Ecole Polytechnique de Lausanne. Switzerland: Publishing in Science, EPFL scientists have
uncovered the unconventional way that the cholera bacterium stabs and
kills other bacteria to steal their DNA, making it potentially more
virulent.
Cholera is caused when the bacterium Vibrio cholerae
infects the small intestine. The disease is characterized by acute
watery diarrhea resulting in severe dehydration. EPFL scientists have
now demonstrated that V. cholerae uses a tiny spear to stab and
kill neighboring bacteria – even of its own kind – and then steal their
DNA. This mechanism, known as “horizontal gene transfer”, allows the
cholera bacterium to become more virulent by absorbing the traits of its
prey. The study is published in Science.
The lab of Melanie Blokesch at EPFL has uncovered how V. cholerae
uses a predatory killing device to compete with surrounding bacteria
and steal their DNA. This molecular killing device a spring-loaded spear
that is constantly shooting out. This weapon is called the “type VI
secretion system” (T6SS) and is known to exist in many types of
bacteria. When V. cholerae comes close to other bacteria, the
spear punches a hole into them, leaving them to die and release their
genetic material, which the predator pulls into itself.
Killing neighbors and stealing genes
This
spear-killing, predatory behavior is triggered by the bacterium’s
environment. The cholera bacterium naturally lives in water, such as the
sea, where it attaches onto small planktonic crustaceans. There, it
feeds on the main component of their shells: a sugar polymer called
chitin. When chitin is available, V. cholerae goes into an aggressive survival mode called “natural competence”. When in this mode, V. cholerae attacks neighboring bacteria with its spear – even if they are of the same species.
Melanie Blokesch set out to explore how V. cholerae
uses this behavior to compete for survival in nature. Her lab tested
different strains of the bacterium from all over the world, most of
which have been implicated in the 7th cholera pandemic, which
began in Indonesia in the 1960’s, spread rapidly to Asia, Europe, and
Latin America, and still affects populations today.
The
researchers grew these bacteria on chitin surfaces that simulated their
natural habitat on crustaceans. What they found was that the tiny spear
is not only part of V. cholerae’s natural survival system, but
it also contributed to the transfer of genes that could make the
bacterium more resistant to threats, even to antibiotics. The
researchers then used genetic and bioimaging techniques to identify, in
real time, which mechanisms are involved in this event, which is called
“horizontal gene transfer”.
“Using this mode of DNA acquisition, a single V. cholerae
cell can absorb fragments containing more than 40 genes from another
bacterium,” says Melanie Blokesch. “That’s an enormous amount of new
genetic information.” This phenomenon is referred to as “horizontal”
gene transfer, as opposed to the conventional “vertical” passage of
genes from parent to offspring.
The importance of this study lies
in the fact that horizontal gene transfer is a widespread phenomenon in
bacteria, and it contributes to the dispersal of virulence factors and
antibiotic resistances. In addition, the chitin-mediated activation of
the spear-killing device most likely renders the bacterium more
dangerous to patients when they ingest it, as this molecular spear might
also kill protective bacteria in the human gut.
The researchers
are now extending their investigation into the interplay between the
chitin-induced production of the spear and horizontal gene transfer. “By
studying this interplay, we can begin to better understand evolutionary
forces that shape human pathogens and maybe also transmission of the
disease cholera,” says Blokesch.
Reference
Borgeaud S, Metzger LC, Scrignari T, Blokesch M. The type VI secretion system of V. cholerae fosters horizontal gene transfer. Science, 2015; 347 (6217): 63 DOI: 10.1126/science.1260064