Vibrio

Genus: Vibrio
Diseases
V. cholerae, the major pathogen in this genus, is the cause of cholera. Vibrio
parahaemolyticus causes diarrhea associated with eating raw or improperly cooked
seafood. Vibrio vulnificus causes cellulitis and sepsis.
Important Properties
Vibrios are curved, comma-shaped, gram-negative rods. V. cholera is divided into two groups according to the nature of its O cell wall antigen. Members of the O1 group cause epidemic disease, whereas non-O1 organisms either cause sporadic disease or are nonpathogens. The O1 organisms have two biotypes, called classic and El Tor, and three serotypes, called Ogawa, Inaba, and Hikojima. (Biotypes are based on differences in biochemical reactions, whereas serotypes are based on antigenic differences.) These features are used to characterize isolates in epidemiologic investigations. V. parahaemolyticus and V. vulnificus are marine organisms; they live primarily in the ocean, especially in warm salt water. They are halophilic (i.e., they require a high NaCl concentration to grow).
1. Vibrio cholerae
Pathogenesis
V. cholerae is transmitted by fecal contamination of water and food, primarily from human sources. Human carriers are frequently asymptomatic and include individuals who are either in the incubation period or convalescing. The factors that predispose to epidemics are poor sanitation, malnutrition, overcrowding, and inadequate medical services. The pathogenesis of cholera is dependent on colonization of the small intestine by the organism and secretion of enterotoxin. For colonization to occur, large numbers of bacteria must be ingested because the organism is particularly sensitive to stomach acid. Persons with little or no stomach acid, such as those taking antacids or those who have had gastrectomy, are much more susceptible. Adherence to the cells of the brush border of the gut, which is a requirement for colonization, is related to secretion of the bacterial enzyme mucinase, which dissolves the protective glycoprotein coating over the intestinal cells. After adhering, the organism multiplies and secretes an enterotoxin called choleragen (cholera toxin). This exotoxin can reproduce the symptoms of cholera even in the absence of the Vibrio organisms. Morbidity and death are due to dehydration and electrolyte imbalance. However, if treatment is instituted promptly, the disease runs a self-limited course in up to 7 days.Clinical Findings
Watery diarrhea in large volumes is the hallmark of cholera. There are no red blood cells or white blood cells in the stool. Rice-water stool is the term often applied to the non bloody effluent. There is no abdominal pain, and subsequent symptoms are referable to the marked dehydration. The loss of fluid and electrolytes leads to cardiac and renal failure. Acidosis and hypokalemia also occur as a result of loss of bicarbonate and potassium in the stool. The mortality rate without treatment is 40%.
Laboratory Diagnosis
During an epidemic, a clinical judgment is made and there is little need for the laboratory. a variety of selective media are used in the laboratory. For diagnosis of sporadic cases, a culture of the diarrhea stool containing V. cholerae will show colorless colonies on MacConkey’s agar because lactose is fermented slowly. The organism is oxidase-positive, which distinguishes it from members of the Enterobacteriaceae. A presumptive diagnosis of V. cholerae can be confirmed by agglutination of the organism by polyvalent O1 or non-O1 antiserum. A retrospective diagnosis can be made serologically by detecting a rise in antibody titer in acute- and convalescent phase sera.
Treatment
Treatment consists of prompt, adequate replacement of water and electrolytes, either
orally or intravenously. Glucose is added to the solution to enhance the uptake of
water and electrolytes. Antibiotics such as tetracycline are not necessary, but they do shorten the duration of symptoms and reduce the time of excretion of the organisms.
Prevention
Prevention is achieved mainly by public health measures that ensure a clean water
and food supply. The vaccine, composed of killed organisms, has limited usefulness; it is only 50% effective in preventing disease for 3 to 6 months and does not interrupt transmission. The use of tetracycline for prevention is effective in close contacts but cannot prevent the spread of a major epidemic. Prompt detection of carriers is important in limiting outbreaks.
2. Vibrio parahaemolyticus
V. parahaemolyticus is a marine organism transmitted by ingestion of raw or undercooked seafood. Little is known about its pathogenesis, except that an enterotoxin similar to choleragen is secreted and limited invasion sometimes occurs. The clinical picture caused by V. parahaemolyticus varies from mild to quite severe watery diarrhea,nausea and vomiting, abdominal cramps, and fever. The illness is self-limited, lasting about 3 days. V. parahaemolyticus is distinguished from V. cholerae mainly on the basis of growth in NaCl: V. parahaemolyticus grows in 8% NaCl solution (as befits a marine organism), whereas V. cholerae does not. No specific treatment is indicated, because the disease is relatively mild and self-limited. Disease can be prevented by proper refrigeration and cooking of seafood.
3. Vibrio vulnificus
V. vulnificus is also a marine organism. It causes severe skin and soft tissue infections (cellulitis),especially in shellfish handlers, who often sustain skin wounds. It can also cause a rapidly fatal septicemia in immunocompromised people who have eaten raw shellfish containing the organism. Hemorrhagic bullae in the skin often occur in patients with sepsis caused by V. vulnificus. Chronic liver disease (e.g., cirrhosis) predisposes to severe infections. The recommended treatment is doxycycline.