Sequencing of marine bacterium will help study of cell communication

The opportunity to annotate the genome of the glow-in-the-dark bacterium, Vibrio fischeri, which lives in symbiotic harmony within the light organ of the bobtail squid, has helped a Virginia Tech microbiologist advance her research on quorum sensing, or how cells communicate and function as a community.

Researchers studying the newly sequenced genome of the marine bacterium V. fischeri, described this week in the Proceedings of the National Academy of Sciences (PNAS), have so far observed both differences and similarities in gene arrangement between it and pathogenic Vibrio species. V. fischeri has a lower GC content than other sequenced Vibrio species, but it is still more closely related to them than other organisms. (Among the four nucleotides that make up DNA – adenine, guanine, thymine, and cytosine (ATGC) – ’A’ pairs with ’T’ and ’G’ pairs with ’C’. The more GC content, the more tightly DNA strands bind.)

Despite the fact that it is a symbiont, V. fischer’s genome contains genes that may have toxin activity. “Analysis of this sequence has revealed surprising parallels with Vibrio cholerae and other pathogens,” said Ann Stevens, associate professor of biology at Virginia Tech.

This sequence research is described in the PNAS online early edition the week of February 7, 2005, (www.pnas.org), in the article, “Complete genome sequence of Vibrio fischeri: A symbiotic bacterium with pathogenic congeners,” by E. G. Ruby, professor of medical microbiology and immunology at the University of Wisconsin, Madison, previously at the University of Hawaii; C. Lupp; J. McCann; D. Millikan; A. Schaefer; and C. Whistler of the University of Hawaii at Honolulu; M. Urbanowski and E. P. Greenberg of the University of Iowa at Iowa City; J. Campbell at Integrated Genomics; A. Dunn and E. Stabb at the University of Georgia at Athens; Marie Faini and Ann Stevens of Virginia Tech; R. Gunsalus of the University of California at Los Angeles; and K. Visick of Loyola University Chicago.

Ann Stevens, associate professor of biology at Virginia Tech, studies how cell-to-cell interaction regulates bacterial processes that include antibiotic or toxin production, biofilm formation, and bioluminescence. One of the longest studied models of quorum sensing is the bioluminescent marine bacterium V. fischeri.

“It was selected by Drs. Ruby and Greenberg to be sequenced exactly because it is so well studied,” said Stevens. “This was also the first time a non-pathogenic Vibrio species has been sequenced and there is the potential for valuable lessons as it is compared with the pathogenic species. I was particularly pleased when my graduate student, Marie Faini, and I were given the opportunity to be members of the team that annotated the sequence.”

Annotation is a complex process that assigns functions to genes. Having access to the genetic sequence in order to perform this chore gave Stevens and Faini early access to specific information about the genes of the bacterium she has been studying for several years.

Many different bacteria use quorum sensing. Stevens explains that cells release autoinducer molecules. The numbers of these signal molecules increase with cell density, until they initiate various reactions, such as light production – or, in a pathogen, the release of toxin once a certain level of bacteria build up.

Stevens studies the regulation system of the cell-to-cell communication. “Our quorum sensing research group at Virginia Tech is working to build a more complete understanding of the molecular processes that occur at the point that a bacterium changes its gene expression pattern in response to quorum sensing,” she said. The genome sequence has allowed Stevens to begin exploring the connection between quorum sensing and other global regulatory networks through a combination of comparative genomics and experimental molecular biology.

Other researchers are focusing on the relationship between the bacterium and its animal host. The PNAS article concludes, “If we are to understand the unifying themes underlying these contrasting bacteria–host interactions, we must begin to use comparative genomic approaches with closely related pathogenic and beneficial microbial species.”

Marie Faini O’Brien of Frederick, Maryland, earned her master’s degree in biology in April 2003.