Culturing Microorganisms On An Industrial Scale

In any bioprocess, what must the microorganisms involved be able to do?

Synthesize or break down the chemical required, work reasonably fast, give a good yield of the product, use relatively cheap nutrients, and not require extreme (and therefore expensive) conditions. It must not produce any poisons or mutate easily into non-functioning forms.

Do we always want the same thing from microorganisms?

No, this varies from one bioprocess to another.

Sometimes, how are the substances we want from the microorganisms formed?

As an essential part of the normal functioning of an organism, for example, ethanol, ethanoic acid, and a range of amino acids and enzymes. They are known as primary metabolites.

What are primary metabolites?

Organic compounds directly involved in the normal growth, development, and reproduction of an organism

What are secondary metabolites?

Substances produced by organisms that are not essential for normal growth, but are still used by the cells.

Once a microorganism has been chosen, and the ideal size and shape of the bioreactor (reaction vessel) decided, what has to be decided?

The organisation of the commercial production.

What are the two main ways of growing microorganisms?

• Batch fermentation.

• Continuous fermentation

What happens in batch fermentation?

• The microorganisms are inoculated into a fixed volume of medium.

• As growth takes place, nutrients are used up and both new biomass and waste products build up.

• As the culture reaches the stationary phase, overall growth ceases but during this phase, the microorganisms often carry out biochemical changes to form the desired end products.

• The process is stopped before the death phase and the products harvested. The whole system is then cleaned and sterilised and a new batch culture started up.

What happens in a continuous culture?

• Microorganisms are inoculated into sterile nutrient medium and start to grow.

• Sterile nutrient medium is added continually to the culture once in reaches the exponential point of growth.

• Culture broth is continually removed - the medium, waste products, microorganisms, and product - keeping the culture volume in the bioreactor constant.

What does continuous culture enable?

Continuous balanced growth, with levels of nutrients, pH and metabolic products kept more or less constant.

What can both methods of operating a bioreactor be adjusted to ensure?

Either the maximum production of biomass or the maximum production of the primary or secondary metabolites. Most sytems are adapted for maximum yield of metabolites.

What do the majority of industrial processes use?

Batch or semi-conservativation.

What is continuous cultivation largely used for?

The production of single-celled protein and in some waste water treatments.

What do all bioreactors produce?

A mixture of unused nutrient broth, microorganisms, primary metabolites possibly secondary metabolites, and waste products. The useful part of the mixture has to be separated out by downstream processing, this is one of the most difficult and expensive parts of the whole bioprocess.

Whether a bioreactor is simply a container containing microbial broth or a complex aseptic fermenter, what is it very important to control and manipulate?

The growing conditions to maximise the yield of product required.

What are the factors that have to be controlled in bioreactors?

• Temperature.

• Nutrients and oxygen.

• Mixing things up.

• Asepsis.

Why must temperature be controlled in a bioreactor?

If the temperature is too low the microorganisms will not grow quickly enough. If the temperature gets too high, enzymes start to denature and the microorganisms are inhibited or destroyed. Bioreactors often have a heating and/or a cooling system linked to temperature sensors and a negative feedback system to maintain optimum conditions.

Why must nutrients and oxygen be controlled in a bioreactor?

Oxygen and nutrient medium can be added in controlled amounts to the broth when probes or sample tests indicate that levels are dropping.

Why must mixing things be up be controlled in a bioreactor?

Inside a bioreactor there are large volumes of liquid, which may be quite thick and viscous due to the growth of microorganisms. Simple diffusion is not enough to ensure that all the microorganisms receive enough food and oxygen or that the whole mixture is kept at the right temperature, so most bioreactors have a mixing mechanism and many are stirred continuously

Why must asepsis be controlled in a bioreactor?

If a bioprocess is contaminated by microorganisms from the air, or from workers, it can seriously affect the yield. To solve this problem most bioreactors are sealed, aseptic units. If the process involves genetically engineered organisms, it is a legal requirement that they should be contained within the bioreactor and not be released into the environmen.