Microbial growth and physiology

Question 1

Nutrient requirements

Answer 1

Energy 
carbon
nitrogen 
inorganic elements 
gases (oxygen and hydrogen) 
water

Question 2

nutrient requirements

Carbon source

Answer 2

Carbon dioxide (autotroph)

Organic compounds e.g. glucose (heterotroph)

Question 3

nutrient requirements

Energy source

Answer 3

Sun i.e. photosynthesis (phototroph)

inorganic elements e.g. NH3
OR
organic compounds oxidised aerobically or anaerobically (chemotroph)

Question 4

nutrient requirements
Electron source
(e removed from donor, passed to acceptor releasing energy; donor is oxidised)

Answer 4

inorganic electron donor e.g. H (lithotroph)

organic electron donor e.g. glucose (organotroph)

Question 5

requirements - oxygen

5 points

Answer 5

1. aerobes - require oxygen
2. obligate anaerobes - must have no oxygen
3. facultative anaerobes - can survive with no oxygen
4. aerotolerant anaerobes - anaerobes tolerate oxygen
5. microaerophiles - require oxygen at very low concentration

Question 6

microbial growth

Answer 6

single celled microorganisms divide by binary fission (doubling)

1 cell = 2.
2 cells = 4. so on…

exponential increase when ready supply of nutrients (increase in size at constantly growing rate)

Question 7

increase in cell number by generation

Answer 7

generation 
1 = 2 to power of 1 = 2
2 = 2 to power of 2 = 4
3 = 2 to power of 3 = 8
4 = 16

Question 8

converting exponential growth curve into straight line

Answer 8

plotting logarithm (log) of the number of cells against time

Question 9

Microbial growth curve

4 phases

Answer 9

1. Lag phase
2. log (exponential phase)
3. stationary phase
4. death phase

Question 10

Microbial growth curve

lag phase

Answer 10

organism adapting to its new environment; enzymes synthesised, cells increasing in size but not yet dividing

Question 11

Microbial growth curve

Log (exponential) phase

Answer 11

cells undergoing exponential growth with constant growth rate and mean generation time. Slope and length of log phase depend on how well medium meets requirements

Question 12

Microbial growth curve

Stationary phase

Answer 12

no net increase in cell number; nutrient (or oxygen) exhausted, environment changed (pH), toxic products accumulate

Question 13

Microbial growth curve

Death phase

Answer 13

cells die (often exponential); no energy, pH damage, toxic products

Question 14

Batch culture

Answer 14

When microbes are inoculated into a fixed volume of growth medium in a closed system

Question 15

continuous culture

4 points

Answer 15

1. Permanently in log phase of growth
2. New nutrient added at same rate that culture mixture removed from growth vessel
3. Should achieve steady state
4. Can maximise production of microbe and its products
   Industrially important

Question 16

chemostat

Answer 16

equipment for continuous culture of micro-organisms

Question 17

Growth of micro-organisms on solid medium

6 points

Answer 17

1. Cannot count cell numbers
2. bacteria colony spreads out to obtain nutrients from agar
3. Change in diameter of colony shows similar growth curve to cell numbers in liquid medium
4. Growth on solid medium usually slower than liquid medium
5. For fungi growth radiates out from initial spore – roughly circular growth
6. Diameter of colony shows similar curve as cell numbers of unicellular micro-organisms

Question 18

biofilms

4 points

Answer 18

1. Microbial growth on solid material in the environment often takes the form of biofilms
2. Need moisture, a source of nutrients and a surface

3. Steps:
Attachment (to surface)
Exopolysaccharide production (cements biofilm together)
Maturation
Dispersal

4. Can excrete molecules which attract other microbes increasing size and complexity of the biofilm
   e. g. microbial growth in pipes or on rocks

Question 19

Counting in liquid media – “direct” count

9 points

Answer 19

1. Use a counting chamber e.g. “haemocytometer”
2. Glass slide has grid with precision engraved squares of known area
3. Add a coverslip and flood slide with culture (may need to be diluted first)
4. Known depth between grid and coverslip
5. Count number of cells in a square (for several squares)
6. Calculate average number of cells per square
7. Calculate volume of liquid above each square
8. Know average cells in a given volume
9. Calculate cells/ml diluted culture

Question 20

Counting on solid media – “viable” count

5 points

Answer 20

1. Use either the pour plate or spread plate technique to enable colonies to be seen
2. Need to dilute original sample to ensure a good number of clearly separated colonies which can be counted
3. Use “serial dilutions”
4. Quantitative so must use accurate volumes
5. Common serial dilutions are 1 in 10 or ten-fold dilutions – each dilution is one-tenth of the concentration of the previous one. [Other common serial dilutions are 1 in 2, 1 in 5, 1 in 100]