Influence of Chemotaxis and Swimming Patterns on the Virulence of the Coral Pathogen Vibrio coralliilyticus.

Chemotaxis, the directed movement toward or away from a chemical signal, can be essential to bacterial pathogens for locating hosts or avoiding hostile environments. The coral pathogen Vibrio coralliilyticus chemotaxes toward coral mucus; however, chemotaxis has not been experimentally demonstrated to be important for virulence. To further examine this, in-frame mutations were constructed in genes predicted to be important for V. coralliilyticus chemotaxis. Most Vibrio genomes contain multiple homologs of various chemotaxis-related genes, and two paralogs of each for cheB , cheR , and cheA were identified. Based on single mutant analyses, the paralogs cheB ₂ , cheR ₂ , and cheA ₁ were essential for chemotaxis in laboratory assays. As predicted, the Δ cheA ₁ and Δ cheR ₂ strains had a smooth-swimming pattern, while the Δ cheB ₂ strain displayed a zigzag pattern when observed under light microscopy. However, these mutants, unlike the parent strain, were unable to chemotax toward the known attractants coral mucus, dimethylsulfoniopropionate, and N -acetyl-d-glucosamine. The Δ cheB ₂ strain and an aflagellate Δ fliG ₁ strain were avirulent to coral, while the Δ cheA ₁ and Δ cheR ₂ strains were hypervirulent (90 to 100% infection within 14 h on average) compared to the wild-type strain (66% infection within 36 h on average). Additionally, the Δ cheA ₁ and Δ cheR ₂ strains appeared to better colonize coral fragments than the wild-type strain. These results suggest that although chemotaxis may be involved with infection (the Δ cheB ₂ strain was avirulent), a smooth-swimming phenotype is important for bacterial colonization and infection. This study provides valuable insight into understanding V. coralliilyticus pathogenesis and how this pathogen may be transmitted between hosts. IMPORTANCE Corals are responsible for creating the immense structures that are essential to reef ecosystems; unfortunately, pathogens like the bacterium Vibrio coralliilyticus can cause fatal infections of reef-building coral species. However, compared to related human pathogens, the mechanisms by which V. coralliilyticus initiates infections and locates new coral hosts are poorly understood. This study investigated the effects of chemotaxis, the directional swimming in response to chemical signals, and bacterial swimming patterns on infection of the coral Montipora capitata Infection experiments with different mutant strains suggested that a smooth-swimming pattern resulted in hypervirulence. These results demonstrate that the role of chemotaxis in coral infection may not be as straightforward as previously hypothesized and provide valuable insight into V. coralliilyticus pathogenesis.
(Copyright © 2018 American Society for Microbiology.)