Ex. 15 Quantifying Bacteria Flashcards
quantitative technique
methods used to determine number of microorganism per ml of sln
2 roles of serial dilution
a) reduces cell density and b) provides framework by which to link the unknown (original cell density) with the known (number of bacteria dilution)
standard plate count (viable plate count)
direct measurement of cell density and reveals info related to live bacteria
spectrophotometric (turbidimetric) analysis
based on turbidity and indirectly estimates all bacterial (cell biomass) dead or alive
method of standard plate count
diluting bacteria into sterile water to be able to count accurately when plated onto solid medium. assumption: each colony risen from one viable cell
why is it more accurate to say colony forming units
bacteria clump, a colony may result from several bacteria clustered together
dilution factor equation
DF= final vol/initial vol
dilution equation
D= initial vol/final vol or 1/DF
bacteria per mL in orignal sln equation
= number of colonies on a plate X 1/dilution of plate
cell mass/ biomass
total amount of both living and dead micro-organism
as turbidity and cell mass of suspension increases
amount of transmitted light decreases
why is turbidity an indirect measurement
based on cell mass and not the number of cells
3 examples of problems with turbidity
a) small microbes may enlarge, scattering more light but number of cells may not increase b) large cells rapidly multiplying may simple divide, increasing microbial numbers without altering cell mass or turbidity c) dead cells increase turbidity while viable cells small
optical density
amount of light scattered by cells in sln
standard curve graph is valid when
samples in same medium, measured at ambient temp, contain same species and examined with same wavelength
