Bacterial Growth Flashcards
define ‘growth’
an increase in biomass, typically accompanied by cell division
what is binary fission?
cell division and separation followed by filamentous growth
how many planes do different shapes of bacteria divide in?
rods divide in one plane; cocci divide in one/two/three planes
what is a batch culture?
- growth medium inoculated with small quantity of culture; incubation with shaking
- bacteria go through growth phases of lag/exponential/stationary/death as conditions change
what is a continuous culture?
- continuous addition of nutrients and removal of waste products in chemostat
- culture maintained in steady state of exponentional growth
- conditions remain constant
what is the lag growth phase?
cells adapting to new conditions; synthesis of new cellular components eg. enzymes for nutrients in medium
what is the exponential growth phase?
maximal growth; population doubles at regular intervals
what is the stationary growth phase?
population remains constant as rate of cell growth = rate of cell death; nutrients are depleted
what is the death phase?
cells die due to starvation and the accumulation of toxic waste products
what is meant by the abbreviation t(gen)?
the average doubling time, also known as the mean generation time; depends upon:
- the uptake of nutrients
- time required to replicate DNA/proteins/cell wall/etc.
what methods can be used to count bacteria?
- haemocytometer
- counting chamber
- serial dilution
what calculation is carried out using a counter chamber?
average number of cells in one square x number of squares x 1/depth x 1000/width
what are some disadvantages of the counter chamber?
tedious; counts both live and dead cells
describe the use of serial dilutions to count bacteria
concentration of original culture = [dilution factor x number of colonies] / volume plated
- to get a reasonably accurate count, choose a plate with several hundred colonies
what is meant by ‘cfu’?
colony-forming units; this is not always equivalent to a single cell, since cells may cluster
what is a dis/advantage of the method of serial dilution?
it is a very slow method
but it counts only live cells
which methods do not rely on counting cells?
- optical density
- Coulter counter
- calculations of biomass
- calculation of MPN (most probable number)
- electrical impedance
describe the method of using optical density as a measure of number of bacteria
- use a spectrophotometer to measure light-scattering
- calibration factor required to calculate the number of cells using the measured absorbance
what is a dis/advantage of the optical density method?
it is a very quick method
but it only works on dense cultures
describe the use of a Coulter counter to count bacteria
- automated counting of individual cells
- a cell suspension is forced through a small hole with electrodes on either side
- every time a microbe passes through the hole, the current drops
what are the disadvantages of the Coulter counter?
- accurate for large cells but not for most bacteria
- problems with debris, filaments, etc. being detected as cells
- cannot distinguish between live and dead cells
describe the use of biomass to count bacteria
harvest cells by centrifugation and filtration and measure wet/dry weight
what is MPN?
most probable number;
- carry out a set of dilutions and score for ‘growth’ or ‘no growth’
- statistical tables are used to estimate the likely cell number
which industry uses MPN?
the water industry
what is electrical impedance?
- measure electrical properties of the sample
- microbial metabolism involves the production of ionic metabolites
- this results in a decreased electrical impedance
which industry uses electrical impedance?
the food industry (for quality control)
