Salmonella and Shigella Flashcards
Relationship between species, strains and serotypes.
A strain is a population of organisms within a species that descends from a single organism - surface components often vary among strains within a species
A serotype is a strain that is differentiated by serological means, based on their surface elements:
- O antigens (polysaccharide component of LPS, attached to lipid A in mebrane)
- H antigens (the flagellar antigen)
- K antigen (polysaccharide capsule component)
Serological tests
- agglutination
- ELISA
- western blot
- flow cytometry
List the three members of the Enterobacteriaciae (“enterics”). How do they gram stain?
Salmonella, Shigella, E. coli
Gram (-)
What are the three clinically distinguishable syndromes related to Salmonellosis in man?
- typhoid or enteric fever
- S. Typhi (1 main and 1 rare serotype) - septicemia
- S. cholerasuis (1 serotype) - acute gastroenteritis
- S. enteriditis or S. typhimurium (>1500 serotypes)
Compare the epidemiology of the Salmonella species.
A: 16 million cases a year and >600K related deaths, common in developing world and rare in NA, Europe and Australia
- HUMANS ONLY (via food or water)
B: rare disease, rarely seen in healthy individuals; inc susc with young age, malaria, AIDS, steroid use, immune suppressive therapy, SS
- SOURCE: swine
C: 30K cases in US per year, 1.4m cases per year with 600 deaths
- SOURCE: poultry, pork, dog food, eggs; fruits and veg. can be contaminated; reptile pets; sandbox
Compare their clinical presentations.
A: typhoid fever, incubation 7-14 days (early GI phase - may be subclinical with positive stool culture)
- wk 1: episodic fever, brachycardia, skin rash (Rose spot) diagnostic, leukopenia, enlarged liver and spleen (Bacteremic phase)
- wk 2-3: intestinal hemorrhage or perforation (late GI phase)
- may hide in GB (chronic phase - 3% of patients)
B: INFECTIOUS DOSE - 1000 organisms, short incubation (6-72 hours), high fever and bacteremia after onset of gastroenteritis, microabscesses can develop in any body tissue
C: symptoms begin within 8-48 hours, sudden onset of headache, chills, abdominal pain, vomiting then diarrhea with fever, lasts 1-4 days
Discuss the pathogenesis of A.
A: low pH resistant, adhesins, endocytosis into intestinal epithelieum, ingestion by macs (survive within phagocytic vacuoles enabled by Vi antigen, a polys capsule), survives acidicity of lysosome, kills mac and diss via thoracic duct to blood (fever and shock), liver, spleen and GB; reinvasion of GI tract via GB, GI bleed and sometimes diarrhea
C:
Discuss the virulence determinants of A
- pathogenicity islands, acquired through horizontal gene transfer, unexpected G+C content
- SPI-1: encodes genes for invasion (T3SS)
- SPI-2: encodes genes for IC survival - endotoxin (lipid A component of LPS)
- at low levels, pathogen clearance
- at high levels, endotoxic shock
fever
inflammation
hypotension
disseminated intravascular coagulation
(DIC) and bleeding
organ failure due to lack of oxygen
- many gram(-) induce local inflam due to LPS, and shock when they grow in bloodstream (sepsis)
What are the pathogenesis and virulence determinants of C?
- LPS release during invasion of epithelial cells o fthe small and large intestines are responssible for many symptoms
- T3SS mediates invasion of epithelial cells
- EC cells produce toxins (including a pertussis-like toxin) that promote inflammation and secretion
Describe prevention and vaccines for A.
PREVENTION:
- control of water supplies and sewage disposal
- food safety
- pasteurize milk
- screen for carries among food handlers
VACCINES
- oral attenuated vaccine (Ty21a)
- ViCPS capsular polysaccharide vaccine (injected)
Steps in diagnosis of Salmonella infections.
- feces (all) or blood culture (S. typhi only)
- glucose fermentation, oxidase negative, reduces nitrate
- non-lactose fermentation
- motile (unlike SHIGELLA) and produce H2S
- urease negative (unlike PROTEUS)
- indole negative (unlike E. COLI)
- serotyping and PCR tests with O and H antigens allow species identification to trace outbreaks
How is A treated?
- fluoroquinolines (e.g. ciproflaxin) or third generation cephalosporin (e.g. ceftriaxone)
- chronic carrier states: 1) ampicillin or ciproflaxin 2) cholecystectomy
- relapse can occur in 10% of patients in endemic areas
How is C treated?
- self-limiting, death uncommon
- fluid and electrolyte replacement
- treat patients with predisposing conditions with appropriate antibiotics:
- Ampicillin
- Trimethoprim sulfa
- third generation cepholosporin
- fluoroquinolone (ciprofloxacin)
- antibiotic resistance testing can be useful
Discuss the 1984 outbreak
US bioterrorism event using C
Shigella - epidemiology
Spread by 4Fs: food, fingers, feces, flies
Children , 18% of US cases
S. sonnei - 75% of US cases
S. boydii - common in India
Shigella - clinical presentation
fever (LPS)
diarrhea and abdominal cramps (Shiga toxin)
bloody diarrhea with mucus (T3SS)
self-limiting, rarely fatal
bacteremia rare
can detect in feces up to 1-4 weeks after recovery (DISSEMINATION)
Shigella - virulence and pathogenesis
low inoculum (100 bugs) - acid tolerance
incubation period of 1-4 days
invade intestinal cells in terminal ileum and colon
uptake by macs via T3SS
T3SS dep cell-to-cell spread
induces apoptosis, reinfect new cells
IL-1 and TNF from monocytic cells (fever and systemic symptoms)
SHIGA TOXIN (from S. dysenteriae):
- intestinal ulceration
- exotoxin with subunits A and B:
A interferes with 60S ribosomal RNA
B binds to receptor on intestinal cells
- diarrhea ensues due to fluid malabsorption
- may lead to apoptosis of mucosal cells and ulceration
Shigella - diagnosis
clinical symptoms not diagnostic
feces sample
detection of PMNs
diagnostic traits
- gluc ferm with NO gas
- lactose nonfermenters (like SALMONELLA)
- no H2S (unlike SALMONELLA)
- nonmotile, no flagella (unlike SALMONEL)
contains O antigen, not H
indole and urease negative (like SALMONELLA)
Shigella - prevention and treatment
PREVENTION
- improve sanitation (human is only host)
Food preparation
Prevent spread among young children
diaper hygiene
hand washing
Avoid swallowing water from ponds, lakes, or untreated pools
- no effective vaccine (live atten not effective)
- recombinant O-antigen conjugated to an inactivated Shiga toxin (distinct from T3SS) is a promising candidate vaccine
TREATMENT
- fluid and electrolyte replacement, esp in young children
- antibiotics in severe cases (sulfa drugs such as ciprofloxin and trimethropin)
- antibiotic susceptibility testing impt
