Salmonella and Shigella Flashcards
Explain the relationship between species, strain, and serotypes.
Species: E. coli, etc
Strain: subpopulation within a species that descended from the SAME PARENT
Serotype: a subset of strains within species that are bound by COMMON ANTIBODIES
—Distinguished by specific antisera that recognize bacterial surface elements
Explain the use of flagellar and LPS antigens as the basis for strain identification. What is the K antigen?
O antigen = polysaccharide component of LPS
—Only in Gm- bacteria
H antigen = flagellar antigen
K antigen = polysaccharide capsule component
Briefly, list the syndromes caused by Salmonella (5) and Shigella (4) species.
SALMONELLA
- S. typhi - typhoid fever (rarely S. paratyphi)
- S. choleraesuis - sepsis syndrome
- S. enteriditis and S. typhimurium - gastroenteritis with diarrhea
SHIGELLA
- S. dysenteriae - most severe; dysentery in developing world
- S. flexneri - some dysentery in US, developing world
- S. sonnei - dysentery in US
- S. boydii - dysentery in India
Explain the basic elements of the Type III secretion system.
NEEDLE COMPLEX: similar to a flagellum; provides passage for molecule through membrane
BASE connects needle with bacterial membranes
Explain the role of endotoxin (Lipid A) in disease. What happens when there is too much?
Lipid A: the part of LPS buried in bacterial outer membrane
Host innate immune system + low [LPS] -> rapid response:
- Macrophages -> TNFalpha in tissue
- Increased phagocyte/lymphocyte migration into tissue -> phagocytosis
- Increased platelet adhesion to blood vessel wall -> occlusion -> drainage to local lymph node
- Removal of infection
Too much Lipid A in blood -> endotoxic shock -> highly lethal
Describe the properties of enterics. (6)
Live in the gut Gm- rods Ferment glucose Oxidase negative Facultative anaerobes Reduce nitrate
Include Salmonella, Shigella, E. coli, Campylobacter, Vibrio, others
What does S. typhi cause? How many serotypes? What else can cause this?
Typhoid (enteric) fever - 1 serotype
Rarely caused by Salmonella paratyphi
What does S. choleraesuis cause? How many serotypes?
Septicemia (bacteremic syndrome) - 1 serotype
What do S. enteritidis and S. typhimurium cause? How many serotypes?
Acute gastroenteritis with diarrhea
- >1500 serotypes
What are the three phases of typhoid fever?
Early GI phase (week 1)
Bacteremic phase (weeks 2-3)
Late GI phase (week 3+)
What characterizes the early phase of typhoid fever? (4)
SUBCLINICAL
Episodic fever
Abdominal pain
Constipation
List the steps in the pathogenesis of typhoid fever. (7)
- Invasion of intestinal Peyers patches
- T3SS-mediated ingestion by macrophages
- Survival/proliferation inside phagocytic vacuoles of macrophages -> spread
- T3SS-mediated killing of macrophage
- Dissemination via thoracic duct to blood, bone marrow, liver, gall bladder
- LPS in blood causes fever/shock
- Re-invasion of GI tract
How does S. typhi infect humans? List non-human vectors and infectious dose.
Contaminated food or water
NO non-human vectors
ID = 1,000-100,000 bacteria (high)
How is S. typhi well suited to colonize the gut? (2)
Resistant to killing by stomach acid
Adhesins promote attachment to intestinal epithelium
What is the incubation period of S. typhi?
7-14 days
What are pathogenicity islands? How are they acquired?
Encode genes for virulence
Acquired through horizontal gene transfer
What characterizes the bacteremic phase of typhoid fever? (5)
Bradycardia Skin rash (rose spots = bacterial emboli) Leukopenia Hepato/splenomegaly Confusion
What characterizes the late GI phase of typhoid fever? (2)
Intestinal hemorrhage (common) Intestinal perforation (rare)
Where does S. typhi hide in its chronic phase? Why is this phase important?
Gallbladder
Presents in 3% of patients
Important source for spreading infections under poor hygienic conditions
What pathogenicity islands does S. typhi have? What do they do?
SPI-1: encodes T3SS genes for INVASION of macrophages
SPI-2: encodes T3SS genes for INTRACELLULAR SURVIVAL
What is a T3SS?
Type III secretion system: protein appendage found in Gm- bacteria that allows bacterium to deliver proteins to host cytoplasm
True/false: Diarrhea is a major feature of typhoid fever.
False. Diarrhea is NOT a major feature of typhoid fever
How is typhoid fever diagnosed?
Stool culture (week 1 or after 3rd week) Blood culture (between 2nd and 3rd week)
How is typhoid fever treated?
Antibiotics (need to access INTRACELLULAR bacteria)
–Fluoroquinolones or ceftriaxone
Chronic carrier state: ampicillin, cipro, cholecystectomy
How is typhoid fever prevented? Are there any vaccines?
