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The Acetate Switch
The acetate switch is a microbial metabolic process that was first identified in E. coli. The switch occurs after bacteria have gone through overflow metabolism (or fermentation). Overflow metabolism occurs when bacteria are growing in new media, and instead of going to the process of aerobic metabolism, cells go through fermentation. Overflow metabolism is theorized to occur because it requires cells to create fewer proteins. However, as cell density increases, by products from fermentation (including acetate) accumulate. This accumulation of fermentation products is potentially damaging to cells because these products are acidic and they lower the pH of the surrounding environment. Therefore, cells begin to import acetate and use it in cellular processes to prevent this lowered pH from damaging the cells.

Incorporation of acetate into either respiration or biosynthetic pathways requires the enzyme Acetyl-CoA Synthetase (Acs), which synthesizes acetyl-CoA from acetate.

Symbiosis and Pathogenesis
The ability to go through the acetate switch has previously been shown to be an important colonization factor in multiple different species of bacteria.

Regulation
The acetate switch is regulated by many different transcriptional regulators. To determine what proteins regulate the acetate switch, researchers often use the expression of the acs gene as a marker for whether or not the acetate switch is occuring. This marker is often accompanied by an analysis of acetate levels.

Quorum Sensing in V. fischeri.
In Vibrio fischeri, a mutual symbiont, the acetate switch is regulated by a quorum sensing system.