Bacteria Chapter 14: Type III Secretion System as VIrulence Factor Flashcards
What are facultative intracellular pathogens?
pathogens that can live within or outside host cells
ie. Salmonella
What are obligate intracellular pathogens?
pathogens that must grow within host cells, and cannot be cultivated outside of their host
ie. Chlamydia
Why are intracellular environments good for bacteria?
- rich source of nutrients and energy
- shields from host’s immune system
- provides protection from many antibiotics that don’t penetrate host cells
What immune response plays an important role in defence against intracellular pathogens?
cell-mediated immunity – T cell effector mechanisms and activation of macrophages
What is Salmonella?
intracellular pathogen that hides from host’s immune defences
What are characteristics of Salmonella?
- Gram-negative
- flagellated
- facultatively anaerobic Bacilli
What are the major antigens of Salmonella?
- H antigen (flagellar)
- O antigen (somatic – cell wall)
- K antigen (capsular)
What is a major antigen of the capsular antigen?
Vi
possessed by only a few serovars of Salmonella – particularly those that cause typhoid fever
How is Salmonella transmitted?
from feces of infected people or animals, to other people or animals via contaminated food or drink
What are the two main diseases associated with Salmonella?
- salmonellosis
- typhoid fever
What is salmonellosis in human?
usually self-limiting gastroenteritis (diarrhea)
What is Salmonella enterica?
serovar of Salmonella
What are the two sub-species of Salmonella enterica?
- Salmonella enterica serovar Typi (Salmonella typhi)
- Salmonella enterica serovar Typhimurium (Salmonella Typhimurium)
Where does Salmonella enterica cause disease?
intestines
Who is more at risk for Salmonella infections
individuals who produce little or no stomach acidity
Salmonella are very acid sensitive
How many bacteria are required to cause Salmonella infection in humans?
- large amount required to infect humans with normal stomach acid
- smaller amount required to infect humans when introduced with bicarbonate
What happens to Salmonella that escape being kill by stomach acid?
pass into small intestine, where invasion into host cell may occur
Salmonella enterica serovar Typhi (S. typhi)
What is typhoid fever?
serious and often-fatal disease characterized by nausea, vomiting, and fever
Salmonella enterica serovar Typhi (S. typhi)
What disease does this cause?
typhoid fever
Salmonella enterica serovar Typhi (S. typhi)
What hosts is S. typhi pathogenic to?
humans only
Salmonella enterica serovar Typhi (S. typhi)
What is the main source of infection of S. typhi?
drinking contaminated water, or consumption of food that is washed or irrigated with contaminated water
Salmonella enterica serovar Typhi (S. typhi)
How long is the incubation period of typhoid fever?
week to a month after initial ingestion of bacteria
Salmonella enterica serovar Typhi (S. typhi)
How does the systemic infection stage of typhoid fever occur?
bacteria can multiply in the spleen and liver, then a large number is released into bloodstream
this stage of typhoid of fever can last 2-3 weeks
- high fever and chills
- flushed appearance
- anorexia
- convulsions
- delirium
Salmonella enterica serovar Typhi (S. typhi)
How might people shed S. typhi into their feces?
bacteria from the liver also moves to gall bladder, where they are shed in the bile and back into intestine – eventually shed as feces
*severe ulceration of the intestine may occur – infection is likely fatal
Salmonella enterica serovar Typhi (S. typhi)
Who are the main spreaders of typhoid fevers? How?
asymptomatic carriers
Salmonella enterica serovar Typhi (S. typhi)
During an outbreak of typhoid fever, what may occur in individuals?
- may become infected, but remain asymptomatic
- may recover, but become chronic carriers – bacteria persist in gall bladder and shed in feces for years
Salmonella enterica serovar Typhimurium (S. typhimurium)
What does S. typhimurium cause in humans?
self-limiting gastroenteritis
- diarrhea
- abdominal cramps
- vomiting
- nausea
What bacteria is a common cause of food-born illness?
S. typhimurium
Salmonella enterica serovar Typhimurium (S. typhimurium)
How long do symptoms of infection last?
appear 6-24 hours after ingestion of bacteria
last up to around 7 days
may be relatively mild, or very severe
Salmonella enterica serovar Typhimurium (S. typhimurium)
What is systemic shock caused by?
(in immunocompromised and elderly)
small number of S. typhimurium enters bloodstream, causing a systemic infection, leading to systemic shock – may be fatal if not treated right away with antbiotics
Salmonella enterica serovar Typhimurium (S. typhimurium)
Does S. typhimurium produce toxins?
no – does not produce any of the enterotoxins that seem to be necessary for pathogenic bacteria to cause diarrhea
Salmonella enterica serovar Typhimurium (S. typhimurium)
How do Salmonella cause the diarrhea and pain associated with salmonellosis?
abdominal pain is likely due to:
- inflammatory response that results from recognition of bacterium’s LPS
- subsequent release of cytokines, as well as that caused by the Salmonella pathogenicity island 1 (SPI1) effects
What does Salmonella diagnosis require?
