300
Chapter 7
7.6.2 Growth of Filamentous Microorganisms
The mechanisms for growth of filamentous microorganisms are very different from those of
unicellular microorganisms, since the cells are connected in so-called hyphal structures (see Fig.
7.19). All cells within these multicellular structures may contribute to the growth process, i.e.,
production of protoplasm, but extension of the hyphae occurs only at the tips. The number of tips in
a mycelium is therefore a characteristic morphological variable. Even though the linear rate of tip
extension has an upper limit, the total length of a mycelium may increase exponentially due to the
formation of new tips along the hyphae. The frequency of formation of new tips is determined by
the rate of production of protoplasm within the mycelium and the number of tips to which the
material is distributed. The ratio between the size of the mycelium and the number of tips is
therefore another characteristic morphological variable, which Caldwell and Trinci (1973) called
the
hyphal growth unit.
They originally defined it as the total mycelium length divided by the
number of tips (called the
hyphal growth unit length
but it may also be defined on the basis of
total mycelium mass (called
hyphal growth unit mass).
At conditions that support rapid growth, a
densely branched mycelium with a large hyphal diameter is observed, whereas a less branched
mycelium with a small hyphal diameter is observed at poor growth conditions (Nielsen, 1992),
where the mycelium will extend itself in the hope o f reaching an environment where the growth
conditions are better, and it therefore forms long threads with very few branch points.
In a hyphal element several cells behind the tip are involved in the tip extension process, since they
supply the necessary cellular material for tip extension, e.g., cytoplasmic material and building
blocks for wall synthesis. These cells are not separated by a septum (the wall between the
individual cells), and they therefore share a common cytoplasm in which the nuclei of all the cells
are found. The part of the hyphal element between the tip (or apex) and the first septum is called the
apical compartment.
The cells just behind the apical compartment have an intracellular
composition very similar to that of the apical cells, and this part of the hyphal element is called the
subapical compartment.
Despite the presence of a septum between the apical and subapical cells,
there may be an exchange of protoplasm since the septa are often perforated. When one moves
further away from the tip, one finds cells containing large vacuoles. These cells do not participate
directly in the tip extension process, but they are believed to be of importance in creating an
intracellular pressure sufficient to ensure transport of protoplasm toward the tip section. This part of
the hyphal element is referred to as the
hyphal compartment.
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