measurements of microbial growth Flashcards
what do direct measures of growth measure
cell numbers
what do indirect measures of growth measure
total mass of the population, and use that to get info on cell numbers
list 4 direct techniques to measure bacterial growth
standard plate count, direct microscopic count, flow cytometry, filtration
is a standard plate count direct or indirect
direct
what does a standard plate count measure
number of viable cells
what are two types of standard plate counts
pour plate or spread plate
what range for colony number do we use for plate counts
25-250 colonies
what does CFU stand for
colony forming units
how do we determine CFU/ml for a standard plate count
count the number of colonies and multiply it by the reciprocal of the dilution used
disadvantages of the plate count method?
takes time, inaccurate counts of the wrong medium is used + if clumps of bacteria aren’t broken up + if bacteria cannot be cultured on solid media + if large colonies are present that take up lots of space, and this technique only allows determination of viable cells
disadvantages of the pour plate method?
hot agar may kill or injure sensitive cells, colonies can form beneath the agar
what thing does direct microscopic counting use
a hemocytometer
describe the structure of a hemocytometer
has a chamber with a known volume + a series of grids
advantages of direct microscopic count
you get both viable and nonviable cells, time efficient bc no incubation is required = immediate results
disadvantages of direct microscopic count
there is no distinction between live and dead cells, must have a high conc. of cells in the original sample or the count won’t be accurate
is flow cytometry direct or indirect
direct
what is flow cytometry used for
used to count microbes
how does flow cytometry work
one cell at a time is passed through a beam of laser light. Each cell causes the light to scatter, allowing the number of cells to be determined
advantages of flow cytometry
quick results
disadvantages of flow cytometry
expensive machine, technical expertise required to use the machine
is filtration a direct or indirect method
direct
when is filtration used to count cell numbers
when numbers are low
describe filtration to count cell numbers
sample is passed through a membrane filter that has pores small enough to retain bacteria. Filter if lifted off and placed on an agar medium –> colonies will arise on the filter’s surface
list 3 selective media that can be used for filtration
nutrient agar, fecal coliform medium, wort medium
after colonies are counted from the filtration method, how are the results of the count recorded
recorded as # bacteria per volume filtered
list 3 indirect techniques for cell counting
dry weight, turbidity, or measure of a cellular substance
describe dry weight as an indirect method
cells are grown in liquid medium, centrifuged, washed, dried and weighed
how are the results of a dry weight method expressed
mass per unit volume
what is the dry weight method of counting useful for (what type of organism)
filamentous fungi
disadvantages of the dry weight method
not very senstive, time consuming
describe turbidity as a method of measuring growth
cells in a medium will become turbid (cloudy), and this can be measured with a spectrophotometer
describe absorbance and % transmittance with a blank
high % transmittance and low absorbance
describe absorbance and % transmittance as the number of bacteria in a sample increases
decrease in %T and increase in A
advantage of turbidity method
easy, fast results
disadvantage of turbidity method
bacterial numbers must be high enough to be read by the spectrophotometer = not useful for samples with low bacterial numbers
describe measuring a cellular substance as a technique for measuring microbial growth
measure the amount of protein/nitrogen/ATP. If the amount is high, the number of bacteria should be high
