Specialised weaponary used to free up naked DNA.

As described in our lecture, chitin induces natural competence in Vibrio cholerae. However, recent findings by Borgeuad et al (2015) show that V. cholera express additional genes during growth on chitinous substrata, which further enhances their adaptive capabilities. They describe the function of a Type iv secretion system (T6SS) and its intrinsic function related to the induction of competence, and additionally explains the role of quorum sensing in this process.

Natural competence is the ability of a bacterium to acquire exogenous DNA, which is then incorporated into the genome via transduction. The attachment of V. cholerae to a chitinous substratum initiates the gene expression of a DNA-binding protein appendage; type iv pilli. This increases the bacterium’s ability to gain genes for adaptation to environmental conditions or colonising the human gut. However, the availability of naked DNA may present a hindrance in the rapid adaptation of these bacteria.

Borgeuad et al (2015) show that in addition to induced competence, attachment to chitin results in the expression of a type iv secretion system (T6SS). The T6SS is a structure made up of number of proteins that looks remarkably like a torpedo, and it has been suggested that they are extended into neighbouring cells; causing cell lysis. The tip of the structure protrudes from the outer membrane of the bacterium, and is homologous to the tail spike of a bacteriophage; which are used for adhesion and may dissociate upon entry into the cell. The shaft of the structure is a protein encoding haemolysin, which can destroy the cell membrane of red blood cells. The innermost protein complex (situated in the inner membrane of the bacterium) is made up of two contractile proteins that appear to provide the mechanical force for entry into cells. Simultaneously, the bacterium produces immunity modules to prevent any damage to itself. In short, the bacterium facilitates its adaptation by causing neighbouring cells to lyse, freeing up potentially valuable DNA. Although using the T6SS would only be useful if there is an abundance of bacteria surrounding it, which provide additional genes.

Quorum sensing is a chemical communication used by bacteria to signify their high density. As the production and use of T6SS is likely to be energetically costly, the process is regulated through quorum sensing to ensure there are enough bacteria in close proximity to lyse. Thus, the gene expression leading to the production of the T6SS only occurs at high cell density. When T6SS is extended into the high-density community on the chitinous substrata, any bacterium without the expression of the immunity modules will be lysed, creating a pool of naked DNA.

The methodology of this paper was interesting and novel, as they fluorescently labelled an allele known to be key in the structure of the T6SS, in order to visualise the structure, and how it extends and contracts. This paper shows how V. cholerae not only create specialised structures to uptake naked DNA, but also develop weapon like protein structures to attack neighbouring cells and free-up DNA, which can be used to its advantage. This clearly shows the adaptability and rapid evolution of these bacteria, highlighting how pathogenicity islands can be passed around.

Reference:

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.