Team jams bacteria 'talk' to boost bio-product yields
In studies that could be vital to an expanding field of industrial biotechnology, scientists at the Center for Biosystems Research are learning to censor what E.coli bacteria are 'talking' about.
Cell-to-cell cross talking by laboratory E. coli strains engineered to produce antibiotics, industrial polymers or other products in fermentation vessels can lead to stress in the culture and severely limit product output. But scientists with CBR and partners have begun to decipher and override stress 'talk' among cells of recombinant E. coli. CBR is part of the University of Maryland Biotechnology Institute (UMBI).
In separate fermentation experiments with strains of E. coli that produce two important model products--interleukin-2 or organophosphorus hydrolase--the team increased product output by three to four times. The first product is an important drug in preventing cancer and HIV. The latter is a detoxifying agent for biowarfare nerve agents.
The scientists achieved higher yields of both products by either conditioning the bacterial cultures with high levels of molecules from a signaling pathway called AI-2, or by splicing the LuxS gene for quorum signaling into the recombinant E. coli.
"This is great," says team leader William Bentley, CBR research professor. "We want to understand what are the receptor molecules on these cells, to understand the communication circuitry," he adds. The experimental techniques are applicable to boosting production of many other products in E. coli cultures. However, the researchers say they are only beginning to understand and control the communications circuitry of E. coli and other bacterial cells.
E. coli is a common enteric bacterium that is one of the most highly studied microbes, with its genome and basic physiology well known. It is also one of the easiest vehicles for genetic engineering and a workhorse for metabolic engineering, an emerging branch of industrial biotechnology. Metabolic engineers study interactions of biological molecules in order to improve the manufacture of cell products and proteins for therapeutic and industrial value.
At CBR, Bentley heads one of a very few metabolic engineering laboratories focused on non-pathogenic E. coli quorum sensing. "We first learned that when you grow E. coli in a fermentor, the resulting product per cell goes down to a third of what it was (on the laboratory bench), but you still make more cells. Why don't they make as much at high cell density as low cell density? We decided to look at what changes are going on." The team also includes Matthew P. DeLisa, formerly of Bentley's laboratory and currently with Cornell University; and James J. Valdes, U.S. Army Soldier & Biological Chemical Command, Aberdeen Proving Grounds, Md.
Scientists have known about chemical cross-talking among cells of bacteria since 1970. Through such communication, bacteria gauge their own population density and respond by altering their expression of specific genes as a group, a process known as quorum sensing. In marine microbes, quorum-sensing molecules speed up reproduction to form bio-fouling films that may colonize on surfaces of boats or piers. Pathogenic bacteria, such as those that cause cholera, salmonella, Lyme disease, tuberculosis and pneumonia, become virulent. Still others such as Anthrax may form spores when they reach a quorum, or make antibiotics to fight off neighboring microbes.
But studies of cell-to-cell 'talking' in E. coli, so important to industrial biotechnology, have lagged behind somewhat. Scientists at Princeton University discovered signaling molecules in E. coli in 1998.
Bentley's team is investigating recombinant E. coli in the context of "an emerging centralized role for quorum sensing where it overlaps with signals for stress and starvation," he says. This new link between quorum sensing, cellular metabolism and stress-responsive circuits raises the possibility of targeting quorum pathways for improving cellular productivity.
They observed that signaling in recombinant E. coli, decreased the yields of a desired protein product. There had been no previously published reports of experiments to use quorum-sensing signaling pathway molecules or genes to improve recombinant protein yield in any bacteria expression system.
The team began two years ago by mapping DNA transcription of four quorum-regulated genes and 20 stress genes of E. coli. They found significant regulatory overlap among several stress and starvation genes and known quorum-sensing genes. "Because quorum-sensing signals turn bacterial genes on and off, deciphering this language will enable us to commandeer it for our own purposes, like controlling protein and metabolic engineering, rewiring cells, you might say, to produce important polymers or try to figure out the coordination of the enzymatic pathways. This is just the very beginning."
Such biological processing (fermentation) to produce products is more desirable than traditional industrial processing because of the use of renewable resources such as E. coli bacteria. It is more energy efficient and cleaner processing.
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