Phytoplankton Promote V. parahaemolyticus Persistence

Vibrio parahaemolyticus, is another malignant Vibrio, causing acute gastroenteritis (and occasionally death) if shellfish in which it has accumulated is under cooked or improperly handled. Ubiquitous in marine waters and distributed worldwide it causes an estimated 4,500 cases in the USA annually. The bacterium has a highly seasonal life-style in brackish coastal waters where it is most common. Increasing in abundance during warm summer months but effectively vanishing in winter by entering a viable but non-culturable (VBNC) state. Like other Vibrio species, it possess a particular affinity for chitin and attaches to chitin-based surfaces such as zooplankton exoskeletons. Chitin is also produced by phytoplankton such as diatoms and is a structural component of their cell walls. Given that increases in diatom abundance have been associated with increased in Vibrio spp. abundances it is possible that adherence to chitin compounds is a mechanisms for Vibrio parahaemolyticus persistence along with VBNC. Frischkorn et al. examined this, firstly by assessing the role that type IV pili pay in attachment to chitin and then examining the association with a real diatom species.

The role in attachment of two type IV pilins was assessed, mannose-sensitive haemagglutinin (MsHA) and chiten-regulated (PilA) pilus. These mediate biofilm formation and cell-cell binding respectively. Additionally the role of GlcNAc binding protein A (GbpA) which facilitates adherence to chitin in V. cholera was examined. Knockouts were created for each gene and biofilm formation on polystyrene and attachment to chitin beads was assayed. Knockouts showed both reduced biofilm formation and attachment compared to wild type bacteria, with double knockouts for both pilins doing worst. Whereas GbpA knockouts were not as affected. This indicated that the pilins act synergistically to facilitate attachment and are more essential for attachment than GbpA. Adherence to the diatom Thalassiosira weissflogii, which produces chitin containing fibrils, was then tested. Attachment to the diatom was assayed throughout the growth curve and increased from the early stationary phase though to death phase. Bacteria were also observed attached to free floating fibrils shed by the diatom. T. weissflogii is known to increase its chitin production during these phases as structural nutrients become starved. Attachment to non-chitinous products was ruled out and knockouts showed that the two type IV pili are important for binding to the diatoms chitin. These results highlight the ability of V. parahaemolyticus to associate with an abundant diatom species during period of the growth phase analogous to a bloom collapse. Potentially providing a method of outbreak prediction by monitoring the dynamics of diatom species, which are relatively easy to identify. This is particularly important in the Pacific Northwest as remote sensing data is currently not good enough to deal with the intricate coastlines and high cloud cover.

I thought this study was excellent, being both highly applied and extremely interesting. A number of future avenues of research spring to mind. Is direct attachment of the bacterium observed in field samples and is the association consistent over other chitin producing diatoms? Overall, I think the study shows that numerous different chitin producing taxa are important in the ecology of Vibrios. Given this, I would not be surprised if associations with other chitin producers e.g. fungi are present. More broadly, the study highlights the link between science for the sake of it and that of an applied ilk. This study is about a public health issue, but none of it would have been possible without an excellent of both the pathogen and the diatom species. This clearly shows that you cannot have one without the other, both are interconnected and to ignore blue skies would be of serious detriment to real world applications. 



Reference: Frischkorn, K.R., Stohanovshi, A. and Paranjpye, R. (2013). Vibrio parahaemolyticus type IV pili mediate interactions with diatom-derived chitin and point to an unexplored mechanism of environmental persistence. Environmental Microbiology, 15, 1416-1427.