Shigella

Question 1

Epidemiology

Answer 1

Shigella dysenteriae- most common species in developing world
Shigella flexneri (18% of the cases in US)- also common in developing countries
Shigella sonnei-most common species in US (75%)
Shigella boydii (common in India)

Question 2

Sources

Answer 2

Spread by food, fingers, feces, and flies (4fs)
Children under 10 more susceptible, also infects adults
No animal reservoir

Question 3

Clinical Syndrome

Answer 3

Fever (LPS), diarrhea and bloody diarrhea with mucus, and ab cramps (due to Shiga toxin)
Usually self-limiting, rarely fatal
Bacteremia rare
Hemolytic uremic syndrome can occur in association with shigella dysenteriae
Can detect organisms in feces up to 1-4 weeks after recovery

Question 4

Virulence determinants and pathogenesis

Answer 4

Low inoculum (100 bugs)- acid tolerance
Incubation period of 1-4 days
Invades intestinal cells in the terminal ileum and colon
Uptake by macrophages into phagocytic vacuoles (induced by T3SS-secreted Shigella protein)
escape from vacuole into cytoplasm and cell-to-cell spread (T3SS dependent)
Induces apoptosis of macrophages, re-infect new cells
IL-1 and TNF from monocytic cells leads to fever and systemic symptoms
Intestinal ulceration due to Shiga toxin in S. dysenteriae
Shiga toxin is an exotoxin with two subunits (A and B)
Subunit B binds to receptor on intestinal cells and A interferes with function of 60S ribosomal RNA inhibiting protein synthesis
may lead to apoptosis of mucosal cells and ulceration
diarrhea ensues due to fluid malabsorption

Question 5

Treatment

Answer 5

Fluid and electrolyte replacement, especially in young children
antibiotics in severe cases
antibiotic susceptibility testing may be important as resistance is increasing

Question 6

Diagnosis

Answer 6

Clinical symptoms not diagnostic
Isolation of microogranism from feces
Detection of PMNs in stool indicative of invasive disease
Important diagnostic traits 
   -no gas with glucose fermentation
   -shigella are lactose non-fermenters 
   -no H2S
   -shigella is nonmotile
   -contains O antigens only, no H
   -indole and urease negative

Kligler Iron agar demonstrates analysis of sugar fermentation, gas production, and H2s production

Question 7

Prevention

Answer 7

Improve sanitation
no effective vaccine
recombinant O-antigen vaccine conjugated to inactivated Shiga toxin is a promising vaccine candidate- not approved ye

Question 8

General properties

Answer 8

Hippocrates provided the first description of dysentery (bloody diarrhea)
Epidemics of bacillary dysentery described as early as 5th century BC
14,000 cases reported annually in US