Cholera Case Study Flashcards
What are the symptoms of cholera?
Profuse watery diarrhoea called rice stools because it looks like the cloudy water after washing rice
Vomiting, fever and severe dehydration
Loss of electrolytes
Death
How is chorea transmitted?
Cholera is a waterborne disease which is endemic in many low income countries
What electrolytes are lost during a cholera infection?
Sodium, potassium, chlorine and bicarbonate
What is the causative agent of cholera?
Cholera is caused by the Gram-negative, rod-shaped bacterium Vibrio cholerae
What is the transmission route for cholera bacteria?
Faecal - oral from infected water or undercooked sea food
Where do the vibrios multiply?
In the alkaline small intestine
They use their flagella to move to the epithelial cells
They produce mucinase which is an enzyme which degrades mucous therefore allowing access for attachment
How is attachment achieved by the vibrio bacteria?
The cholera vibrio bacteria is an A-B structure with 5 B subunits which facilitate attachment and 1 A u it which is the enzyme.
The bacteria does not enter the cell, the cholera toxin is released by the bacteria and taken up by receptor mediated endocytosis.
The B subunits bind to ganglioside receptors GM1
What happens inside the cell once the B subunits have bound to the epithelial cell?
The toxin enters the cell via receptor mediated endocytosis when the B subunit binds to the ganglioside receptor on the cell surface
The A subunit is released in the cell and causes an ADP- ribose molecule to be transferred from a NAD molecule to the alpha subunit of a G protein.
This minding of the ADP - ribose molecule prevents the action of the G protein.
The G protein normally inhibits the production of adenylate cyclase which is cleaved in the cell to produce a secondary messenger called cAMP (cyclic adenosine monophosphate) which regulates the opening of ion channels in the epithelial cell membrane
Chlorine and bicarbonate ions are released through the dysregulated ion channels and sodium is prevents from being taken up by the cell
Potassium also exits the cell due to the action of the B subunits which open the potassium on channels
Water then exits the cell via osmosis due to the increased concentration of ions in the gut epithelium.
How is cholera diagnosed using an antiserum?
Cholera bacterium have h antigens on their flagella
Material containing Bacterial flagella are injected into an animal so that an immune response is initiated. Blood is en taken from the animal which contains h antigen antibodies. When these antibodies are added to a wet sample of this bacteria isolated from a stool sample, the antibodies bind to the h antigens and render the bacteria immobile.
How is cholera diagnosed using a faecal sample?
A faecal sample is placed into an alkaline peptone water broth which allows vibrio cholerae bacteria to grow whilst inhibiting other faecal bacteria. Once incubated the broth is plated onto thiosulphate - citrate - bile salts - sucrose agar
Once colonised there are two indicators used to identify the cholera bacterium:
1 - sucrose and pH indicator. Vibrio bacteria ferment sucrose for energy and produce acid as a result. The area surrounding the bacteria turns the pH indicator from blue to yellow
2 - gas indicator. Vibrio bacteria do not produce hydrogen sulphide gas whereas others do. When the gas is present the bacteria turn black. Cholera bacteria will remain yellow.
How is cholera diagnosed using a pure sample?
Vibrio cholerae bacteria are oxidase positive which means that they have a similar electron transport chain as mitochondria where the end product is cytochrome oxidase. This enzyme oxidises oxidase reagent and turns the sample purple when the oxidase regent is added to the bacterium sample.
This is called the oxidase test
How is cholera diagnosed using the slide agglutination test?
A culture is placed on a slide with antiserum which recognises O1 antigens
If the sample clumps the agglutination has occurred and the O1 antigens have been bound to one another and the bacterium is identified as cholera
If no clumping has occurred the. There are no O1 antigens and the sample is not vibrio cholerae
How are the two bio types of O1 vibrio cholera distinguished?
They are distinguished by their metabolic
activities
1 - their different haemolytic activity,
2 - relative resistance to the antibiotic polymyxin B,
3 - different susceptibilities to bacteriophage.
The two biotypes are called classical and El Tor
The classical biotype is further divided into two serovars (also known as serotypes), based on the antisera that recognise them, and named after the
place where they were rst isolated: Inaba and Ogawa.
Describe the immune response to cholera?
Immune response is initiated in the but associated lymphoid tissue called payers patches
B cells are stimulated to differentiate into plasm cells which produce antibodies
IgM and IgA - both of these bind to the lipopolysaccharides on the bacteria as well as the B subunit of the toxin inactivating it
IgG - confined to the blood
What are the three types of vaccine which have been produced in the prevention of cholera?
Killed whole cell vaccine
Killed whole cell plus subunit vaccine
Live attenuated vaccine
None are cost effective at the moment as more than one dose is usually required
They offer only short lived protection
