block 8- cholera Flashcards
(30 cards)
cholera
- common waterbourne disease
- Cholera is an acute diarrhoeal infection caused by ingestion of food or
water contaminated with the bacterium Vibrio cholerae.
-Provision of safe water and sanitation is critical in reducing the impact of
cholera and other waterborne diseases. so is a problem in poorer countrys
the genus vibrio
-Gram-negative straight or curved rods, motile
by means of a single polar flagellum
-Vibrios are aquatic organisms, they occur in both marine and fresh water
habitats. Are capable of both respiratory and fermentative metabolism
-V. cholerae, V. parahaemolyticus and V. vulnificus are pathogens of humans
vibrio cholerae
-a severe
diarrheol disease caused by the bacterium Vibrio cholerae.
-The Infectious Dose- ID50 varies depending on the pH of the stomach. In healthy
volunteers 108 bacteria produces infection, after neutralization of stomach acid 104 bacteria can cause disease( basically need a very high number of the bacterium to get the disease which lowers when the stomach is neutralized)
-The small intestine is the primary site of infection. V. cholerae colonise the
epithelium without invasion or apparent damage. This is an extracellular
pathogen that does not invade host tissue.
nomenclature of v.cholerae
-V. cholerae has been classified according to its O antigens. These O
antigens allow for the classification of V. cholerae strains into serovars or
serogroups. At least 155 different V. cholerae serogroups had been
identified
-Serogroups can be divided into even smaller groups of classification
known as biotypes. There are two biotypes, El Tor and classical. Biotypes
are determined based on specific biochemical properties and the
bacteriophage susceptibility of the isolate.
principles of identification of cholerae
-isolates from primary culture are identified by colonial appearance,
Gram’s stain, serology (agglutination with specific antisera) and
biochemical testing.
-Thiosulfate Citrate Bile Sucrose
Agar (TCBS) is used for the
isolation and selective cultivation
of Vibrio cholerae and other
enteropahtogenic vibrios.
Growth of V. cholerae on TCBS
clinical symptoms of cholera
- voluminous diarrhoea and dehydration
-Hypovolaemic shock occurs when the volume of the circulatory system is too
depleted to allow adequate circulation to the tissues of the body
rehydration treatment of cholera
-Oral Rehydration Salts (ORS)
-Packets of Oral Rehydration Salts Distributed by WHO, UNICEF
-Dissolve in 1 L water
NaCl, KCl, NaHCO3, glucose
In severe cases a drip is needed to
start the process
An effective antibiotic can reduce the volume and duration of
diarrhoea and the period of Vibrio excretion. Tetracycline is the usual
antibiotic of choice, but resistance to it is increasing. Other antibiotics
that are effective when V. cholerae are sensitive to them include
cotrimoxazole, erythromycin, doxycycline and chloramphenicol
cholera models
-humans are the only accurate model
-rabbit ligated ideal loops: Adult rabbit ileum is
divided into 5-10-cm segments by surgery, and live
bacteria or other test material injected into
individual loops. After 12-24h, animal is sacrificed;
loop length and fluid volume measured as ml/cm readout.
Infant mice (most common current model):
Newborn mice are inoculated with bacteria; at
sacrifice, stomach and entire gut are removed and
-weighed; gut wt/carcass wt = fluid accumulation
ratio. Bacteria can be cultured in vivo, and
competitive index determined.
cholera toxin
-the toxin is the
major mediator of the cholera syndrome. Ingestion of only 5μg of purified toxin resulted in production of 1-6L of diarrheal stool in volunteers
cholera pathogenicity
-All strains belonging to the species V. cholerae are not equally
pathogenic. Pathogenic strains are those strains capable of causing
disease.
* Toxigenic strains are those strains that possess the ability to
produce cholera toxin. Not all V. cholerae strains are capable of causing
cholera, because not all V. cholerae strains are capable of producing
cholera toxin.
* In order for a strain of V. cholerae to be pathogenic it must
encode the ctxA and ctxB genes, and it is dependent upon the
interaction between multiple genes including genes encoding one or
more virulence factors; these genes include the CTX phage and the TCP
pathogenicity island.
-essentially pathogenetic strain is the same as toxigenic strains
virulence factors for cholera
Key Virulence Factors:
Cholera toxin (CT): Enterotoxin responsible for the symptoms of cholera.
Toxin-coregulated pili (TCP): Surface structure required for intestinal colonization.
Mobile Genetic Elements:
Both CTXΦ and TCP are mobile genetic elements.
CTXΦ (a filamentous bacteriophage) encodes:
ctxA and ctxB genes, which produce cholera toxin.
Toxigenic strains contain CTXΦ; non-toxigenic strains lack it.
Pathogenic Evolution:
Non-toxigenic strains can become toxigenic through horizontal gene transfer.
TCP not only aids pathogenicity but also acts as the receptor for CTXΦ.
CTXΦ Phage Characteristics:
Replicates as a plasmid in toxigenic strains.
Self-transmissible, producing DNA particles encoding the CTXΦ element.
In summary, CT and TCP are essential, coordinately regulated elements for V. cholerae virulence, with CTXΦ enabling toxigenicity through gene transfer and TCP supporting colonization and phage
relationship between CTX0 and TCP
-in order for the transmission of CTXΦ to occur, the TCP must be present in the genome as it acts as the receptor for the CTXΦ.
*If the TCP is not expressed in a strain the CTXΦ will be unable to be transferred as the receptor required for transfer is not present.
