Notes on Salmonella and Shigella

Enterobacteriaceae Overview
  • Salmonella and Shigella: Notable families of non-lactose fermenters within the Enterobacteriaceae family.
Salmonella
  • Classification: Contains two main species:
    • Salmonella typhi
    • Salmonella enteritidis
  • Serotypes: Over 2,000 serotypes exist with about 50 commonly found in the U.S., differentiated by O (somatic) and H (flagellar) antigens.
Salmonellosis
  • Pathogen: Primarily caused by Salmonella enterica.
  • Incubation Period: 12 - 36 hours; pathogens need to invade intestinal mucosa and multiply.
  • Symptoms: Moderate fever, intestinal cramps, diarrhea, nausea.
  • Prognosis:
    • Self-limiting in healthy individuals, symptoms may resolve within a week.
    • Organisms can be shed in feces for up to 6 months.
    • Risk of severe illness in very young, elderly, or immunocompromised (may invade bloodstream).
  • Antibiotic Treatment: Not usually recommended unless bloodstream infection occurs.
Common Sources of Salmonella
  • Transmission:
    • Frequently linked to poultry and eggs; FDA advises against consuming raw eggs, including lightly cooked sunny-side up eggs.
    • Homemade foods like eggnog, Caesar salad dressing, and mayonnaise made from raw eggs are risky.
    • Transovarial transmission occurs where eggs can be contaminated even before laying.
  • Reptiles: Common carriers, especially pet turtles.
    • Note: A 1975 ban on selling small turtles aims to reduce transmission risk.
  • Statistics: Approximately 79,000 salmonella-related illnesses and 30 deaths in the U.S. annually from contaminated eggs.
Typhoid Fever
  • Pathogen: Caused specifically by Salmonella typhi, a subtype of Salmonella enterica.
  • Characteristics:
    • Human-only carrier: No animal hosts.
    • Longer incubation period (~2 weeks).
    • Symptoms include high fever, headache, confusion, and variable diarrhea or constipation.
  • Mechanism: Bacteria proliferate in phagocytic cells and disseminate throughout the body.
  • Epidemiology: Rare in the U.S. but prevalent worldwide in areas with poor sanitation.
    • Chronic carriage in ~1-3% of patients post-recovery; common in the gallbladder.
  • Famous case: "Typhoid Mary" (Mary Mallon) was an asymptomatic carrier linked to multiple outbreaks in the early 20th century.
Prevention and Treatment of Typhoid Fever
  • Vaccines: Killed vaccines are available for high-risk groups (e.g., lab workers and military).
  • Antibiotic Treatment: Third-generation cephalosporins (e.g., ceftriaxone) are common if treatment is necessary.
    • Recovery from infection results in lifelong immunity.
Shigella
  • Species Overview: Includes four species that cause bacillary dysentery.
  • Characteristics:
    • Prolific production of Shiga toxin inhibits protein synthesis in host cells.
    • Infective dose is extremely low (fewer than 10 organisms) and resistant to stomach acidity, leading to rapid proliferation in the intestines.
  • Disease Impact: Damages intestinal mucosa; blood and mucus may be present in stools.
  • Treatment: Oral rehydration and antibiotics (fluoroquinolone like ciprofloxacin) for severe cases.
  • Epidemiology of Shigellosis:
    • Approximately 450,000 shigellosis cases per year in the U.S.
    • Outbreaks often associated with unsanitary food/water, daycare centers, and specific populations (MSM).
    • Symptoms include diarrhea (often bloody), fever, and cramps, typically resolving within 5-7 days; can lead to hospitalization in vulnerable populations (young, elderly).