isolation of organisms from clinical specimens (ie. blood or stool)
Salmonella Diagnosis – Procedure
- clinical specimen is plated onto selective and differential media
- identification of suspicious colonies is done by biochemical tests
- identification of serotype is done by antigenic analysis of O (cell wall antigen) and H (flagella antigen) using polyvalent and specific antisera
What is the Widal test for diagnosing Salmonella infections?
patient’s serum is tested fro presence of antibodies against Salmonella H and O antigens
useful when facilities for culturing Salmonella bacteria are unavailable
Widal Test – Procedure
- Salmonella H and O antigens are prepared as stained suspensions
- Suspensions are mixed with patient’s serum
Interpreting Results:
- agglutination = positive test result
- if positive reaction is observed with 20 μl of test sample, this indicates presence of clinically significant levels of corresponding antibody in patient serum
What is the usual treatment for Salmonella?
nothing – infections are usually resolved within a week
What can Salmonella be treated with?
- rehydration therapy – for severe diarrhea
- antibiotic therapy (ampicillin, gentamicin, trimethoprim, ciprofloxacin) – for infections that have spread from intestines to blood
What is the treatment of choice for S. typhi?
antibiotics – chloramphenicol, ceftriaxone
What vaccines are currently available for prevention of Salmonella infections?
- whole dead bacteria vaccine (injected)
- capsular material with Vi antigen vaccine (injected)
- live attenuated/avirulent strain of S. typhi vaccine (oral)
What are virulence factors of Salmonella?
- adhesins
- type I fimbriae
- plasmid-encoded fimbriae
- LPS
- capsules
- type III secretion systems
- ability to invade and replicate inside host cells
Where are genes that encoded Salmonella virulence factors?
pathogenicity islands (PAIs) in Salmonella genome
What are pathogenicity islands (PAIs)?
large regions of DNA that encode clusters of genes associated with virulence
- toxin genes
- adhesion genes
- invasion genes
- etc.
How are genes that encode Salmonella virulence factors obtained?
may have been acquired during evolution by HGT
are often associated with bacteriophage or prophage genes
Where are PAIs not found?
in genomes of non-pathogenic members of the same or related species
What are type III secretion systems?
protein structures that pathogenic bacteria can use to inject proteins into a host cell’s cytoplasm or endomembrane system
What are Salmonella’s two type III secretion systems?
- one encoded by genes in SPI1
- one encoded by genes in SPI2
What is an injectisome?
type III secretion system –delivery system
type III secretion system has hollow needed that spans the plasma membrane of the host cell to inject proteins into the host cell
SPI1
What genes does SPI1 contain?
- genes associated with virulence
- genes involved in regulation of virulence genes
SPI1
What are the inv genes?
responsible for:
- disruption of normal cell function
- promotion of membrane ruffling associated with invasion (internalization) of Salmonella by host cells
SPI1
Protein Injection by SPI1 System – Process
- effector protein is injected
- triggers rearrangement of actin inside host cell
- results in membrane ruffling of epithelial cell
- formation of pseudopods engulf bacterium and internalize it inside vacuole
- extensive actin rearrangement in vicinity of invading bacterium
- however, surface of host cell and actin filaments in the region return to normal
SPI2
Protein Injection by SPI2 System – Process
- SPI2 activated after internalization by intestinal epithelial cell
- SPI2 injects several protein effectors into cytoplasm and endomembrane system of the cell
- proteins alter structure of membrane to maintain vacuole, and prevent fusion of lysosomes with vacuole - Salmonella-containing vacuole (SCV) gradullay matures through interactions with host endocytic pathway into a specialized compartment
- Salmonella replicate in compartment
- as bacteria replicate, vacuole sprouts thin filaments that stretch out from the walls of the surface
Is SPI2 essential for virulence of bacteria? Why?
yes – mutant strains of Salmonella are not able to maintain structure of SCV, and are released into cytoplasm of host cell where they are quickly killed
Inside the Intestinal Epithelium
S. typhimurium uses specific set of effector proteins, which are delivered into cells via its SPI1 injectisome to activate responses such as NF-κB signalling pathways, which closely mimic those induced by the stimulation of innate immune receptors such as TLR
therefore, intestinal epithelial cells can produce cytokines and chemokines to recruit neutrophils to infected area
neutrophils produce prostaglandin that acts on intestinal epithelial cell to increase internal concentration of cAMP
increased levels of cAMP cause increase in secretion of Cl– ions and Na+ ions into lumen of intestine
as concentration of NaCl increases in lumen of intestine, water is lost from tissues into lumen, resulting in diarrhea
infection is eventually cleared from host by combined action of diarrhea and shedding of infected epithelial cells
What is SPI1 secretion system required for?
invasion into host tissues
What is SPI2 secretion system required for?
survival inside intestinal epithelial cells
SP1 secretion system can activate signalling pathways that produce what?
pro-inflammatory cytokines and chemokines that recruit neutrophils to site of infection
What does the production of prostaglandin by neutrophils do?
increases internal concentration of cAMP, resulting in secretion of Cl- ions and Na+ ions into lumen of intestine
What triggers diarrhea associated with Salmonella infections?
host’s immune response
