UNDERSTANDING MICROBIAL POPULATIONS

One of the weaknesses in carrying out fermentation technology is due to poor understanding of the behaviour and characteristics of microbial populations especially in the fermentor. Understanding the role and function of the microorganisms is more than just knowing what type of microorganisms we are dealing with or knowing the physical structure and dimension of microbial cells. There is more to that, and this lack of knowledge about the behaviour and composition of microbial populations in the fermentors are often regarded as the " Achilles Heel" of the fermentation process.

Whether we want to admit it that although the unit of microorganisms is the single microscopic cell but their behaviour are often characterised in a "multicellular fashion" in terms of the microbial population. Even microbiologists expressed the activity of the unicellular organisms in terms of the microbial population. The growth curve of the microorganisms is often expressed as microbial population growth against time :))

The concept of microbial population is complex in terms of its structure and composition. The properties of the microbial population is always dynamic and changing with time. We cannot and must not regard the microbial particles as innate or static particles in the fermentor. Everything during the fermentation process is in a state of dynamic flux with microorganisms affecting the fermentor functions and vice versa

In the study of microbiology there are two states of growth.
1First the real growth of the single cell which increase in size and biomass before dividing into two new daughter cells and
2 Second the population growth where a number of cells will divide to form high number of cells.

In the fermentor we are more concerned with the growth behaviour of the population of microorganisms or the microbial inocula introduced into the fermentor

The growth of the initial microbial population (inocula) is affected by the following factors:
1 Supply of nutrients
2 Diffusion of waste products
3 Mass transfers between the inocula and the broth environment

All though within the same inocula the initial composition of microbial cells within the inocula show different composition temporally and spatially. This to a degree depend on the time period of the inocula and the conditions of the inocula. Even though it is deemed that the inocula is taken at log phase that does not mean all the cells in the inocula are uniform occurring as young active cells


POPULATION COMPOSITION WITH TIME
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At the onset of the fermentation period after the microorganisms have adapted to the environment, there will be higher proportion of new young cells compared to old and dying cells. However on reaching the stationary phase there will be a shift in the composition of the cells with higher percentage of the population consisting of old and dying cells

To maintain a high proportion of young cells or for it to be continually in the log phase for most of the population either fresh substrate need to be added or a wastage of cells need to be carried periodically

PRESENTATION OF MICROBIAL POPULATIONS IN FERMENTORS
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While it is true microorganisms are unicellular and capable of existing independently, but in a population of microbes in the fermentor these microbial cells do not occur in a uniform unicellular suspension. In reacting to the stress of the fermentor environment such as intense aeration and shearing forces, these microorganisms tend to occur in the form of microbial aggregates such as a microbial floc, pellets, mats, granules or as biofilms.

These microbial aggregates will not only complicate the rheology and mass transfer processes but also the fermentation process on the whole. New considerations must be taken in operating the fermentor under such conditions such as stirring and supply of oxygen