Microbial pellet formation

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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 + M01/ 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.