How different forms of cholera microbe increase infectivity

Various forms of cholera microbe

Researchers have uncovered a novel way in which Vibrio cholerae, the aquatic microbe that causes cholera, may increase its competitive fitness, and the likelihood of creating pandemic strains of the bacteria.

The finding was revealed by examining the type VI secretion system, or T6SS for short, a secretion system used by Vibrio cholerae to inject toxic proteins into neighboring bacterial cells, killing them and increasing its survival advantage.

The researchers studied a gene cluster within the T6SS called auxiliary cluster 3 (Aux3). The gene cluster exists in two states: one, which is highly mobile, is found in environmental strains of V. cholerae--these do not cause disease--and a second, which is a less mobile and truncated form of the gene cluster. This second, less mobile form of the gene cluster is "locked" in the pandemic V. cholerae genome and potentially helped create the pandemic strains.

Environmental Aux3 can be readily excised from and integrated into the genome via site-specific recombination, whereas pandemic Aux3 recombination is reduced.

"We wanted to look at how a harmless environmental strain of Vibrio cholerae acquires the unique traits needed to become the pandemic strain of the bacteria," said the senior author. "With this in mind, we located an island in the chromosome of the bacteria that only exists in the pandemic strain. Unexpectedly, we found a much larger form of the DNA island in a couple strains in the environment that seems to be jumping from one strain to another."

The lead author added, "Even though we may not have a cure for cholera, this research contributes to our understanding of how these pathogens evolve and become pandemic."