Horizontal gene transfer (HGT) occurs in bacteria and
archaea through many different processes. Natural competence for transformation
is a common mode of HGT and allows bacteria to take up free DNA from the
environment. Vibrio cholerae is a
mainly aquatic organism mostly found in marine habitats associating with
plankton blooms as well as shellfish and is considered by the WHO to be the
most important Vibrio sp. to humans.
When growing on chitinous surfaces, V. cholerae can initiate natural competence
through the production a regulatory protein, TfoX, due to the presence of
chitin and its degradation products. The type VI secretion system (T6SS) is the
most recently discovered mechanism for effector secretion that is now
understood in gram-negative bacteria. Its structure and function resembles
intracellular and membrane-attached phage tails and can pierce neighbouring
bacterial and eukaryotic cells. A study by Borgeaud et. al. (2015) demonstrated the use of the T6SS by V. cholerae as part of the TfoX
competence regulon and its expression when on chitinous surfaces.
V. cholerae was
grown in the absence and presence of chitin-induced expression of TfoX.
RNA-sequencing was then used to understand the extent of the competence regulon
and enable an accurate assessment of the bacterial transcriptome. Two strains
of V. cholerae were also used to
simulate a predator-prey relationship to investigate whether competence induced
T6SS-mediated killing affects transformation. Live-cell fluorescence microscopy
imaging was used to visualise prey lysis and the transfer of genetic material.
Three T6SS encoding gene clusters were observed to be
up-regulated upon the induction of TfoX, the major gene cluster and two
auxiliary clusters. They concluded that the RNA-seq data suggests TfoX
initiates the transcription of these clusters in the presence of a chitinous
substrate and the T6SS genes were elevated after growth of V. cholerae on chitin flakes. The functionality of the T6SS was
then assessed in an interspecies killing assay. V. cholerae strains with an inducible copy of TfoX were shown to
have a significant killing behaviour toward Escherichia
coli due to the activation of T6SS. They also found upon TfoX induction, in
predator-prey relationship tests, natural transformants were readily obtained
in predator cells and never or rarely found in T6SS-defective strains. In
conclusion, upon competence induction V.
cholerae induced the T6SS, this lead to the killing and lysis of non-immune
neighbouring bacteria. This caused the release of DNA which transforms
competent predatory cells. This technique resembles bacterial fractricide as
described in Streptococcus pneumonia but,
in contrast, V. cholerae seems to
target strains with no compatible effector-immunity and is dependent on
contact. Live-cell fluorescence microscopy showed high T6SS activity in
predator cells followed by cell rounding and lysis of the prey. The close
proximity observed of competent bacteria to lysed cells with the distinctive
formation of certain proteins is indicative of DNA translocation into the
predator cell. These gene transfer events were only observed in T6SS-positive
strains.
These findings show the importance of the T6SS as a
method of HGT in V. cholerae. Similar
to S. pneumoniae and other bacteria,
this uptake of DNA from the environment could lead to the emergence of
multidrug resistant strains. This could have particular significance with
antibiotics entering the water column, environmental V. cholerae could become resistant to these leading to cholera
outbreaks which could be harder to fight and control. However, this papers
findings could be used to develop new ways to fight V. cholerae; if a method could be found to interfere with the HGT
pathways, similar to the effects of CRISPR on Staphylococcus epidermidis, then this may significantly reduce its
evolution and thus the spread of antibiotic resistance. Borgeaud et. al also suggests that chitin-induced
expression may also enhance the virulence potential of this pathogen in the
human gut. Therefore, any method found to reduce the HGT of V. cholerae may have an added bonus of
also reducing the severity or its ability to cause disease.
Borgeaud, S., Metzger, L. C., Scrignari, T., & Blokesch, M. (2015). The type VI secretion system of Vibrio cholerae fosters horizontal gene transfer. Science, 347(6217), 63-67.
Special thanks to Prof. Blokesch for access to this report
Hi Ben,
ReplyDeleteIn terms of new methods to fight V. cholerae, do you think there could just be a way to reduce or block competence of it for the chitin? Perhaps by preventing adhesion to the surface? Although even if it could work, I'm not sure how it could be implemented on a large scale!!
I agree that on a large scale it wouldn't feasible/appropriate to try and control V. cholerae natural competence. If this was possible then it could have wide environmental consequences, maybe affecting chitinous organisms or other bacteria associated with them. In order to reduce any possible evolution of antibiotic resistance in environmental populations I think the best method is to control the amount of antibiotics entering the water e.g. from farm animals. If a method was devised, such as blocking receptor sites or denaturing the TfoX protein etc, then I feel this would have much more use when used in conjunction with regular antibiotics combatting infections in the human gut. This would mean that V. cholerae would have far less potential to become resistant to this antibiotic making the drug more effective as well as, as mentioned above, possibly reducing its virulence. I hope this answers your question!
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