Bacterial Growth

Question 1

define ‘growth’

Answer 1

an increase in biomass, typically accompanied by cell division

Question 2

what is binary fission?

Answer 2

cell division and separation followed by filamentous growth

Question 3

how many planes do different shapes of bacteria divide in?

Answer 3

rods divide in one plane; cocci divide in one/two/three planes

Question 4

what is a batch culture?

Answer 4

• growth medium inoculated with small quantity of culture; incubation with shaking
• bacteria go through growth phases of lag/exponential/stationary/death as conditions change

Question 5

what is a continuous culture?

Answer 5

• continuous addition of nutrients and removal of waste products in chemostat
• culture maintained in steady state of exponentional growth
• conditions remain constant

Question 6

what is the lag growth phase?

Answer 6

cells adapting to new conditions; synthesis of new cellular components eg. enzymes for nutrients in medium

Question 7

what is the exponential growth phase?

Answer 7

maximal growth; population doubles at regular intervals

Question 8

what is the stationary growth phase?

Answer 8

population remains constant as rate of cell growth = rate of cell death; nutrients are depleted

Question 9

what is the death phase?

Answer 9

cells die due to starvation and the accumulation of toxic waste products

Question 10

what is meant by the abbreviation t(gen)?

Answer 10

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.

Question 11

what methods can be used to count bacteria?

Answer 11

• haemocytometer
• counting chamber
• serial dilution

Question 12

what calculation is carried out using a counter chamber?

Answer 12

average number of cells in one square x number of squares x 1/depth x 1000/width

Question 13

what are some disadvantages of the counter chamber?

Answer 13

tedious; counts both live and dead cells

Question 14

describe the use of serial dilutions to count bacteria

Answer 14

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

Question 15

what is meant by ‘cfu’?

Answer 15

colony-forming units; this is not always equivalent to a single cell, since cells may cluster

Question 16

what is a dis/advantage of the method of serial dilution?

Answer 16

it is a very slow method

but it counts only live cells

Question 17

which methods do not rely on counting cells?

Answer 17

• optical density
• Coulter counter
• calculations of biomass
• calculation of MPN (most probable number)
• electrical impedance

Question 18

describe the method of using optical density as a measure of number of bacteria

Answer 18

• use a spectrophotometer to measure light-scattering

- calibration factor required to calculate the number of cells using the measured absorbance

Question 19

what is a dis/advantage of the optical density method?

Answer 19

it is a very quick method

but it only works on dense cultures

Question 20

describe the use of a Coulter counter to count bacteria

Answer 20

• 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

Question 21

what are the disadvantages of the Coulter counter?

Answer 21

• 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

Question 22

describe the use of biomass to count bacteria

Answer 22

harvest cells by centrifugation and filtration and measure wet/dry weight

Question 23

what is MPN?

Answer 23

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

Question 24

which industry uses MPN?

Answer 24

the water industry

Question 25

what is electrical impedance?

Answer 25

• measure electrical properties of the sample
• microbial metabolism involves the production of ionic metabolites
• this results in a decreased electrical impedance

Question 26

which industry uses electrical impedance?

Answer 26

the food industry (for quality control)