Cholera – a bacterial infection caused by drinking food or water contaminated with Vibrio cholerae bacterium – is responsible for up to 120,000 global deaths each year. In a new study, researchers say they have uncovered a mechanism by which V. cholerae becomes more virulent, allowing a better understanding of the transmission of cholera and other diseases.
Image credit: Graham Knott and Melanie Blokesch/EPFL
Melanie Blokesch and colleagues, from the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, found that V. cholerae stabs and kills nearby bacteria – even those of the same strain – using a spring-loaded spear-like weapon.
The bacterium then takes the DNA of its victim, allowing it to spread further throughout its host.
The researchers publish their findings in the journal Science.
V. cholerae naturally lives in water and binds to tiny planktonic crustaceans. It feeds on chitin – a sugar polymer found on the shells of the crustaceans. As it does so, V. cholerae enters “natural competence” – a type of survival mode in which the bacterium attacks neighboring pathogens with its spear-like weapon, called the “type VI secretion system” (T6SS).
In this study, Blokesch and colleagues wanted to find out more about the natural competence action of V. cholerae and how it uses it to survive.
The team assessed a variety of V. cholerae strains, including some that were involved in the cholera pandemic that began in the 1960s and that still affects present-day populations.
In order to monitor these strains in their natural environment, the researchers grew them on the shell surface of crustaceans, allowing them to feed on chitin.
Using genetic and bioimaging techniques, they found that as well as entering the natural competence phase – in which T6SS stabs and kills nearby bacteria, including its own species – it steals their DNA.
This process is known as “horizontal gene transfer” and is activated by the environment in which V. cholerae resides, the researchers explain. This technique allows V. cholerae to become more virulent.
“Using this mode of DNA acquisition, a single V. cholerae cell can absorb fragments containing more than 40 genes from another bacterium,” explains Blokesch. “That’s an enormous amount of new genetic information.”
The team says their findings are significant as horizontal gene transfer occurs in an array of bacteria and is known to play a role in speeding up pathogens’ resistance to antibiotics.
What is more, the fact that T6SS is activated by a chitin-related environment means V. cholerae may pose a higher risk to individuals who ingest it; the spear-like device could also kill protective bacteria that lives in the human gut.
The researchers now plan to investigate the relationship between the production of the spear and horizontal gene transfer. Blokesch adds:
“By studying this interplay, we can begin to better understand evolutionary forces that shape human pathogens and maybe also transmission of the disease cholera.”
In 2013, Medical News Today reported on a study in which researchers claimed to reveal why some people are more susceptible to cholera than others.