Quorum-sensing Agent Has Dual Role

In findings with potential drug discovery implications, a bacterial quorum-sensing compound has been found to convert chemically to a product with a different function. The compound–N-(3-oxododecanoyl)homoserine lactone (3O-C₁₂-HSL)—undergoes a spontaneous intramolecular reaction by a previously undocumented route to form a product that kills other bacteria.

Bacterial quorum-sensing compounds, also called autoinducers, are analogous to animal pheromones. Bacteria use them to communicate with each other to sense when enough of their peers are around to warrant turning on genes for virulence, spore formation, biofilm formation, luminescence, and other functions.

3O-C₁₂-HSL is a quorum-sensing agent produced by the pathogen Pseudomonas aeruginosa. The compound belongs to a family of quorum-sensing agents called acyl homoserine lactones (AHLs).

Previously, professor of microbiology and immunology Barbara H. Iglewski of the University of Rochester School of Medicine & Dentistry and coworkers established that 3O-C₁₂-HSL has a second function in addition to quorum sensing: It also acts as a virulence factor that can alter the way P. aeruginosa infects its hosts [J. Bacteriol., 184, 1132 (2002)]. However, the compound performs this function without prior chemical conversion.

Now, chemistry professor Kim D. Janda and coworkers at Scripps Research Institute have found that the same autoinducer can undergo a spontaneous intramolecular chemical reaction to form a product with a different second function--bactericidal activity [Proc. Natl. Acad. Sci. USA, 102, 309 (2005)]. "This novel chemical reaction has been missed for at least 15 years or so" in studies of the 3O-C₁₂-HSL system, Janda says. "The newly formed molecule is very stable and can basically keep other bacteria off its turf, so to speak."

The bactericidal agent, a tetramic acid, is produced from 3O-C₁₂-HSL by a Claisen-like condensation reaction. Tetramic acid scaffolds "are found in numerous natural products known to be relevant in medicinal settings," Janda says, and the findings could therefore have drug discovery applications.

The tetramic acid doesn't harm the producing microorganism, P. aeruginosa, which is gram-negative. (Bacteria are classified as gram-negative or gram-positive on the basis of cell wall differences.) Instead, the study by Janda and coworkers indicated initially that the tetramic acid only attacks and kills gram-positive bacteria. Therefore, the producing bacterium may use the conversion product to protect itself from other bacteria. The researchers also found that this tetramic acid binds iron tightly and may therefore be a bacterial siderophore--a compound that chelates iron.

THE DISCOVERY of the conversion product's new function "is an exciting finding," says assistant professor of biology Eric S. Gilbert of Georgia State University. His research focuses on microbial biofilms, the formation of which may be regulated by quorum sensing. "To date," he says, "AHLs have been primarily appreciated because of their role in bacterial cell signaling or quorum sensing. Janda and coworkers have demonstrated that these compounds could also be important for other ecological functions besides gene regulation. This means that scientists who detect these compounds will need to think more broadly about their activities or whether they have multiple functions."

"I believe that this is an interesting and potentially significant finding--a first of its kind," says chemistry professor Dehua Pei of Ohio State University, Columbus. Pei is a specialist in autoinducer biosynthesis. "It surely will stimulate further research in this area, and it could lead to new strategies for antibacterial drug design." However, he adds, "it is unclear at this point whether this is a general phenomenon," as other gram-negative species use different quorum-sensing agents, and it remains to be seen whether those compounds have reaction products that exhibit antibacterial activity. In addition, the tetramic acid agent exhibits only a modest level of potency and is active only against gram-positive bacteria, "which seems to be rather limiting if the goal is to ward off competing bacteria," Pei says.

Janda replies that the tetramic acid has adequate potency to impart an advantage to 3O-C₁₂-HSL-producing bacteria in real-world situations--such as in biofilms, where autoinducers can be found in high concentrations. And a preliminary study by his group suggests that tetramic acids may also kill certain gram-negative bacteria in addition to gram-positive species, contrary to the initial findings, he adds.

Chemical biology and biotechnology professor Hiroaki Suga of the University of Tokyo, whose interests include cellular communication in P. aeruginosa, comments that the bactericidal selectivity of the tetramic acid "is quite interesting, although the mechanism remains elusive." The Scripps findings suggest that screening of autoinducer-analog libraries "may yield new antibacterial agents," Suga says.