Vibrio harveyi

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. 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. Adherence Strains of V. harveyi have been documented to have different pili genes such as, mshB and pilA. Research has suggested that some V. harveyi infections in fish may be the result of an initial C. irritans infection, although the exact nature of the relationship between the two organisms is unclear. It has been hypothesized that V. harveyi might be beneficial to C. irritans by either improving C. irritans pathogenicity or by dysregulating the immune response of the host organism . == Quorum sensing ==

Groups of V. harveyi bacteria communicate by quorum sensing to coordinate the production of bioluminescence and virulence factors. Quorum sensing was first studied in V. fischeri (now Aliivibrio fischeri), a marine bacterium that uses a synthase (LuxI) to produce a species-specific autoinducer (AI) that binds a cognate receptor (LuxR) that regulates changes in expression. Coined "LuxI/R" quorum sensing, these systems have been identified in many other species of Gram-negative bacteria. Despite its relatedness to A. fischeri, V. harveyi lacks a LuxI/R quorum-sensing system, and instead employs a hybrid quorum-sensing circuit, detecting its AI through a membrane-bound histidine kinase and using a phosphorelay to convert information about the population size to changes in gene expression. Since their identification in V. harveyi, such hybrid systems have been identified in many other bacterial species. Qrr RNA molecules are responsible for controlling regulator translation, repressing and promoting factors dependent on cell density. V. harveyi uses a second AI, termed autoinducer-2 or AI-2, which is unusual because it is made and detected by a variety of different bacteria, both Gram-negative and Gram-positive. Thus, V. harveyi has been instrumental to the understanding and appreciation of interspecies bacterial communication. Previous research has characterized this quorum sensing (QS) system in V. harveyi as a "parallel circuit" due the system's architecture where multiple chemical signals are integrated to coordinate the production of bioluminescence. The three-channel sensory architecture V. harveyi utilizes three distinct autoinducers (AIs) and three cognate membrane-bound receptors, all functioning in parallel, in order to channel information into a singular shared regulatory pathway: System 1 (Intraspecies) This system uses HAI-1 (Harveyi-Autoinducer 1), an acyl-homoserine lactone (AHL) produced by LuxM and detected by LuxN. This signal enables communication between V. harveyi members. System 3 (Intrageneric) This system uses CAI-1 (Cholerae-Autoinducer 1), produced by CqsA and detected by the CqsS sensor. This signal is shared among members of the Vibrio genus, thus enabling them to monitor the composition of the surrounding community. For instance, the luciferase enzyme was found to be the responsible catalyst for bioluminescence. Light is only produced in V. harveyi when a reduced flavin mononucleotide (FMNH2) and long-chain aliphatic aldehyde are oxidized in the presence of O2. This oxidation reaction in turn releases energy as blue-green light, with a peak emission near 490 nm. The luxCDABE genes found in the lux Operon encode this system. More specifically, luxA & luxB form the luciferase subunits while luxC, D, and E encode the fatty acid reductase complex responsible for regenerating the aldehyde substrate, ultimately enabling V. harveyi bioluminescence. In vivo insights The "parallel circuit" architecture described in V. harveyi, enabled it to act as a "coincidence detector" where the total concentration of LuxR, thus light intensity, is a result of the synergistic integration of all three signals. Metabolic cost Bioluminescence is energetically expensive, consuming significant amounts of oxygen and reducing power. However, the QS system serves as a powerful evolutionary "switch" by ensuring light is only produced when the population is large enough to be biologically functional, such as in symbiotic or pathogenic interactions. Regulatory breath Beyond light production, this system was also found to regulate other energy-expensive tasks such as metalloprotease production. In vivo dominance In vivo studies, specifically using brine shrimp, have also underlined that AI-2 & CAI-1 are the dominant signals driving QS (and virulence) during infection. Whereas HAI-1 has often been found to have little effect on pathogenicity. Diagnostic tool Bioluminescence also serves as a powerful way for researchers to monitor, in real-time, both when and where bacteria reach a quorum threshold within a host. == Ecology & climate ==

The distribution and survival of V. harveyi are heavily influenced by specific environmental parameters, with recent research highlighting how fluctuating conditions in both natural and aquaculture settings drive the pathogen's prevalence and ability to colonize diverse marine niches. Habitat preference & environmental conditions V. harveyi has been shown to exhibit distinct spatial & temporal dynamics, primarily driven by factors such as water temperature, salinity, and depth. Temperature & pH V. harveyi bacteria seem to exhibit a robust capacity for adaptation across a diverse range of environmental conditions. For instance, while water temperature seems to remain a primary driver of growth, its impact is significantly modulated by pH. Most notably, V. harveyi show optimal fitness at specific temperature-pH combinations, even though their ability to maintain cellular homeostasis remains challenged when both parameters shift simultaneously toward extremes. Metabolic flexibility Under such fluctuating conditions, V. harveyi are capable of adjusting their metabolic rate and protein expression. For instance, statistical investigations seem to indicate that these physiological shifts enable V. harveyi to remain competitive under intense aquaculture environments, despite rapid and significant variability in dissolved oxygen concentrations and nutrient availability. Climate change Recent epidemiological traits of V. harveyi have identified a strong link between global climate change and the expansion of the pathogen's geographic range, and subsequent virulence. Warming oceans The steady increase in global sea surface temperatures has been directly associated with the increased frequency of V. harveyi outbreaks. Due to the thermophilic nature of these bacteria, warming waters appear to provide a larger "thermal window" for their growth, thus enabling them to persist and reach QS thresholds in regions that had previously been too cold. Epidemiological shifts Subsequently, climate change has been altering the epidemiology of V. harveyi infections by creating ideal "bloom conditions". For instance, extreme weather events that alter coastal salinity, such as droughts or heavy rainfall, combined with the global rise in sea surface temperatures, seem to facilitate the spread of V. harveyi into new latitudes that had previously been unsuitable for their growth. This has led to an increased risk of vibriosis in both wild marine ecosystems as well as commercial fisheries. == Impact & management ==

Aquaculture and marine organisms diseases Many studies show that V. harveyi is associated with multiple diseases in wild and farmed species with species living in warm water environments becoming more vulnerable. White Syndrome includes multiple diseases such as 'White Band', 'White Plague', and 'Shut Down Reaction'. Prevention and control Given the widespread impacts of V. harveyi on marine and aquaculture organisms, several control alternatives have been explored. Vaccines Various vaccines have been developed and largely marketed to control the shrimp and fish disease associated with V. harveyi. This includes whole-cell vaccines in which injection into barramundi fish (Lates calcarifer) has succeeded in making this species produce antibodies. Lastly, a DNA vaccine also has been developed and results in a wider range of RPSs. == References ==