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Industrial Manipulation of Quorum Sensing Systems
- The History, Biochemistry, and Potential of Quorum Sensing

Quorum sensing has immediate obvious uses in medicine and improving human quality of life. But QS systems may also be manipulated for industrial uses. While QS antagonists can be used in industries such as aquaculture as an antibacterial agent, QS can also be manipulated for increased metabolite production for industrial purposes. While strain selection and genetic cloning are already techniques used in industry, QS may become easier, cheaper, and even more efficient than existing practices.

Acetic acid is a simple carboxylic acid, best known as the main ingredient in vinegar. Besides being a popular salad dressing, acetic acid is the precursor to many other products including Polyethylene Terephthalate (often called PET), a partially recyclable thermoplastic used in food and drink packaging. Acetic acid is normally produced synthetically from petrol products. Historically, humans have used bacteria to produce acetic acid, and acetic acid for vinegar is still produced biologically today. Oxidation of ethanol by Acetobacter produces acetic acid in aerobic conditions. Some anaerobes such as Clostridium can metabolize sugars straight to acetic acid, without the ethanol intermediate, making biologically produced ethanol much more efficient. It's very difficult to increase acetic acid yield in biological production because the product becomes toxic to cells, and will inhibit growth. The best available method to increase yield is to select strains that have a high resistance to acetic acid. Strains can also be genetically modified to produce high activity enzymes and growth; but it seems that a negative QS system is the biggest limiting factor in overall production.

Gluconacetobacter intermedius is an obligate aerobe, and produces acetic acid oxidatively from an alcohol precursor. By genetically manipulating cells to overexpress aldH from Gluconacetobacter polyoxogenes, cellsshow an increase in overall acetic acid production (3). aldH encodes a high activity aldehyde dehydrogenase - the enzyme responsible for acetic acid synthesis. aco from Acetobacter aceti, which encodes an aconitase can also be cloned to improve acetic acid production. Aconitase synthesizes isocitrate, an eventual precursor molecule for acetic acid.

Recently, it has been shown that genes that limit the overall acetic acid production in     G. intermedius are controlled by a quorum sensing system (5). The quorum sensing system affects fermentation negatively, decreasing fermentation, and therefore energy yield and growth rate. Biologically, decreasing growth rate decreases production of acetic acid before it becomes toxic to the cells. This control mechanism helps in preventing a toxic level of acetic acid. A higher product yield can also be achieved by inhibiting the negative QS regulating cell growth. Inhibition of the QS system gives a higher product yield than genetic manipulates such as aldH cloning (1.4 fold compared to 2.4 fold, respectively)(5).

By combining existing genetic techniques with new QS-level manipulation product yield could be even higher. While these techniques would most certainly be popularized in the vinegar industry, the plastic industry may stick to common petrol chemistry. Ideally, with further enhancement and on a larger scale, biological production may be a possible "green" solution to petrol reaction synthesis. Many industrial products have been known to be regulated by cell growth and density. QS may provide a way to molecularly trick cells into producing a desired product.

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