Vibrio harveyi is a Gram-negative, bioluminescent, marine bacterium in the genus Vibrio that was first discovered by Johnson and Shunk in their 1963 publication "An interesting new species of luminous bacteria". V. harveyi is rod-shaped, motile (via polar flagella), facultatively anaerobic, halophilic, and competent for both fermentative and respiratory metabolism. It is typically found in aquatic ecosystems — particularly in warmer tropical waters (optimum growth: 30° to 35 °C) — as a free-living bacterium. However, V. harveyi can also live commensally with other marine life, form biofilms on marine surfaces, and act as a pathogen in organisms like coral and oysters. This bacterium is tolerant to fluxes in environmental conditions, a characteristic attributed to its rapid adaptation caused by mutations and Horizontal Gene Transfer. This tolerance allows V. harveyi to thrive in environments affected by climate change. Certain strains of V. harveyi can cause disease in marine life, such as luminous vibriosis — a disease that causes commercially farmed penaeid prawns to glow in the dark. The pathogenicity of these strains can be enhanced in environments affected by climate change, primarily due to the weakening of marine hosts. Despite its harmful effects, V. harveyi plays a beneficial role in nutrient cycling by using chitin — a building block of marine invertebrate exoskeletons — as a carbon source. It does so by breaking chitin down into simpler molecules that are used and eventually returned to the aquatic system to be taken up by other organisms. This species is also thought to be the cause of the milky seas effect, in which, a uniform blue glow is emitted from seawater during the night. Some glows can cover nearly 6,000 sq mi (16,000 km²).
Genome & taxonomy
Phylogeny
Taxonomy classification Vibrio harveyi belongs to the bacterial kingdom under the Pseudomonadota phylum and is classed as a Gammaproteobacteria. More specifically, it belongs to the Vibrionaceae family and therefore the Vibrio genus. It also belongs to the Harveyi clade alongside Vibrio campbellii, Vibrio natriegens, Vibrio alginolyticus, and Vibrio parahaemolyticus.
Relations to other Vibrio species Although closely related to V. campbellii — with a DNA similarity of 61% to 74% — V. harveyi has also been found to contain similar genes to other bacteria outside its clade such as Vibrio cholerae, specifically thought to have occurred through Horizontal Gene Transfer. This was hypothesized after ToxR, a regulator for the cholera toxin gene, was found to exist within all Vibrio species within the Harveyi clade.
Genome structure The genomes of five different strains of V. harveyi — ATCC 33843, FDAARGOS_107, QT520, WXL345, and WXL538 — were sequences and analyzed, finding the following information:
Genome size The genome size of the five different strains of V. harveyi ranges from 5.88 to 6.18 Mb in length.
G-C bases in DNA Out of all the DNA sequenced in the five different strains of V. harveyi, the percent of G-C bases was found to be anywhere from 44.75% to 45.05%.
Large circular chromosome (Chr1) This chromosome contains the genes that encode for cellular components that handle the maintenance of everyday functions, such as ribosomal proteins.
Small circular chromosome (Chr2) This chromosome contains the genes that encode for more specialized functions such as antibiotic resistance, pathogenicity, and adaptation.
Plasmids Several V. harveyi isolates were analyzed with long-read sequencing resulting in the identification of different kinds of plasmids existing within the isolates.
Plasmid use
These plasmids act as storage for important genes V. harveyi can use to increase its pathogenicity, antibiotic-resistance, and adaptation. They also play a major role in Horizontal Gene Transfer, meaning that individuals of a strain are able to transfer genes and gain genes from individuals of another strain, by a process called conjugation. This exchange of traits from other bacteria allows V. harveyi to adapt rapidly as it does not have to solely wait for evolution or mutation to gain access to new traits like many other species.
Discovered effects of plasmids in V. harveyi Two isolates of V. harveyi — Vh-14 and Vh-15 — were discovered to be completely (100%) lethal towards Barramundi fish due to the unique characteristics of their plasmids. Both strains possessed a large conjugative plasmid made up of ~105,412 base pairs that was found to carry major virulence genes such as Type III Secretion System genes. The lethality of these isolates is in part due to the size of their plasmids. The size allowed them to carry more genes specialized in pathogenicity, effectively increasing the pathogenic capabilities of the isolates.
Virulence factors & pathogenicity
Virulence factors
Exotoxins V. harveyi is able to produce a variety of exotoxins that can degrade host tissues and fluids, especially during the exponential growth phase of the bacteria. Some of these exotoxins include lipases, phospholipases, and hemolysins. V. harveyi also produces several types of proteases which break down peptides within the host. There is one cysteine protease that is the main driver of lethality in giant tiger prawns as it prevents the prawns' hemolymph from clotting. Several types of chitinase enzymes like chitobiase are also produced by V. harveyi in order to expose host crustaceans to further degradation. V. harveyi that has grown on chitin will typically express more chitinase enzymes. The composition of the chitin subunits itself will influence the type of chitinases expressed as well.
Iron acquisition V. harveyi is able to acquire iron by producing iron chelating agents and they are mostly used to acquire iron from the bodily fluids of vertebrate hosts. Specific strains of V. harveyi has also been noted to have iron transport proteins sitA, sitB, sitC, and sitD which have not been previously seen in any other Vibrio species.
Adherence Strains of V. harveyi have been documented to have different pili genes such as, mshB and pilA. These pili are used to adhere to surfaces on hosts that might otherwise be difficult to attach to, like mucosa.
Biofilm formation LuxR is a gene found in V. harveyi that plays a role in quorum sensing. This can allow V. harveyi to form biofilms that are resistant to certain antimicrobial compounds and coordinate the activation of virulence genes once enough bacteria have been established in the host organism.
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