Cholera Flashcards
What is the causative agent of Cholera? What is Cholera?
= Vibrio cholerae
Cholera
= human intestinal disease
= occurs at mucosal surface with no invasion into deeper tissue
= disease symptoms primarily due to the cholera toxin
= originates from Ganges delta, was confined to India until 19th C
= seven pandemics to date
= big problem in developing world (although case fatality rate is falling)
How was Vibrio cholerae discovered?
John Snow
= water borne transmission of cholera
= broad street pump
= found clustering of cases around pump
= pump removed, ending epidemic
= theory cholera spread in dirty water
(epidemiological and geographical analysis)
Filippo Pacini
= discovered causative agent
= BUT ignored due to miasma theory
= bacterium later named Vibrio cholera Pacini
Robert Koch
= re-discovered it after Pacini (independent discovery)
= identified Vibrio bacterium
= was more creditable
What is the physiology of V. cholerae
= facultative anaerobic
= gram negative
= curved rod
= asporogenous (no spores)
= growth stimulated by NaCl
= pH 6-10, acid labile
= temperature 18-37 oC
= polar monotrichous (sheathed)
= can enter a viable but not culturable state
What are the serotypes of V. cholerae?
= differences in the sugar composition of heat-stable surface somatic “O” antigen
Outbreaks = 2 serogroups
= O1 = majority of outbreaks
= O139 = confined to South-East Asia
Where is V. cholerae in the environment?
= found in coastal water + estuaries (free-living)
= often associated with zooplankton + shellfish in water
(can use chitin as a carbon and nitrogen source)
= can persist in a free-living state / associated with phytoplankton, zooplankton or biotic / abiotic detritus
= also affected by seasonality (higher in summer - blooms + chitin)
How does V. cholerae associate with zooplankton?
Seasonal plankton blooms
= correlate with ocurence of cholera in Bangladesh
V. cholerae
= found attached to chitin of crustacean zooplankton
= can secrete chtitinase enzymes (?potential nutrient source)
= remain metabolically active = even in high acidic environment
(Without losing viability or virulence)
= abundance of chitin during blooms = important in V. cholerae life cycle and season transmission
How does the V. cholerae genome give pathogenicity?
2 chromosomes (unique)
Chr I
= VPI = attachment pilus
= CTXφ = enterotoxin (exotoxin)
(= they are inserted by a virus)
+ lots of housekeeping genes
After ingestion
Needs to be resistant to gastric acid
(need large amount - high infectious dose)
Can colonise small intestine
What are some V. cholerae virulence factors?
= CTXφ (CTX prophage / cholera toxin)
= VPI-1-toxin coregulated pilus
= Quorum sensing (AI-1/2/CAI-1)
= Accessory colonise factor (ace)
= VPI-2-sialic acid (SA) catabolism
= Vibrio seventh pandemic island 1 (VSP-1) (increased fitness)
= VSP-2 (increased fitness)
= Repeats-in-toxin (RTX) TOXINS
= Mannose-sensitive haemagglutinin pilus (MSHA pilus) (adhesion)
= Haemolysin genes/cytotoxins
= Outer membrane protein / defense (OmpU)
= Type VI secretion system (T6SS)
= Haemagglutinin (protease)
= Virulence gene expression (transcriptional activator - ToxR)
How does V. cholerae use quorum sensing?
Cell-to-cell communication to precisely control pathogenicity and biofilms
= production of colonisation factors and toxins inside human host, biofilm formation
= genetic excahnge
(+ many important cellular processes)
= at least 4 sensory inputs function in parallel to regulate quorum sensing in V. cholerae
What are the V. cholerae infection and environmental cycles?
Interacts with gut microbiome
Commensal microbial function influence chemical cues used by V. cholerae to time gene expression during early vs late infection states
Early infection
= biofilm dispersal
= mucosal penetration
= virulence genes induced
= antivirulence gene repression
Late infection
= quorum sensing on
= virulence genes repressed
= late induced genes induced
= mucosal escape
= priming for environmental entry
What are the V. cholerae virulence regulatory networks?
e.g. Master regulator ToxT
= activates virulence genes involved in synthesis of key virulence determinants: TCP and CT
= expression regulated by TcpP and ToxR
There are also autoinducers
= LuxPQ, CqsS, CqsR, VpsS
(activation dependent on whether high or low levels of autoinducers)
Low = biofilm formation + virulence factor production
High = repression of biofilm / virulence factor production
What are stage in infection of V. cholerae?
Secrete enterotoxin
Enterotoxin binds to intestinal cells
Chloride channels activated
Release large quantities of electrolytes + bicarbonates
Fluid hypersecretion
Diarrhea
Dehydration
What are the key elements for Virulence in V. cholerae?
Genes for cholera toxin carried by CTXφ
= temperate bacteriophage inserted into the V. cholerae genome
= can transmit cholera toxin genes from one strain to another (via horizontal gene transfer = transduction, conjugation, transformation - enhanced by chitin)
AB5 exotoxin (also present in other pathogens - e.g. E.coli, shigella)
= A comprises 2 subunits
Genes for toxin corregulated pilus
= coded by VPIφ pathogenicity island
What is the mode of action of the cholera toxin?
= AB5
- Toxin complex binds the ganglioside GM1 on host membrane lipid rafts
- Toxin is endocytosed
- Phagosome taking to endoplasmic reticulum
- A1 subunit removed from B subunits and exported into cytoplasm
- A1 peptide attached an ADP-ribose to an amino acid within host G protein (regulates adenylate cyclase)
- Cyclic AMP levels rise and activate ion transport systems, causing electrolyte imbalance
Water from cell follows ion = causing diarrhea
How does V. cholerae infection occur?
= faecal-oral route
(faecal = discharge, oral = entry)
= person-to-person possible (indirect)
= food (water, alkaline foods, fruit/veg)
= carriers (houseflies / other insects)
= 1-3 day incubation period
Symptoms
= starts as mild diarrhoea = suddenly severe diarrhoea
= muscle cramps
= scaphoid abdomen
= vomiting
= loss of skin turgor
= weak pulse
