Prokaryotic Growth

Question 1

PROKARYOTIC GROWTH

Answer 1

• Growth refers to an increase in cellular
  constituents that may result in:
  – An increase in cell number.
  – An increase in cell size.
• Growth refers to population growth rather
  than growth of individual cell.

Question 2

THE GROWTH CURVE

Answer 2

• A characteristic growth curve is observed
  when microorganisms are cultivated in batch culture (a culture incubated in a closed vessel with a single batch of medium).
• Usually plotted as logarithm of cell number versus time.
• Growth curve has four distinct phases: lag,
  exponential, stationary, death.

Question 3

Viable Counting Methods

Answer 3

• Spread and pour plate techniques:
  – Diluted sample of bacteria is spread over
  solid agar surface or mixed with agar and poured into Petri dish.
  – After incubation, the number of organisms
  is determined by counting the number of colonies multiplied by the dilution factor.
  – Results expressed as colony forming units (CFU).

Question 4

Measurement of Cell Mass

Answer 4

Dry weight (time consuming and not very
sensitive).
* Quantity of a particular cell constituent (e.g. protein, DNA, ATP, or chlorophyll).
– Useful if amount of substance in each cell
is constant.
* Turbidometric methods (based on light
scattering by cells): quick, easy and sensitive.

Question 5

Turbidity as a Measure of Microbial Mass

Answer 5

Turbidity of a culture often expressed as the
OPTICAL DENSITY at a particular wavelength, e.g. 600nm (OD600nm)
A = 2 - log10 %T

Question 6

Requirements for Carbon, Hydrogen, and Oxygen

Answer 6

• A microorganism that requires the same
  nutrients as most of the NATURALLYOCCURING members of its species is a PROTOTROPH.
• If a prototroph mutates and is no longer able to synthesise a molecule necessary for
  growth and reproduction, it will require that
  molecule to be supplied as a nutrient.
• Such a mutant is an AUXOTROPH

Question 7

Growth Factors

Answer 7

All growth factors are organic compounds.
* Growth factors are essential cell components
(or their precursors) that the cell cannot
synthesise.
* Must be supplied by environment if cell is to survive and reproduce.

Question 8

Classes of Growth Factors

Answer 8

1. Amino acids (needed for protein synthesis).
2. Purines and pyrimidines (needed for nucleic acid synthesis).
3. Vitamins (function as enzyme cofactors).
   An organism unable to produce the growth
   factors it needs is called an auxotroph.

Question 9

Some Media Components

Answer 9

Peptones (protein hydrolysates prepared by
partial digestion of various protein sources).
* Extracts (aqueous extracts, usually of beef oryeast).
* Agar (sulphated polysaccharide used to
solidify liquid media; most microorganisms
cannot degrade it). Agar is extracted from
seaweeds

Question 10

Functional Types of Media

Answer 10

General purpose
Enriched
Selective
Differential

Question 11

Functional Types of Media (continued)

Answer 11

• Supportive or general purpose media
  – support the growth of many types of
  microorganisms (e.g. tryptic soy agar).
• Enriched media – general purpose media supplemented by blood or other special nutrients (e.g. blood agar).

Question 12

Selective Media

Answer 12

Favour the growth of some microorganisms
and inhibit the growth of others.

Question 13

Differential Media

Answer 13

• Distinguish between different groups of
  microorganisms based on their biological
  characteristics:
• e.g. Blood agar (allows differentiation between haemolytic and non-haemolytic bacteria). Blood aga is both an enriched medium and a differential medium).

Question 14

INFLUENCE OF ENVIRONMENTAL FACTORS
ON GROWTH

Answer 14

• Most organisms grow under moderate
  environmental conditions.
• Extremophiles grow under harsh conditions that would kill most other organisms.