How to Optimize Nutrient Transport to Bacteria

Essay for course SQES 2147: Microbial Physiology & Biochemistry

The bacterial cell, in general, has 4 modes of transporting nutrients through its cytoplasmic membrane into the cell itself:

1. passive diffusion
2. facilitated diffusion
3. group translocation
4. active transport

In industry, it is important to optimize nutrient uptake into bacterial cells via these four modes of transport so as to maximize production. Some factors to be considered:

1. The physical nature of raw materials used

The bacterial cell can only transport solutes, both polar and non-polar, through its cytoplasmic membrane. These molecules may be either fat-soluble or water-soluble. Therefore, any substances supplied to the bacterial culture must be in solution form. (it would be ridiculous to hope that the bacterial cell will consume solid particles such as grain!)

2. The type of raw materials used

The carbon source supplied to the bacterial culture must not only be a solute – it must also be suitable for bacterial utilization. Sources such as acetate are not favoured since the energy yield is too low, and the bacterial cell will not readily uptake such nutrients unless they are under high environmental pressure. Also, different carbon sources have different reducing potentials. Anaerobic bacteria require low redox potential energy sources (approximately -200 mV; such energy sources are electron acceptors) while aerobic bacteria are better suited for positive redox potential conditions. Therefore the carbon source used must be selected with considerations to the bacteria that is using it.

3. The modes of transportation

While it is true that in general small non-polar and fat-soluble molecules readily enter the cell via passive diffusion and that most amino acids and organic acids are taken into the bacterial cell via active transport, nevertheless the fact remains that one transport system serves many substances and that certain substances may enter the bacterial cell via more than one way. Therefore, whenever possible, a particular substance that can enter the cell in more than one way ought to be chosen. (consider the passive diffusion mode of transport, for example, which is a slow process, and it will be clear as to why it would be more wise to choose a substrate that can enter the cell through various ways)

4. The quality of raw material added in

We must also take into consideration the size factor: the size of the bacterial cell, the size of the carbon source molecules, and the volume of the container in which these two things are mixed. A bacterial cell will only uptake substances that it collides with, and since passive diffusion of a particular solute is an interminably slow process, if the quantity of raw material (for the bacteria’s usage) is small, chances are that the number of collisions between bacteria and substance will be low and uneconomical consequently. Also, there will be a considerable distance between the bacterial cell and its nutrients. To minimize this distance and maximize collision and uptake, the quantity of nutrients supplied must be increased. However, the bacterial cell physiology must be taken into account, as well as the water content of the liquid in which the bacterial cell is suspended, since osmosis will occur if there is a water concentration gradient, and the bacterial cell will plasmolyse if the liquid is extremely hypertonic.

5. The optimum pH of transport systems involved

If it is true that in facilitated diffusion it is the permease enzymes that transports molecules across the membrane, then such enzymes would be greatly influenced by the pH of its environment. Thus the Nutrient uptake into the bacterial cell can be increased by making sure that the pH of the substance is optimum for the transport system involved. Also the pH of the environment (ie the liquid in which the bacterial cells are suspended) will change as the bacteria metabolize the raw materials into the target products, and it might be necessary to add buffers to the system to keep the pH constant at its optimum level. Otherwise, the transport system will fail and nutrient uptake will decline, and at extreme pH levels, the membrane of the bacterial cells would collapse, resulting in cell lysis.

6. The temperature of the system

It is a thermodynamic fact that the higher the temperature, the more kinetic energy a body will gain. At higher temperatures, the Brownian movement exhibited by both bacterial cells and molecules (of the carbon source) will increase in velocity. The cells and molecules will move at greater speed in the same random fashion, resulting in more frequent collisions, and subsequently, higher nutrient uptake. Therefore it would be of economical importance to raise the temperature of the system in order to increase nutrient uptake. However, since proteins play such an important role in transportation, it would not be wise to raise the temperature beyond 40ºC, since proteins denature at temperatures higher than that. Also, higher kinetic energy would mean harder, more forceful collisions that might damage the bacterial cell. In the case of the energy sources used, certain substances and solutes might either associate or dissociate at certain temperatures to form entirely different molecules that the bacterial cell might not be able to utilize. Therefore the temperature parameter must be taken into serious consideration in optimizing conditions for nutrient uptake.

7. Selecting nutrients and solutions

While the bacterial culture might uptake the specified nutrients supplied to it, it will also inevitably take in other solutes available in the liquid in which the nutrients are dissolved. The uptake of these ‘unnecessary’ molecules and/or ions will result in a decrease in the uptake of the nutrients the cell needs to produce the desired metabolite. (and the metabolism of such substances would yield unwanted products that would contaminate the system – both wanted and unwanted metabolites would be mixed together with no way of separating them) Therefore it is important to ensure that the nutrient liquid supplied is not contaminated by other solutes, or by making sure that these other solutes are substances that may also be used as energy sources.

1999

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Booster’s Comments:

Very Good!!

Note:

1. The type of metabolism the bacterial carries out: Anaerobic? Fermentation?
2. Try getting substrates easily hydrolysed and close to the metabolite culture (at least I think that’s what he wrote. Booster’s handwriting is atrocious)
3. Chemical nature of substrate: hydrophilic/hydrophobic?