Control of water supplies and sewage disposal
Pasteurization of milk
Two vaccines:
- Ty21a - oral attenuated
- ViCPS - Vi capsule antigen vaccine
What does S. choleraesuis cause?
Bacteremic syndrome - high fever, bacteremia after onset of gastroenteritis, microabscesses
Sources/hosts of S. choleraesuis?
Swine
Contaminated food
Infectious dose of S. choleraesuis?
1000 organisms
Incubation period of S. choleraesuis?
6-72 hours
What pathogenicity islands, etc. does S. choleraesuis have?
SPI-1 and SPI-2
Virulence/antibiotic resistance plasmid
Who is most susceptible to S. choleraesuis?
Young
Pts with sickle cell anemia
Cancer patients
How is S. choleraesuis treated?
Appropriate antibiotics
What do S. enteriditis and S. typhimurium cause? How many serotypes?
Acute gastroenteritis (1-4 days) with diarrhea, confined to GI tract
Low fever
Rare bacterimia, more likely in young or old
> 2200 serotypes
Sources/hosts of S. enteriditis and S. typhimurium?
Reptiles and poultry (contact with droppings)
Raw and undercooked eggs, other foods
Incubation period of S. enteriditis and S. typhimurium?
8-48 hours
How is S. choleraesuis diagnosed?
Blood culture
How are S. enteriditis and S. typhimurium disagnosed?
Stool culture
How are S. enteriditis and S. typhimurium treated?
Self limiting -> fluid replacement
Vulnerable patients treated with antibiotics (ampicillin, trimethoprim, ceftriaxone, cipro) to prevent/control bloodstream infections
List the general components in the infectious process of S. enteriditis and S. typhimurium. (3)
T3SS-mediated invasion of epithelial cells
LPS
Toxins and host-damaging proteins
How is Shigella spread?
4Fs: Food, Fingers, Feces, Flies
Who is most susceptible to Shigella?
Children under 10
Sources/hosts of Shigella?
No animal reservoir
Infectious dose of Shigella?
100 bugs (LOW)
Incubation period of Shigella?
1-4 days
Symptoms of Shigella (and their causes)? (3)
Fever - LPS
Diarrhea and abdominal cramps - Shiga toxin
Bloody diarrhea with mucus - T3SS effectors
Self-limiting
Rare hemolytic uremic syndrome
Intestinal ulceration with S dysenteriae
How is Shigella diagnosed?
Stool sample (up to 1-4 weeks after recovery)
List the steps of pathogenesis of Shigella infections. (7)
- Oral ingestion
- Invade intestinal cells in terminal ileum/colon
- T3SS-induced uptake by macrophages into phagocytic vacuoles
- T3SS-dependent escape from phagocytic vacuole -> cytoplasm of macrophages
- Apoptosis of macrophages
- Infection of new cells
- IL-1/TNF secretion from monocytic cells -> fever, systemic symptoms
How does Shigella promote its own cell to cell spread?
T3SS toxins -> actin polymerization -> zooms across, into, and out of cells
How exactly does Shiga toxin in S. dysenteriae work?
- Subunit B binds receptor on intestinal cells
- A subunit interferes with 60S rRNA -> inhibits protein synthesis
- Fluid malabsorption -> diarrhea
- Apoptosis of mucosal cells -> ulceration
How is Shigella treated?
Fluid and electrolyte replacement
Cipro, trimethoprim if severe
Antibiotic susceptibility testing should be considered
How is Shigella prevented? Are there any vaccines?
Improve sanitation
No vaccines - but one with O-Ag conjugated to Shiga toxin has promise
Shigella and salmonella: glucose fermentation test
Positive for both
Shigella and salmonella: color on MacConkey agar
Colorless for both (neither ferments lactose)
Shigella and salmonella: ACID production with glucose
Positive for both
Shigella and salmonella: GAS production with glucose
Salmonella positive, shigella negative
Shigella and salmonella: motility
Salmonella positive, Shigella negative
Shigella and salmonella: H2S production
Salmonella positive, Shigella negative
How is an oxidase test performed? What does it test for?
1% tetramethyl-p-phenylnediamine HCl (TMPD reagent) reacts with cytochrome oxidase -> turns PURPLE
Tests for presence of cytochrome oxidase
Shigella and salmonella: oxidase test
Negative for both (NOT purple)
How can indole or urease tests distinguish Salmonella from other Gm- bacteria?
Salmonella is both indole negative and urease negative
What is the Klinger Iron Agar test? What is it used for
Has lactose, glucose, and iron, plus pH indicator
- Acid = yellow (fermentation)
- Alkaline = red (NO fermentation, or top of glucose fermentation)
- Gas production = bubble at bottom of tube
- H2S production = BLACK
Distinguish between Salmonella and Shigella:
- Salmonella = black with a bubble
- Shigella = red with yellow bottom
How are immunochromatographic assays used for diagnosis of Shigella?
Detects Shiga toxin 1 and 2
Does Shigella have H antigen? Salmonella?
Shigella = NO Salmonella = yes