*If the TCP is present in a non-toxigenic strain and the CTXΦ can be transferred by horizontal gene transfer. This can allow for the ctxA and ctxB
genes to be passed from toxigenic to non-toxigenic strains, which poses a
great threat as it can lead to the development of new toxigenic strains.
*This is an example of co-evolution of certain genetic elements that can lead
to the transduction of virulence genes.
ctx genetic summary
-CTX genetic element
– Cholera toxin genes are
encoded on a lysogenic
filamentous bacteriophage
– The phage can infect strains
of V. cholerae missing toxin
genes
– The receptor for the phage is
Toxin co-regulated pilus (TCP)
– Transfer can occur in the
gastrointestinal tract or in the
aquatic environment
cholera toxin mechanism of action
-Binding and Endocytosis: Cholera toxin (CTX) binds to intestinal cell surface receptors via its B subunits, triggering endocytosis of the toxin.
Activation of A1 Subunit: The A subunit of CTX is cleaved into A1 and A2 domains. The A1 domain becomes enzymatically active.
ADP-Ribosylation of G Protein: The A1 fragment enters the cytosol and ADP-ribosylates the Gsα subunit of the G protein, locking it in its active, GTP-bound form.
Stimulation of Adenylate Cyclase: The locked G protein continuously activates adenylate cyclase, leading to elevated cAMP levels.
Activation of CFTR: High cAMP levels activate the CFTR channel, causing a massive efflux of ions (e.g., chloride) and water from intestinal cells.
Result: This ion and water loss causes severe diarrhea, a hallmark of cholera infection.
cholera:TCP
– Long filamentous pili termed the toxin-coregulated pilus (TCP) that
form bundles on bacterial surface is essential for colonisation
– Mutants lacking TCP are avirulent in human volunteers and in animal
models
– Genes necessary for production of TCP are organized in an operon that
contains 15 genes which is part of
Vibrio pathogenicity island (VPI)
* Major pilin subunit
* Assembly
* Secretion
virulence factors regulation
-Transcription of the ctxAB operon is regulated by a number of environmental signals,
including temperature, pH, osmolarity, and certain amino acids.
-The ToxR protein is a regulatory protein which functions as an inducer in a
system of positive control.
ToxR interacts with ToxS in order to sense some change in the environment
and in response to induce the transcription of genes for attachment (pili
formation) and toxin production.
It is reasonable to expect that the environmental conditions that exist in the
GI tract (i.e., 37o temperature, low pH, high osmolarity, etc.), as opposed to
conditions in the extraintestinal (aquatic) environment of the vibrios, are
those that are necessary to induce formation of the virulence factors
necessary to infect.
structure of cholera toxin
- key factor responsible for profuse diarroea
-* It is composed of two polypeptides, A subunit and 5 B Subunits. - The biological activity of CT is dependent on binding of the B pentamer to specific
receptors on the eukaryotic cell. - The B oligomer binds with high affinity exclusively to GM1 Ganglioside (Ganglioside
is a molecule composed of glycosphingolipid with one or more sialic acids; it is a
component of a cell plasma membrane).
life cycle of pathogenetic vibrio cholerae
Toxigenic strains of Vibrio cholerae persist in aquatic environments alongside non-
toxigenic strains, aided by biofilm formation on biological surfaces and use of chitin as a
carbon and nitrogen source. On ingestion of these aquatic-environment-adapted
bacteria in contaminated food or water, toxigenic strains colonize the small intestine,
multiply, secrete cholera toxin and are shed back into the environment by the host in
secretory diarrhoea. The stool-shed pathogens are in a transient hyperinfectious state
that serves to amplify the outbreak through transmission to subsequent hosts.
cholera vaccine development
-different vaccines have been tested and all failed
-Two recently developed oral vaccines for cholera are licensed and available in some
countries:
Dukoral®, Biotec AB (contains a mixture of inactivated V. cholerae bacteria and a
non-toxic component of the toxin), and Mutacol®, Berna (live attenuated).
Both vaccines appear to provide somewhat better immunity and fewer side-effects
than the previously available vaccine. However, neither of these two vaccines is
currently recommended for travellers
salmonenella
-Enterobacteriaceae family
- Motile Gram-negative
facultative anaerobes
- Non-lactose fermenting
- Resistant to bile salts
classification of salmonella
DNA homology shows only two species Salmonella enterica
(six subspecies) and S. bongori
· Most pathogens in S. enterica ssp. enterica. nomenclature still evolving
-Salmonella enterica subsp. enterica serotype Typhimurium
(Salmonella Typhimurium)
Salmonella serotypes are normally divided into two groups on the basis of host range: host adapted and ubiquitous
salmonellosis
-generic term for disease.
salmonella Eneritis
Most common form of salmonellosis with major
foodborne outbreaks and sporadic disease
High infectious dose (108 CFU)
Poultry, eggs, etc. are major sources of infection
6-48h incubation period
Nausea, vomiting, non-bloody diarrhoea, fever,
cramps, and headache common
S. enterica serotypes (e.g., S. typhimurium)
Zoonotic disease
salmonella enteric fevers
S. typhi causes typhoid fever
S. paratyphi A, B and C cause milder form of
enteric fever
Infectious dose ~ 106 CFU
Fecal-oral route of transmission
· Person-to-person spread by chronic carrier
· Faecally-contaminated food or water
10-14 day incubation period
Initially signs of sepsis/bacteremia with sustained
fever (delirium) for > one week before abdominal
pain and gastrointestinal symptoms
