Cholera is a diarrheal disease caused by an infection of the small intestine by the bacterium Vibrio cholerae. The severe dehydration can kill within hours if untreated. Now, researchers show that Vibrio cholerae injects toxins into surrounding cells using a tiny spear and then steals their DNA -- a brilliant move that helps fuel its antibiotic resistance and virulence. The findings were published in Science this week.
Bacteria often take up DNA from other cells and incorporate it into their own genomes. The process is called horizontal gene transfer, as opposed to the conventional "vertical" transmission of genes from parent to offspring. The cholera bacteria, however, have a particularly murderous way of accomplishing this: They use a long tube to pierce a hole in neighboring competitor cells (including their own kind), and then they fill them with toxins and leave them to die. This spring-loaded spear is called the "type VI secretion system" (T6SS), and this predatory behavior is triggered by the surrounding environment. Vibrio cholerae lives in water where it attaches to tiny crustaceans, feeding on the chitin that makes up their shells. Whenever that sugary polymer is available, the bacterium goes into an aggressive survival mode, attacking nearby bacteria with its spear.
To explore how Vibrio cholerae compete for survival, an Ecole Polytechnique Fédérale de Lausanne team led by Melanie Blokesch grew different strains of the bacterium from across the world on chitin surfaces similar to those of the planktonic crustaceans in their natural habitats. Then they examined them using genetic and bioimaging techniques. That survival system, they found, is co-regulated by genes that prompt the DNA ingestion and integration, guiding the bacterium to assemble the predatory killing device. After the stabbed cells release their genetic material, the killer bacterium scoops them up, and this transfer of genes help make the bacterium more resistant to threats, including antibiotics.
"Using this mode of DNA acquisition, a single V. cholerae cell can absorb fragments containing more than 40 genes from another bacterium," Blokesch says in a news release. "That's an enormous amount of new genetic information."
This chitin-mediated activation of a killing device, they say, likely renders the bacterium more dangerous to people when ingested since the spear might also kill helpful gut microbes. The team is now studying the interaction between the chitin-induced production of the spear and horizontal gene transfer. "By studying this interplay,” Blokesch says, “we can begin to better understand evolutionary forces that shape human pathogens and maybe also transmission of the disease cholera.”
