Vibrio Cholerae (Student Notes)

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Vibrio is a genus consisting of bacteria that are closely related to Enterobacteriaceae. The cholera species are the most important species, and they are short gram-negative curved rods. 

They are facultative anaerobic bacteria brackish rivers and coastal waters. Especially in sea organisms, such as crustaceans, planktons, and protozoa. Vibrio Cholerae is motile with the use of their single polar flagellum. (1)

Laboratory features

The bacterium can be cultivated on a selective Thiosulfate-citrate-bile-sucrose (TCBS) agar. This medium contains sodium thiosulfate and sodium citrate, which inhibits the growth of Enterobacteriaceae. It also contains bile salts, which inhibit the growth of gram + bacteria. This makes the medium selective.

Sugar and protein nutrients are provided by saccharose and peptone, while Bromothymol blue is added as an indicator of pH change. When cultivated Vibrio Cholera will ferment the carbohydrates, producing acid, which converts the color of the indicator yellow which will also stain the colonies yellow. Therefore, colonies on the medium can be seen as convex, smooth, round, yellow colonies. (2)

Of its biochemical properties, VC is oxidase-positive, which is important as it differentiates it from the related Enterobacteriaceae. They possess both O and H antigens, however, only the O antigens are useful for distinguishing between serogroups.

Of the numerous serogroups, serogroups O1 and O139 are significant, as these are responsible for outbreaks of epidemic cholera. Furthermore, serogroup 01 has 2 biotypes, classic, and El tor. The El tor serogroup is associated with the production of hemolysins, longer survival in water as well as a higher carriage rate. (1)

Virulence factors

The most important virulence factors of Vibrio cholerae are their adhesive pili and their ability to produce the cholera toxin. These result in the bacterium being able to colonize the human gastrointestinal tract, inducing hypersecretion from the intestinal epithelial cells which result in watery diarrhea

The adhesive pili are known as the Toxin Coregulated Pilus (TCP). They facilitate binding to GI epithelial cells. These pili are expressed coordinately along with the cholera toxin.

The cholera toxin is a protein toxin composed of 2 subunits, A and B. The B unit facilitates transport into the gastrointestinal epithelial cell. Once inside, the A subunit catalyzes a reaction that activates membrane-bound Gs G-proteins. This leads to the activation of adenylyl-cyclase which catalyzes the formation of cAMP. The Increased intracellular cAMP levels will result in hypersecretion of ions and water by the epithelial cells, which in turn results in watery diarrhea. (3)

Disease/Infection

Vibrio Cholerae is contracted by ingestion of contaminated water or seafood, after which it can be transmitted by the fecal-oral route, usually through contaminated water. Cholera is known to cause epidemics, most frequently in areas with poor sanitation, in which drinking water can become contaminated from the feces of infected individuals. 

While the frequency and scale of epidemics have decreased dramatically in the last century. Epidemics can still occur after natural disasters or in conflict areas, as the water sanitation dramatically decreases. One recent example is the outbreak that occurred after the Haiti earthquake in 2010.

Once ingested, the bacterium can colonize the intestines. Typically, the incubation period ranges from a few hours to a few days. Once colonized, individuals can develop cholera, which is characterized by watery diarrhea with a typical appearance known as rice water diarrhea. 

The infection itself is not fatal, but the heavy loss of water and electrolytes can eventually lead to dehydration and hypovolemic shock, which can result in multiorgan failure and death.

In full-blown cholera infections, often caused by epidemic serogroups O1 and O139, infected patients can lose between 15-20 L of fluid a day, and if untreated, has a mortality rate of over 50% as patients are unable to replenish the water lost.

Other serogroups often cause a milder type of cholera, which is similar to the traveler’s diarrhea observed with certain E.Coli infections. (1)(3)(4)

Diagnosis

Vibrio cholerae can be sampled from the stool of infected individuals as well as from infected water/food sources. After sampling, it can be inoculated and grown on standard media, as well as on the selective TCBS agar. 

After cultivation, microscopic observation and serological tests can confirm diagnosis. The bacterium can also be visualised directly after sampling using a dark-field microscope under which its rapid motility can also be seen.

As mentioned, the Vibrio cholerae is oxidase positive. This is of importance as it enables us to differentiate it from members of the Enterobacteriaceae family in the lab.

Of even greater importance is slide agglutination tests. This enables us to detect the presence of the bacterium as well as serological identification of specific serotypes. This is enabled by the test serum containing antibodies against specific antigens which are then added to the sample with the bacterium on the test slide. If agglutination is seen on the slide, the test is positive. These tests are important for differentiation between pandemic serotypes O1 and O139. (1)(3)(4)

Treatment

Treatment of VC infections generally only requires fluid and electrolyte replacement which must be initiated as soon as possible, even before bacteriologic diagnosis. Due to heavy fluid loss, patients often require IV fluid replenishment.

Milder forms of cholera infections are usually self-limiting, however, more severe infections might require treatment with antibiotics. For such infections, doxycycline is the antibiotic of choice.

The most important measures against cholera outbreaks are public health measures to improve water sanitation, which reduces the risk for fecal-oral contamination of water supplies and food. Also, adequate cooking of seafood in areas known to have cholera is important to prevent outbreaks in the first place.

There is a vaccine available against Vibrio cholera, however, it is only modestly effective (around 50%) and only gives protection for about 6 months. (1)(3)(4)

References