Vibrio vulnificus: Review of Mild to Life-threatening Skin Infections 

Vibrio vulnificus is a member of the Vibrio genus. Most Vibrio species are nonpathogenic in humans; however, V vulnificus is one of the pathogenic strains.¹ In Latin, the term vulnificus means “wounding,” and V vulnificus can cause life-threatening infections in patients. The mortality rate of V vulnificus infections is approximately 33% in the United States.²Vibrio vulnificus is a gram-negative bacterium that was first isolated by the Centers for Disease Control and Prevention in 1964 and was given its current name in 1979.^3-6 It has been found in numerous organisms, including oysters, crabs, clams, shrimp, mussels, mullets, and sea bass.⁴ The vast majority of infections in the United States are due to oyster exposure and consumption.^2,7Vibrio vulnificus is responsible for more than 95% of seafood-related deaths in the United States and has the highest mortality rate of all food-borne illness in the United States.^2,5 It also has the highest per-case economic impact of all food-related diseases in the United States.¹

What distinguishes a pathogenic vs nonpathogenic Vibrio isolate remains unknown; Vibrio species rapidly undergo horizontal gene transfer, making DNA isolation difficult.¹ Some characteristics of V vulnificus that may confer virulence are the capsular polysaccharide, lipopolysaccharide, binding proteins, and tissue-degrading enzymes.^1,5 First, encapsulated strains are more virulent and invasive than unencapsulated strains.¹ The mucopolysaccharide capsule protects the bacterium from the immune system, allowing it to evade immune surveillance, cause more severe infection, and invade into the subcutaneous tissue.^3,5 Second, production of sialic acid–like molecules alter the lipopolysaccharide, allowing for motility and biofilm formation.¹ This allows the bacterium to survive in marine waters and within the bloodstream, the latter leading to sepsis in humans. Third, production of N-acetylglucosamine–binding protein A allows for adhesion to chitin. Shellfish consume chitin, and chitin accumulates in shellfish. N-acetylglucosamine–binding protein A also binds mucin; this may be how V vulnificus binds to mucin in the gastrointestinal tract in humans, causing gastroenteritis.¹ Binding to the human mucosae also may allow the bacteria to gain access to the blood supply, leading to septicemia.⁴ Finally, tissue-degrading enzymes such as proteases are responsible for necrotizing wound infections associated with V vulnificus, as the enzymes allow for invasion into the skin and subcutaneous tissues. Proteases also increase vascular permeability and lead to edema.³ Hence, these virulence factors may provide V vulnificus the pathogenicity to cause infection in humans.

Three biotypes of V vulnificus have been discovered. Biotype 1 is the most common and is found worldwide in brackish water.⁸ It can cause the entire spectrum of illnesses, and it has a case fatality rate of 50% in humans. Biotype 1 is presumably responsible for all infections in the United States. Biotype 2 is found in the Far East and Western Europe; it inhabits a unique niche—saltwater used for eel farming. It typically causes infection in eels, but rarely it can cause wound infections in humans. Biotype 3 is found in freshwater fish farming in Israel, and it is a hybrid of biotypes 1 and 2.It can cause severe soft tissue infections in humans, sometimes requiring amputation.⁸

Epidemiology

Vibrio vulnificus is a motile, gram-negative, halophilic, aquatic bacterium.^1,4,5,8,9 It is part of the normal estuarine microbiome and typically is found in warm coastal waters.^1,5,10 The ideal conditions for growth and survival of V vulnificus are water temperatures at 18 °C (64.4 °F) and water salinities between 15 to 25 parts per thousand.^2,8,9 These conditions are found in tropical and subtropical regions.²Vibrio vulnificus is found all over the world, including Denmark, Italy, Japan, Australia, Brazil, and the United States,² where most infections come from oyster exposure and consumption in the Gulf of Mexico.^2,8,11 The incidence of infection in the United States is highest between April and October.^8,11

Some populations are at a higher risk of infection. Risk factors include male sex, liver cirrhosis, hemochromatosis, end-stage renal disease, immunosuppression, and diabetes mellitus.^1,8,11 Healthy patients with no risk factors account for less than 5% of US V vulnificus infections.⁸

Male Predilection
Men are 6 times more likely to be affected by V vulnificus than women.Some hypotheses for this discrepancy are that estrogen is protective againstV vulnificus and that women may be less likely to engage in risky water activities and seafood handling.⁵ Additionally, older males (aged >60 years) are most often affected,^1,8 likely due to the association between increasing age with number of comorbidities, such as diabetes mellitus, heart disease, and chronic disease.⁸

Iron Levels
Iron appears to play an important role in V vulnificus infection. Iron is essential for bacterial growth, and the ability to obtain iron from a host increases the organism’s pathogenicity.³Vibrio vulnificus rapidly grows when transferrin saturation exceeds 70%.⁸ Additionally, iron overload decreases the inoculum needed to cause sepsis in animal studies, which could play a role in human pathogenesis.⁴ Iron levels are elevated in patients with hemochromatosis due to increased iron absorption, cirrhosis and chronic liver disease due to impaired iron metabolism, and end-stage renal disease, especially in patients receiving parenteral iron.⁸