Microbial pellet formation
Shown by culture of filamentous/mycelial
organisms such as molds and fungi
• Mycelial organisms which show apical growth
• Also grow exponentially.
• Filamentous fungi have a 'growth unit‘
– is composed of the apex of the hypha and
– a short length of supporting hypha.
• The total hyphal length of a mycelium and the
number of tips increased exponentially at
approximately the same rate.
• When the volume of the hyphal growth unit
exceeds a critical volume a new branch, and
hence, a new growing point, is initiated
• This is equivalent to the division of a single cell
when the cell reaches a critical volume.
• The rate of increase in hyphal mass, total length and
number of tips is dictated by the specific growth rate
:
• The rate of increase in hyphal mass
dx/dt = μ X,
• The rate of increase in total length
dH/dt = μ H,
• The rate of increase in number of tips
dA/dt = μ A
Where,
• x is biomass of hypha, H is total hyphal length and A
is the number of growing tips.
• Kinetics and dynamics of pallet
formation.
– Described by Pirts in 1975.
– In submerged culture (shake flask or fermenter) a mycelial
organism may grow as dispersed hyphal fragments or as
pellets.
– The growth of pellets will be exponential until the
density of the pellet results in diffusion limitation.
– Under diffusion limitation the central biomass of
the pellet will not receive a supply of nutrients,
nor will potentially toxic products diffuse out.
• The growth of the pellet proceeds from the outer
shell of biomass which is the actively growing zone
and was described by Pirt in1975 as:
M 1/ 3 = k.t + M0 1/ 3
Where
– Mo and M are the mycelium mass at time 0 and t,
respectively
– A plot of the cube root of mycelial mass against time will
give a straight line, the slope of which equals k.