cell growth 9/28

Question 1

what can measurement of microbial growth measure changes in

Answer 1

number of cells in a population and mass of population

Question 2

how are direct cell counts done

Answer 2

–counting chambers

–electronic counters – flow cytometry

–on membrane filters

Question 3

what are counting chambers

Answer 3

• Easy, inexpensive, and quick
• Useful for counting both eukaryotes and prokaryotes
• Cannot distinguish living from dead cells

Question 4

what are direct counts on membrane filters and how do they work

Answer 4

• Cells filtered through special membrane that provides dark background for observing cells
• Cells are stained with fluorescent dyes
• Useful for counting bacteria
• With certain dyes, can distinguish living from dead cells

Question 5

what is flow cytometry

Answer 5

• Microbial suspension forced through small orifice with a laser light beam
• Movement of microbe through orifice impacts electric current that flows through orifice
• Instances of disruption of current are counted
• Specific antibodies can be used to determine size and internal complexity

Question 6

is it easy to determine if a cell is alive or dead?

Answer 6

no

Question 7

what are spread and pour plate techniques for counting viable cells

Answer 7

–diluted sample of bacteria is spread over solid agar surface or mixed with agar and poured into Petri plate

–after incubation the numbers of organisms are determined by counting the number of colonies multiplied by the dilution factor

–results expressed as colony forming units (CFU); each CFU comes from 1 viable cell

Question 8

what is the membrane filter technique

Answer 8

–bacteria from aquatic samples are trapped on membranes

–membrane soaked in culture media

–colonies grow on membrane

–colony count determines # of bacteria in sample

Question 9

what is a serial dilution and how do they work

Answer 9

• series of sequential dilutions used to reduce a dense culture of cells to a more usuable concentration
• since each dilution reduces the concentration of bacteria by a specific amount calculating the total dilution over the series allows you to know how many bacteria you started with
• multiply number of colonies times dilution (ex:10³)

Question 10

if a microbe cannot be cultured on plate media what is done

Answer 10

–dilutions are made and added to suitable liquid media

–turbidity determined to yield the most probable number (MPN)

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Question 11

what are turbidometric measures

Answer 11

• light scattering
• quick, easy, sensitive

Question 12

what has to be done to relate a direct cell count to a turbidity value

Answer 12

•a standard curve must first be established which relates optical reading to either total # of cells or total # of viable cells (plated)