Vibrio vulnificus (Vv) inhabits coastal marine environments worldwide and has been isolated from several sources, including fish, seafood, water among many others. Its geographic distribution is increasing due to global warming. Vv is linked to fish mucosae from wild and farmed fish.
Vv is the only species of the Vibrio genus with true zoonotic potential. It is the food-borne human pathogen of the highest mortality rate and also causes in fish (Anguilla anguilla is the most susceptible host) a hemorrhagic septicemia called fish vibriosis. Therefore, it is a species of interest for both WHO (World Health Organization) and aquaculture.
In humans, it is a secondary pathogen and can be introduced to the body by raw seafood intake (causing diarrhea) and skin wounds (which causes an acute infection). In risk patients (mainly those with high iron levels in the blood), the bacteria can get through the blood and develop septicemia, leading the host to death despite the entrance route.
Human and animal vibriosis have several things in common:
- An iron dependence (mainly in humans)
- The disease is transmitted in the same way (ingestión or contact)
- The most severe form is a hemorrhagic septicemia with a high probability of death in less than 24 hours
Lyfe cycle and iron
- Resuscitation and induction of the viable but nonculturable state (VBNC). As a free-living form, the pathogen swifts between a VBNC and a vegetative state depending on nutrient availability as well as on water temperature and salinity.
- Capsule and flagellum production in the environment. Vegetative bacteria produce a capsule and a polar flagellum when iron, and probably other nutrients, are available. The flagellum production is also controlled by temperature.
- Host colonization. Motile/nonmotile bacteria could be attracted by blood/mucus (chemotaxis) from their susceptible hosts (eels and humans with high iron levels in the blood) and colonize a wound or fish mucus. Alternatively, bacteria can be uptaken by filtering organisms and infect humans by ingestion and colonize the intestine or can infect humans by diseased fish handling.
- Septicemia. From the wound or mucosal tissue, the pathogen arrives at the bloodstream; in case of humans with high iron levels, the pathogen produces a capsule, multiplies and secretes the toxins VvhA and RtxA1 that cause the death by a toxic sepsis; in case of an eel, only the cells with the plasmid produce two iron-regulated outer membrane proteins, Fpcrp (fish phagocytosis complement resistance protein) and Ftbp (fish transferrin binding protein) that protects against innate immunity (in addition to an envelope enriched in O-antigen), multiply and secrete VvhA, which lyses erythrocytes, increases iron levels and, indirectly, actives the production of RtxA1, which causes the death of the fish by a toxic sepsis.
- Shedding bacteria to water. Diseased fish liberate bacteria to water. If water is rich in iron, bacteria can infect humans (zoonosis).
- Biofilm formation and dispersion. Bacteria could be attached to surfaces (including fish mucosae) and to form biofilms under iron restriction. Under iron excess, bacteria will be dispersed from the biofilms as capsulated motile bacteria.
Using Next Generation Sequence, we were able to get some new genomes of Vv. With these genomes and those present in databases (such as NCBI) we performed a phylogenomic analysis to unravel the evolutionary process in the species. In order to achieve that, we got a core genome for the species. We found out that the species is subdivided into 5 phylogenetic lineages plus one pathovar (piscis) (Roig et al., 2018), which invalidates the subdivision into biotypes.
The piscis pathovar is defined by a virulence plasmid for fish transmitted by conjugation in the fish farming environment (Lee et al., 2008). It includes a zoonotic clonal complex that is distributed worldwide (Sanjuan et al., 2011) Eel mucosae is an ideal environment for horizontal gene transfer (HGT) (Carda-Dieguez et al., 2018)
This plasmid contains two genes that encode resistance to fish innate immunity (Pajuelo et al., 2015 y Hernández et al., 2019). The fcprp gene encodes an outer membrane lipoprotein that confers resistance to fish complement and phagocytosis. The gene ftbp encodes a receptor for fish transferrin that enables the bacterium to resist and multiply under the iron restriction conditions imposed by the host immunity (nutritional immunity)
Analyzing the genes that conform the core genome, we observed that many virulence genes (e.g., rtxA1, vvhA among others) are present in every strain, so all strains might be treated as potentially virulent. Nevertheless, there are several polymorphisms in specific genes that allow the detection of those strains dangerous in Public Health (Roig et al., 2009) There are also several plasmid and chromosomal genes specific for the zoonotic clonal complex and/or pathovar piscis that can be used to identify the strains that are dangerous for public (targeted gene pilF) (Roig et al., 2009 y Baker Austin et al., 2011) and animal (ftrp) health (Sanjuan y Amaro, 2007).
One of the main virulence factors is the toxin RtxA1, encoded in the zoonotic strains both in plasmid and chromosome 2. Both copies of rtxA1 are identical (Roig et al., 2011) The toxin in the zoonotic strains is called RtxA13. The toxin is related to an early cytokine storm in mice (model to test human vibriosis) and hypothetically in eels (model to test fish vibriosis) (Murciano et al., 2016).
We are interested in unraveling the mechanisms by which the pathogen is able to cause disease in hosts as evolutionary distant as humans and fish/seafood.