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.
I didn't know diatoms produced chitin; according to one paper, chitin synthesis is possibly very widespread in diatom groups. Diatoms form long chitin fibres which protrude from their cell walls and they seem to up regulate chitin synthesis when silica or nutrient levels are low. I suppose maybe they use chitin as a back up cell wall component during stressful times; so peak diatom chitin availability to V. parahaemolyticus must coincide with conditions which would induce VBNC states.
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