Vibrio
Vibrio is a genus of gram-negative bacteria commonly found in aquatic environments, including both saltwater and freshwater, and is known for its distinctive curved rod shape and motility due to a polar flagellum. The most well-known species, Vibrio cholerae, is the causative agent of cholera, a severe gastrointestinal disease characterized by profuse watery diarrhea, often referred to as "rice water diarrhea." Cholera primarily affects humans, attaching to the intestinal lining and releasing toxins that cause significant fluid loss, leading to rapid dehydration and potentially high mortality rates during outbreaks. Other pathogenic species include Vibrio parahaemolyticus, which can cause gastroenteritis from consuming infected seafood, and Vibrio vulnificus, associated with wound infections and consumption of raw oysters, particularly in warmer coastal waters.
Vibrio bacteria can be transmitted through direct contact with contaminated water or through ingestion of contaminated shellfish. While many infections are self-limiting with adequate hydration, antibiotic treatment may be necessary in severe cases, particularly for wound infections or bacteremia. Resistance to common antibiotics has been observed, making some infections more challenging to treat. Understanding the characteristics, transmission routes, and clinical significance of Vibrio is crucial for prevention and management, particularly in regions where these infections are endemic.
Vibrio
- TRANSMISSION ROUTE: Direct contact, ingestion
Definition
Vibrio, one of the most common forms of bacteria in the Earth's surface waters, is a motile aerobic rod that causes the human disease cholera, other forms of gastroenteritis, and some extraintestinal infections.
Natural Habitat and Features
Vibrios are gram-negative bacteria with a curved rod shape. They are aerobic and motile, with a polar flagellum. Vibrios are common in salt water and fresh water around the world. The bacteria can be carried by numerous animals that live in the sea and may be ingested through human consumption of crabs, clams, and oysters. V. cholerae is the most medically important species, as it causes the disease cholera, which is endemic to India and Southeast Asia.
Vibrios produce smooth, rounded colonies that are opaque when exposed to light. They grow well at 98.6° Fahrenheit (37° Celsius) on thiosulfate-citrate-bile-sucrose agar and on many other media. When cultured over time, the curved rods may become straight and resemble gram-negative intestinal bacteria. Cholerae grows rapidly on blood agar with a pH (acidity) level near 9. Typical colonies are identifiable in about eighteen hours.
Vibrios are differentiated from other intestinal gram-negative bacteria by being oxidase positive. A positive oxidase test is important in the preliminary identification of cholerae and other vibrios. They grow well in high pH but are rapidly killed by acid. For this reason, any condition or medication that decreases stomach acidity may predispose a person to infection.
Antigenic structures of vibrios include a single heat-labile flagellar H antigen. Cholerae has O lipopolysaccharides that define serologic specificity. (A minimum of 139 O-antigen groups exist.) Antibodies to O antigens may protect some animals from infection. Enterotoxins produced by cholerae can cause prolonged hypersecretion of water and electrolytes in humans, causing profuse diarrhea. Cholera enterotoxin may stimulate the production of neutralizing antibodies. Although an attack of cholera may be followed by immunity, the duration and degree are unpredictable.
Pathogenicity and Clinical Significance
Cholerae is pathogenic only in humans. It is not an invasive infection. It remains in the intestinal tract attached to the microvilli of intestinal epithelial cells, where it releases toxins but does not enter the bloodstream. Up to 75 percent of infections may be asymptomatic. The development of symptoms depends on the size of the inoculum. When symptoms do occur, they follow an incubation period of one to four days. Typically, symptoms develop suddenly and may include cramping, abdominal pain, nausea, vomiting, and profuse diarrhea. The term “rice water diarrhea” is used to describe the stools, which contain copious mucus, epithelial cells, and large numbers of vibrios. Diarrhea and vomiting lead to rapid depletion of fluid and electrolytes.
Untreated cholera may result in profound dehydration, anuria, circulatory collapse, and death. Mortality rates as high as 50 percent may be seen during an epidemic. Diagnosing epidemic or endemic cholera is not difficult, but sporadic or isolated cases may be confused with other causes of gastroenteritis.
Other vibrios that cause human disease include parahaemolyticus, which causes gastroenteritis following ingestion of Vibrio-infected seafood. Incubation is twelve to twenty-four hours, which is followed by nausea, vomiting, fever, and bloody diarrhea. The symptoms usually subside without treatment in one to four days. Parahaemolyticus does not produce a toxin. It is present worldwide and may infect humans who eat raw seafood.
Vulnificus is a free-living organism found in ocean estuaries worldwide. In the United States, vulnificus is found predominantly along the Gulf Coast. People swimming in these waters with an open wound may become infected, and the infection may cause sepsis. Vulnificus may be found in oysters during warm months and cause gastroenteritis if eaten raw. Although wound infections may be mild, vulnificus may have a mortality rate as high as 50 percent if sepsis develops. Other vibrios that may cause diarrhea include mimicus, hollisae, and fluvialis. Other vibrios that cause wound infections include damsela and alginolyticus.
Drug Susceptibility
Vibrio gastroenteritis is usually self-limited, and most people will recover as long as adequate hydration and nutrition are available. Most vibrios are sensitive to antibiotics, but antibiotic therapy may not shorten the course of intestinal illness. Antibiotics are more important in vibrio wound infection or bacteremia. In these cases, intensive medical therapy, including intravenous antibiotics, management of septic shock, and aggressive surgical debridement, may be needed.
Antibiotics that are effective against cholera include tetracyclines, such as doxycycline, and fluoroquinolones, such as ciprofloxacin, although there has been increasing resistance to tetracyclines. Azithromycin has also shown high effectiveness. In wound infections and bacteremia caused by non-cholera Vibrio species, the combination of doxycycline, ceftazidime, and a broad-spectrum type of penicillin, such as ticarcillin, is the treatment of choice. Many Vibrio species have developed resistance to commonly used antibiotics. Vibrios have been found to be susceptible to several novel antibiotics, such as tigecycline, daptomycin, and linezolid.
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