Deep-sea Vent Bacteria Provide Insight into Evolution of Ulcer-Causing Relatives

Genes responsible for virulence in certain pathogenic bacteria, including strains associated with stomach cancer and ulcers in humans, also allow related deep-sea vent bacteria to thrive in their environment, according to new research published by the Proceedings of the National Academy of Sciences.

Researchers sequenced the genomes of previously unidentified species of Sulfurovum and Nitratiruptor obtained from samples collected at a hydrothermal vent in the Iheya North field near Japan at a depth of 1000 meters. The bacteria, like many other deep-sea vent bacteria, are chemolithoautotropic-that is, they make their own food from carbon dioxide using energy from inorganic molecules. As vent communities occur at such great depths that they receive no sunlight and cannot support photosynthetic organisms, it is instead the chemolithoautotrophic bacteria which form the base of the ecosystem. Many of these bacteria, including the two species sequenced, belong to a group called the epsilon-proteobacteria. Curiously, the epsilon-proteobacteria also include a number of animal pathogens, such as Helicobacter pylori, which is associated with ulcers and stomach cancer in humans, and the food-borne, diarrhea-causing Campylobacter jejuni.

Analysis of the two new deep-sea vent bacteria genomes revealed that they possessed copies of genes considered important to virulence in the pathogenic epsilon-proteobacteria. Among them were a cluster of genes responsible for evasion of host immune systems in the pathogenic bacteria; the authors suggested that the genes arose in ancestral deep-sea vent bacteria symbiotic with invertebrates (as many modern vent epsilon-proteobacteria are today), and were pre-adaptive to the evolution of pathogenic epsilon-proteobacteria.

The researchers also found that both the deep sea and pathogenic bacteria had relatively few DNA-repair genes, which lead to more variability in the organisms. They also suggested that this variability and tendency to mutation was adaptive to both groups of bacteria as they encountered enviromental gradients. This was important in the pathogens as it grants them their ability to cause persistant infections despite variable conditions within their hosts, and in the deep-sea bacteria as it allowed them to live in differing chemical environments depending on their distance from hydrothermal vents. These results provide insight into the surprising origin and evolution of important pathogens of humans and other animals.

While scientists have previously cultured and sequenced the genomes of several pathogenic epsilon proteobacteria (including several species of the aforementioned Helicobacter and Campylobacter), the study of their deep-sea vent relatives was facilitated by recent advances in culture techniques which allowed researchers to grow up sufficent numbers of cells for genome sequencing. The project was funded by a grant from the National Science Foundation.

References:

Satoshi Nakagawa, "Deep-sea vent epsilon-proteobacterial genomes provide insights into emergence of pathogens." Abstract URL: "http://www.pnas.org/cgi/content/abstract/0700687104v1?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=deep+sea+vent+genome&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT"