Growth Kinetics On Solid Media

  • Here we're concerned primarily with mycelial fungi.
  • While it's relatively easy to determine the biomass of a fungus growing in a liquid medium, it's more difficult to estimate biomass when a fungus is growing in all three dimensions over and through a solid medium.
  • For this reason we usually express the growth of a colony in terms of the RADIAL EXTENSION OF THE COLONY (i.e. we measure colony radius).
  • If we plotted colony radius v. time we might observe the following four stages ........
Radial extension v. time:
Graph of colony radial extension against time.
  • The LAG PHASE follows inoculation of the plate. During this phase spores are beginning to germinate (spore inoculum) or severed hyphae are beginning to branch (mycelial inoculum).
  • The EXPONENTIAL PHASE lasts until the colony radius is only approx. 100µm (0.1 mm). Because readings of colony radius are often made at daily intervals this phase is often missed.
  • During the LINEAR PHASE the rate of increase in colony radius is constant. The slope of this region represents the COLONY RADIAL GROWTH RATE = Kr.
  • A DECELERATION in colony extension occurs as the colony approaches the margin of the Petri dish and staling metabolites begin to accumulate within the growth medium.
  • The specific growth rate of the colony = µ = Kr/W, where Kr = the colony radial growth rate (obtained from the linear region of the graph; µmh-1), and W = the width of the peripheral growth zone (in µm).
Peripheral Growth Zone:
Diagram illustrating estimation of PGZ.
  • The Peripheral Growth Zone is represented by the length of hypha needed to maintain maximum growth rate of the leading hyphae at the colony margin.
  • It may be estimated by comparing the growth rates of leading hyphae at the colony margin BEFORE and AFTER cutting diagonally across the colony margin.
  • A cut too close to the apex will kill a hypha - so it will stop growing.
  • A cut further back will cause the hypha to grow more slowly.
  • Eventually a distance from the apex is reached at which the cut has no effect on growth rate of the hypha - the length of hypha back to this point from the apex represents the Peripheral Growth Zone.
Hyphal density:
Although determining the rate of colony extension will provide us with a measure of growth, it will not necessarily be equivalent to the increase in biomass of the fungus because hyphae will be growing down and through the agar medium as well as across its surface. For this reason it's usually a good idea to estimate HYPHAL DENSITY as well, i.e. the number of hyphal branches produced per unit area.
REMEMBER: two colonies may have the same radius but vary significantly in hyphal density and, therefore, biomass! See section on hyphal branching.

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