Vibrio vulnificus is a species of Gram-negative, motile, curved rod-shaped (vibrio), pathogenic bacteria of the genus Vibrio. Present in marine environments such as estuaries, brackish ponds, or coastal areas, V. vulnificus is related to V. cholerae, the causative agent of cholera. At least one strain of V. vulnificus is bioluminescent. Increasing seasonal ocean temperatures and low-salt marine environments like estuaries favor a greater concentration of Vibrio within filter-feeding shellfish; V. vulnificus infections in the Eastern United States have increased eightfold from 1988 to 2018. Infection with V. vulnificus leads to rapidly expanding skin infections by entering a wound causing cellulitis or even sepsis. V. vulnificus is also a source of foodborne illness. It was first isolated as a source of disease in 1976.
Strains Vibrio vulnificus is a species of gram-negative, motile, curved rod-shaped (bacillus), pathogenic bacteria of the genus Vibrio. Present in marine environments such as estuaries, brackish ponds, or coastal areas, V. vulnificus is related to V. cholerae, the causative agent of cholera. The most harmful strains of V. vulnificus documented have been observed in three different forms. The first is when there is a layer of protective sugar molecules surrounding the bacteria called an anti-phagocytic polysaccharide capsule. By encapsulating the bacteria, phagocytosis and opsonization can not occur, thus allowing the bacteria to continue throughout the organism it is in. The second way that V. vulnificus has been most harmful is with some of the toxins that it creates. These toxins are not part of the infection that V. vulnificus causes but instead they are part of a secondary infection in the gastrointestinal tract that most certainly will lead to systemic infection. Lastly, V. vulnificus has been seen to cause more harm in patients who have higher levels of iron.
Genome size The genome size of V. vulnificus is approximately 5.3 Mbp. The genome is organized into two circular replicons, similar to that of V. cholerae, which also has a large and a small chromosome. However, the genome of V. vulnificus is at least one megabase bigger.
Natural transformation Natural transformation is a bacterial adaptation for DNA transfer between individual cells. V. vulnificus was found to become naturally transformable during growth on chitin in the form of crab shells. The ability to now carry out transformation experiments in the laboratory should facilitate molecular genetic analysis of this opportunistic pathogen.
Biotypes and genotypes V. vulnificus has at least 3 biotypes. Biotype 1, which was the first to be isolated, is the biotype responsible for virtually all human infections. Within biotype 1, two distinct genotypes have been isolated, the C-genotype (clinical) and the E-genotype (environmental). The C-genotype is associated with human clinical cases, and carries specific genes that increase virulence and resistance to serum killing. The E-genotype has been isolated from oysters and water, and possesses unique genes associated with enhanced environmental survival. This discovery has led to the proposal that V. vulnificus exists as two divergent ecotypes. Biotype 2, which has rarely been isolated in humans, primarily infects farm-raised eels, causing a fatal septicemia. Biotype 3 is thought to be a hybrid of biotypes 1 and 2, and has only been isolated in human wound infections after an outbreak in an Israeli tilapia aquaculture in 1996.
Pathogenesis
Capsule V. vulnificus has a capsule, made of polysaccharides, and is thought to protect against phagocytosis. The capsule also aids the bacteria in escaping opsonization. Different strains of the bacteria are capable of shifting through the unencapsulated and encapsulated forms. Mouse models have shown that the unencapsulated forms are avirulent. These same strains, however, are more likely to be in their encapsulated form when taken up by oysters. Varying levels of oxygen determine the amount of capsular production. Oxygen levels increase the amount of capsule the bacteria can make. Out of the two genotypes the strain commonly found in the environment showed a higher level of capsular production than the one found in human infections. In low oxygen conditions the capsule appears thin and translucent, while it is normally supposed to be thicker and more opaque.
Biofilm V. vulnificus creates less biofilm under anaerobic conditions, when in most bacteria the opposite is the case. Genotypes that are found more in the environment show this correlation more than the genotypes found in human infection. Environmental strains such as temperature change also play a role in the formation of biofilms, with lower temperatures increasing the production. Strains found in human infection showed more biofilm formation at temperatures of 24 °C than environmental strains, showing these strains are adapted for their environments.
Endotoxin Like all gram-negative bacteria, V. vulnificus has LPS (lipopolysaccharide) as the major component of its outer membrane. However, the LPS the bacteria produces isn't as efficient at triggering the immune system's release of tumor necrosis factor (TNF) alpha and other cytokines that produce shock syndromes. The capsular proteins the bacteria express, however, are capable of producing an immune response contributing to shock syndrome.
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