprivation or toxic wastes. (Death of population is usually logarithmic. Death rate decreases faster if sample is bigger)
Is the growth rate constant in the exponential phase and give an example to show this
Yes because each microorganism is dividing at constant intervals. O if the culture medium is inoculated with one cell that divides every 20 mins. The population will be 2 cells after 20 mins, 4 cells after 40 mins, 8 cells after 1 hour etc.
What is the formulae to work out the population after a certain amount of time?
Nt (population at time t) = N0 (initial population number) x 2n (number of generations in time t) this shows the number of cells present after a period of exponential growth
Using the formulae in the question before how would you calculate this using log?
Log Nt = logN0 + n. Log 2
N= log Nt - log N0/ log2
What does the mean growth rate constant (k) measure?
The number of generations per unit time (generations per hour in units h-1)
K= n/t = logNt - logN0 / log2 x t
