What is vibrio?

Definition

Vibrio, among the most common forms of bacteria of
the surface waters of the earth, 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 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 is 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 60 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. Diagnosis of epidemic or endemic cholera is not
difficult, but sporadic or isolated cases may be confused with other causes of
gastroenteritis.

Other vibrios causing 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 diarrhea that may be bloody. 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 may
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 in 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 tetracycline and
ciprofloxacin. In wound infections and bacteremia caused by noncholera
Vibrio species, the combination of doxycycline, ceftazidime,
and a broad-spectrum type 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.

Bibliography

Brooks, George F. and Carroll, Karen C. Jawetz, Melnick, and Adelberg’s Medical Microbiology. 25th ed. New York: McGraw-Hill, 2010. Chapter 17 of this textbook gives an excellent introduction to the vibrios including their morphology, identification, antigenic structure, and growth characteristics.

Fauci, Anthony, et al., eds. Harrison’s Principles of Internal Medicine. 17th ed. New York: McGraw-Hill, 2008. Chapter 149 covers cholera and other vibrio infections. Includes a good discussion of vibrio infections other than cholera gastroenteritis.

Ho, Hoi. “Vibrio Infections.” Available at http://emedicine.medscape.com/article/232038-overview. Examines vibrio infection, including aggressive treatment of noncholera wound infection and bacteremia.

Vaseeharan, B., et al. “In Vitro Susceptibility of Antibiotics Against Vibrio spp. and Aeromonas spp. Isolated from Penaeus monodon Hatcheries and Ponds.” Journal of Antimicrobial Agents 26 (2005): 285-291. Provides a good background on antibiotic susceptibilities of Vibrio species and the emergence of resistant strains.