measurements of microbial growth

Question 1

what do direct measures of growth measure

Answer 1

cell numbers

Question 2

what do indirect measures of growth measure

Answer 2

total mass of the population, and use that to get info on cell numbers

Question 3

list 4 direct techniques to measure bacterial growth

Answer 3

standard plate count, direct microscopic count, flow cytometry, filtration

Question 4

is a standard plate count direct or indirect

Answer 4

direct

Question 5

what does a standard plate count measure

Answer 5

number of viable cells

Question 6

what are two types of standard plate counts

Answer 6

pour plate or spread plate

Question 7

what range for colony number do we use for plate counts

Answer 7

25-250 colonies

Question 8

what does CFU stand for

Answer 8

colony forming units

Question 9

how do we determine CFU/ml for a standard plate count

Answer 9

count the number of colonies and multiply it by the reciprocal of the dilution used

Question 10

disadvantages of the plate count method?

Answer 10

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

Question 11

disadvantages of the pour plate method?

Answer 11

hot agar may kill or injure sensitive cells, colonies can form beneath the agar

Question 12

what thing does direct microscopic counting use

Answer 12

a hemocytometer

Question 13

describe the structure of a hemocytometer

Answer 13

has a chamber with a known volume + a series of grids

Question 14

advantages of direct microscopic count

Answer 14

you get both viable and nonviable cells, time efficient bc no incubation is required = immediate results

Question 15

disadvantages of direct microscopic count

Answer 15

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

Question 16

is flow cytometry direct or indirect

Answer 16

direct

Question 17

what is flow cytometry used for

Answer 17

used to count microbes

Question 18

how does flow cytometry work

Answer 18

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

Question 19

advantages of flow cytometry

Answer 19

quick results

Question 20

disadvantages of flow cytometry

Answer 20

expensive machine, technical expertise required to use the machine

Question 21

is filtration a direct or indirect method

Answer 21

direct

Question 22

when is filtration used to count cell numbers

Answer 22

when numbers are low

Question 23

describe filtration to count cell numbers

Answer 23

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

Question 24

list 3 selective media that can be used for filtration

Answer 24

nutrient agar, fecal coliform medium, wort medium

Question 25

after colonies are counted from the filtration method, how are the results of the count recorded

Answer 25

recorded as # bacteria per volume filtered

Question 26

list 3 indirect techniques for cell counting

Answer 26

dry weight, turbidity, or measure of a cellular substance

Question 27

describe dry weight as an indirect method

Answer 27

cells are grown in liquid medium, centrifuged, washed, dried and weighed

Question 28

how are the results of a dry weight method expressed

Answer 28

mass per unit volume

Question 29

what is the dry weight method of counting useful for (what type of organism)

Answer 29

filamentous fungi

Question 30

disadvantages of the dry weight method

Answer 30

not very senstive, time consuming

Question 31

describe turbidity as a method of measuring growth

Answer 31

cells in a medium will become turbid (cloudy), and this can be measured with a spectrophotometer

Question 32

describe absorbance and % transmittance with a blank

Answer 32

high % transmittance and low absorbance

Question 33

describe absorbance and % transmittance as the number of bacteria in a sample increases

Answer 33

decrease in %T and increase in A

Question 34

advantage of turbidity method

Answer 34

easy, fast results

Question 35

disadvantage of turbidity method

Answer 35

bacterial numbers must be high enough to be read by the spectrophotometer = not useful for samples with low bacterial numbers

Question 36

describe measuring a cellular substance as a technique for measuring microbial growth

Answer 36

measure the amount of protein/nitrogen/ATP. If the amount is high, the number of bacteria should be high