The gram-negative marine bacterium Vibrio parahaemolyticus commonly accumulates in filter feeding shellfish and can cause gastroenteritis in humans when the shellfish are ingested. V. parahaemolyticus is often isolated from coastal brackish waters where it occurs in higher abundance during the summer months. V. parahaemolyticus, like many other species within this genus, are not culturable when the environmental conditions prevent optimal growth (VBNC state).
Entering the VBNC state was previously assumed to be the main mechanism by which the bacteria persist in the environment until their attachment to plankton or other substrate has been identified as another mechanism for overwintering in the environment. The adherence to substances of many other Vibrio sp. is reliant on pili such as type IV pili and in V. vulnificus, type IV have been found to enable the bacteria to persist during seasons by binding to substrates. V. parahaemolyticus has genes for two types of IV pilins; one of which is involved in attachment to surfaces and biofilm formation (MshA), whereas the other pili enhances cell-to-cell interactions and connection one cell to another (PilA). PilA expression was found up-regulated when the bacteria were exposed to chitin rich conditions. As mentioned in recent blog posts, Vibrio sp. are often associated with chitinous zooplankton, and this study was one of the first to investigate the interactions of diatom-derived chitin and the efficiency of type IV pili adhering V. parahaemolyticus to substrates.
Bioassays were used to measure the ability of V. parahaemolyticus to form biofilms, as well as the adherence of the bacteria to the diatom Thalassiosira weissflogii. Results showed that the bacteria were able to attach themselves to the diatom-derived chitin, and additionally, adherence increased when the diatoms entered the stationary and lag phases of growth. For testing the importance of the two above mentioned genes for the type IV pili, genetic mutants were created and it was shown that mutations of the genes were less effective at attaching the bacteria to the diatoms compared to the wild types.
In summary, this study pointed out the importance of diatoms for the attachment and persistence of V. parahaemolyticus in the environment in addition to prior findings of zooplankton adherence and the VBNC state. Results could be of significant importance for the prediction of dangerous accumulations of the human pathogen in shellfish by monitoring algal blooms as findings gave evidence for an increased accumulation of V. parahaemolyticus at a certain abundance of the algae. The adherence of the bacteria was shown to be dependent on the type IV pili and the ability to attach to chitin excreted by diatoms could be an important factor enhancing their persistence in the environment.
Frischkorn et al. (2013).Vibrio parahaemolyticus type lV pili mediate interactions with diatom-derived chitin and point to an unexplored mechanism of environmental persistence. Environmental Microbiology 15(5), pp. 1416-1427.