Introductory Microbiology (1+1)

Measurement of fungal growth

• Kinetic analyses show that fungal filamentous growth can be interpreted on the basis of a regular cell cycle, and therefore encourage the view that mycelial growth and morphology can be described mathematically. Here, mathematical models that attempt to describe fungal growth and branching in the vegetative (mycelial) phase are presented.

Measurement methodologies:
In order to describe and quantofy hyphal growth and branching, measurement of the parameters such as hyphal diameters (hd) and hyphal length (hl) is essential. These allow hyphal volume (hv), to be calculated, which when multiplied by the average density of the composite hyphal material (p), gives an estimate of biomass (X). If these measurements are taken over a series of time intervals it is possible to calculate hyphal extension rate (E), and hence rate of increase of biomass. Currently, automated image analysis systems permit real time analysis of these microscopic parameters. Some of these analyses suggest that hyphal tips grow in pulses, although this has been contested, particularly because the observations use video techniques and the pixelated image generated by analogue and digital cameras will cause pulsation artefacts.
The most important macroscopic parameters is total biomass. Total hyphal length is proportional to total biomass, if hd and p are assumed to be constant, but measurement can be difficult. Non destructive direct mass measurement is rarely feasible, in most cases due to the technical difficulty encountered in physically separating the mycelium from the substrum.

Describing hyphal branching:
A germ tube hypha will initially grow in length exponentially, at a rate that increases until a maximum, constant extension rate is reached, and that thereafter, it will increase in length linearly. The primary and subsequent branches will behave similarly. Thus, there develops a scenario in which individual hyphae extend linearly yet the biomass of the whole mycelium increases exponentially. It was due to the exponential increase in tips due to branching. Hyphal branching is calculated by equation(1) E = µ max G Where E is the mean tip expression rate, µ max, is the maximum specific growth rate, and G is the hyphal growth unit. G is defined as the average length of a hypha supporting a growing tip according to equation(2)

G=   LT  

        NT

Where Lt is total mycelial length, and Nt is the total number of tips. The hyphal growth unit is approximately equal to the width of the peripheral growth zone, which is a ring shaped peripheral area of the mycelium that contributes to radial expansion of the colony. In a mycelium that is exploring the substratum, branching will be rare and thus G will be large. G is therefore an indicator of branching density. Prosser and Trinchi (1979) studied for exponential growth and branching in fungal mycelia. The process was modeled in two steps:
1) Vesicles were produced in hyphal segments distal to the tip and were absorbed in tip segments.
2) Vesicles flowed from one segment to the next, towards the tip.
Apical branching initiated when the concentration of vesicles in the tip exceeded the maximum rate that the apex could absorb the new material. Varying the ratio of these steps produced different flow rates and branching patterns. The model also incorporated the concepts of the ‘duplication cycle’. This was achieved by increasing the number of nuclei in the model mycelium at a rate proportional to the rate of biomass increase. Septa were then assumed to form in growing hyphae when the volume of the apical compartment per nucleus branched a threshold level.
• This provided for initiation of lateral branches by assuming that vesicles accumulated behind septa to a concentration comparable to that which initiated apical branching. This model achieved good agreement for total mycelial length, number of hyphal tips and hyphal growth unit length in Geotrichum candidum.

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