3. Factors Affecting Microbial Growth

Question 1

FACTORS AFFECTING GROWTH AND ACTIVITY OF MICROORGANISMS

Answer 1

Nutrients
Available Moisture
pH
inhibitory Substances
Temperature
Oxygen Concentration
Pressure
Radiation

Question 2

how nutrients affect growth and activity of microorganisms

Answer 2

Cells need macro and micronutrients, carbon and energy sources for growth

Different organisms require different levels of these compounds;

Question 3

how available moisture affect growth and activity of microorganisms

water is needed for

Answer 3

chemical and enzymatic reactions
transfer of compounds (nutrients and wastes) across membranes

Some microorganisms can remain alive in dry conditions in dormant forms (e.g. endospores) but all require moisture for growth

Question 4

Expressed water activity (aw ) is equal to

Answer 4

1/100 x relative humidity or P solution/P pure water .

Question 5

In hypotonic environments cells can burst due to

Answer 5

increased internal pressures

Question 6

In hypertonic environments cells may undergo

Answer 6

plasmolysis

Question 7

Most organisms maintain their internal osmotic concentrations slight higher than the environment so that

Answer 7

the plasma membrane rests firmly against the cell wall

Question 8

Osmotolerant organisms may grow over a wide range of

Answer 8

water activities e.g. Saccharomyces rouxii, which may grow at aw of 0.6 or less;

Question 9

Halophilic bacteria such as Halobacterium have an obligate requirement for

Answer 9

high salt concentration (2.7 – 6.2M NaCl) for growth.

Question 10

how inhibitory substances affect growth and activity of microorganisms

Answer 10

Many substances are antagonistic to microbial growth
1) Antibiotics
–>Inhibit processes such as cell wall synthesis (penicillin)m Protein synthesis (macrolides), nucleic acid synthesis (quinolones), folic acid synthesis (sulfonamides)
–>Disrupt membranes (polymyxins)

2) Chemical disinfectants and preservatives
3) Metals e.g. Manganese and Aluminum

Some compounds may promote the growth of microorganisms
E.g. exopolysaccharide exudates from legume roots, which stimulate the proliferation of the root nodule bacteria Rhizobium.

Question 11

how temperature affect growth and activity of microorganisms

Answer 11

Important for enzymatic reactions

Decreased activity below optimal temperature

denaturation of proteins and distruption of membranes occur after a certain point

Prokaryotic organisms can generally grow at higher temperatures than eukaryotic organisms;

Each organism have minimal, optimal and maximum temperatures for growth;

Question 12

Psychrophiles- optimal temperature

Answer 12

optimal temperature <15oC, minimum ≤0oC and maximum around 20oC.
Growth at low temperatures may be due to a protein factor that allows protein synthesis at 0oC and increased amounts of unsaturated fatty acids in their membranes

Question 13

Psychrotrophs optimal temperature

Answer 13

can grow at 0oC, but optimal temperature>20oC and maximum temperature around 35oC

Question 14

Mesophiles- optimal between

Answer 14

20 - 45oC, minimum 15 – 20oC and maximum around 45oC

Question 15

Thermophiles- optimal temperautre

Answer 15

> 55oC, minimum around 45oC and maximum between 55 - 65oC.

They can grow at high temperatures due to heat stable enzymes and protein synthesis systems and membranes with more saturated lipids

Question 16

Hyperthermophiles - optimal temperature

Answer 16

83 - 113oC and do not grow well below 55oC

Question 17

how oxygen concentration affect growth and activity of microorganisms

Answer 17

Different microorganisms require different levels of oxygen (Obligate aerobes, obligate anaerobes, facultative anaerobes; microaerophiles, aerotolerant)

Certain destructive compounds can be formed when oxygen is reduced in organisms growing in the presence of oxygen
–> superoxide radical O2-,
–> hydrogen peroxide, H2O2
–> hydroxyl radical, OH˙)

Microbes produce various enzymes to get rid of these destructive compounds such a superoxide dismutase and catalase.

Question 18

how radiation affect growth and activity of microorganisms

Answer 18

Some organisms such as Deinococcus radiodurans and certain endospores can tolerate relatively high dosage of radiation.

Question 19

how pressure affect growth and activity of microorganisms

Answer 19

Some deep sea microbes can live in pressures up to 600 –1100 atmospheres. Some may be barophilic and some may be barotolerant;