MICROBIAL GROWTH FORMS IN FERMENTOR.

Microorganisms in nature behave differently compared to when they are in laboratories or under fermentor conditions. In nature, microorganisms are generally not well off and are often occurring under the feast or famine environment. Their growth form is often affected by these conditions such as formation of spores and diffusive hyphal growth.

It is often said that for microorganisms living in the laboratories or fermentors, they are having a comfortable and luxurious life surrounded by rich nutrients and optimal environment. Their growth forms are therefore different compared to their microbial kins in nature. ( There are however microorganisms in the laboratory suffering cryogenic freezing…hehe)

In reality, those microorganisms living in the rich and optimal environment of the fermentors are in fact suffering and living under physiological stress. They are facing extreme over crowding and high density living being crammed within such limited volume. Their environment is often very viscous to their comfort. They face a lot of shearing forces especially those in the proximity of the shearing blades of the impellers. Exploding bubbles and hydrodynamic shearings attack them non stop for comfort. And physiologically they are forced to breed and multiply to fulfill their master’s wishes to produce more and more fermentation products till they die!

Under such stressful conditions their growth and growth forms are affected. One of the most common growth forms commonly encountered in fermentor growth are the formation of microbial granules or pellets. Granules in this context do not refer to the cytoplasmic inclusions but a form of microbial aggregate.

Despite the common occurrence of granulation in fermentors many are still are not aware of the causes of their formation. This is the result of dichotomy of approach and making quick conclusions from a few research observations.

Before we go further let us look at the nature of the microbial granule or aggregate.
Microbial granules are pellet like structures in which microorganisms are entrapped. These granules are basically EPS in nature and most times dense or compact. In terms of composition they are similar to microbial film thus indicating similar physiology and biochemistry of formation. Their unique morphology are more due to the strong and complex hydrodynamic conditions surrounding their environment. While the nature of microbial films is surface dependent, microbial granules are independent entities although they might have same origin or biogenesis

There are four important points to consider in the formation of granules:

1 The nature of microorganisms
2 Its physiology
3 Nutrient environment
4 Hydrodynamic condition of fermentation broth

In microbial granulation mostly involve bacteria. In a way it does not reflect that fungi can’t form granules as fungi are known to be important component in floc formation. It is most attributed to fungi unable to live in anaerobic conditions or that they are easily broken up by the hydrodynamic forces. Fungi are also not really EPS producers.

The fact that higher amount of microbial granules are associated with anaerobic reactors do say a lot about how they are formed. You cannot say that there are a lot of competition in the anaerobic environment as only few anaerobic bacteria succeeds in establishing themselves there. There is higher number and diversity of microorganisms in microbial flocs than in anaerobic granules

The formation of EPS is a physiological response to stress. During stress the first reaction of microorganisms is self preservation. Although it is seen the formation of EPS is a waste of carbon, energy and protein, it must confer advantages to the producers. Living in the fermentor is stressful as said earlier. While the EPS is seen as a shield or barrier to protect the organism, it is in itself a barrier to diffusion of nutrients

The strong hydrodynamic conditions not only shear cells but it also help in the pelletization process itself. The forces help in forming rounding and compacting the granules. The steady state size of the granules reflect the point of stability of the structure undr the existing operating conditions.

I cannot really see how the microorganisms in the pellet enhanced the bioprocess as the tight granules hinder the diffusion or mass transfer of nutrients. I do agree that it helps in the settlement and clarification of the wastewater treatment.



Type rest of the post here.