Lecture 6: Human Pathogens

Human Pathogens Part 3

Types of Bacteria

• Primary pathogens: capable of causing disease in healthy individuals (e.g., Salmonella, Shigella, Klebsiella pneumoniae, Escherichia coli)
• Opportunistic pathogens: cause disease in individuals with compromised immune systems
• Non-pathogenic bacteria: do not cause disease

General Characteristics of Enterobacteriaceae

• Gram-negative rods with no specific arrangement
• Non-spore forming
• Aerobic and facultative anaerobic
• Motile by peritrichous flagella or non-motile
• Capsulated with ill-defined slime layer or non-capsulated
• Fimbriae or pili present in most species
• Lactose fermenter
• Catalase positive
• Reduce nitrate to nitrite
• Grow on peptone or meat extract media without sodium chloride or other supplements

Antigenic Structure

• O antigens: in the polysaccharide of the cell wall (LPS)
• H antigens: flagellar protein
• K antigen: polysaccharide capsule (only for some species)
• Vi antigen: virulence factor (only for Salmonella species)

Escherichia coli

• >150 serotypes
• Typed by O and H antigens
• Example: O157:H7 (EHEC)

Shigella

• 4 different species: S. dysenteriae, S. flexneri, S. boydii, S. sonnei
• Each have different serogroups: A, B, C, D
• No capsules

Epidemiology of Enterobacteriaceae

• Fecal-oral transmission: contaminated food or water, or from an animal
• Through the gastrointestinal tract
• Can cause diarrhea, urinary tract infections, and other diseases

Campylobacter spp.

General Information

• Gram-negative, comma-shaped bacteria
• Motile with a single polar unsheathed flagellum
• Non-spore former
• Microaerophilic and capnophilic
• Require special growth conditions

Transmission

• Intestinal colonization in chickens
• Contaminated food, water, or milk
• Unpasteurized milk and undercooked poultry are high-risk sources

Pathogenesis and Virulence Factors

• Attachment: uses fimbria-like filaments and cell surface proteins
• Invasion: drills into mucosa using spiral shape and flagella
• Releases cytotoxins, causing cell damage and inflammation
• Complications: toxic megacolon, bacteremia, Guillain-Barré syndrome, reactive arthritis

Symptoms

• Incubation period: 1-7 days
• Initial symptoms: fever, muscle pain, malaise, headache
• Gastrointestinal symptoms: crampy abdominal pain, watery diarrhea, bloody diarrhea

Shigella spp.

General Information

• Family: Enterobacteriaceae
• Species: Shigella dysenteriae, Shigella flexneri, Shigella boydii, Shigella sonnei
• Gram-negative, rod-shaped bacteria
• Facultative anaerobe
• Motility: non-motile
• Non-spore former
• Urease and oxidase negative
• H2S negative

Pathogenesis

• Ingestion and initial infection
• Invasion and immune response
• Spread within the host
• Shiga toxin (specific to Shigella dysenteriae serotype 1)

Symptoms

• Incubation period: 1-3 days
• Typical symptoms: severe abdominal cramps, watery diarrhea, fever, vomiting
• Complications: hemolytic uremic syndrome (HUS), seizures, reactive arthritis

Salmonella spp.

General Information

• Family: Enterobacteriaceae
• Species: Salmonella enterica (with 6 subspecies)
• Gram-negative, rod-shaped bacteria
• Facultative intracellular pathogen
• Motile with flagella
• Non-spore former
• Oxidase negative
• H2S positive

Pathogenesis

• Ingestion and targeting
• Immune evasion
• Survival and replication
• Systemic spread

Symptoms

• Onset: 1-2 weeks after infection
• Typical symptoms: high sustained fever, abdominal pain, constipation, diarrhea, rose or salmon-colored spots on chest and abdomen
• Complications: osteomyelitis, chronic infection, and carrier state

Escherichia coli

Characteristics

• Gram-negative, thin peptidoglycan layer
• Rod-shaped bacteria
• Catalase-positive
• Lactose fermenter
• Encapsulated with a polysaccharide layer
• Motile with flagella
• Facultative anaerobe

Pathogenic Strains

• Attachment: uses fimbriae to attach to host cells

• Serotype classification: based on antigens (O, K, H)

• Pathotypes: based on the disease-causing mechanism and virulence factors### Effacement

• Injects effector proteins via Type III secretion system, causing actin cytoskeleton rearrangement

• Leads to pedestal formation and microvilli effacement

• Impairs absorption, causing watery diarrhea, primarily in children under two years of age

Uropathogenic E. coli (UPEC)

• Colonizes periurethral area and ascends urinary tract
• Adheres to uroepithelial cells using type 1 fimbriae and P fimbriae
• Invades and replicates within bladder cells
• Produces alpha and beta hemolysins, causing lysis of urinary tract cells

Infections and Symptoms

• Cystitis: Infection of the bladder, causing dysuria and frequent urination
• Pyelonephritis: Ascends to kidneys, causing flank pain and more severe symptoms
• Diarrhea: Abdominal cramps, vomiting
• STEC: Bloody diarrhea, low-grade fever, potential HUS with symptoms like body swelling, confusion, and jaundice
• ETEC: Watery diarrhea, possible fever, and bloating
• EIEC: Bloody diarrhea and chills
• EPEC: Severe dehydration in children, prolonged diarrhea leading to malnutrition
• UPEC: UTI symptoms like dysuria, urinary frequency, and possible flank pain if kidneys are affected

Diagnosis

• Gram Staining: On stool or urine sample
• Culture: On eosin methylene blue agar
• HUS Diagnosis: Identifying Shiga toxin in the blood

Treatment

• Diarrhea: Hydration and rest, antibiotics in severe cases (e.g., doxycycline, cotrimoxazole)
• Hemolytic Uremic Syndrome (HUS): Supportive care (dialysis, corticosteroids, blood transfusions, plasmapheresis)
• UTIs: Antibiotics (cotrimoxazole, nitrofurantoin, fluoroquinolones, e.g., ciprofloxacin)

Intracellular Multiplication

• Invades and multiplies within epithelial cells, causing cell destruction
• Triggers a strong inflammatory response, leading to widespread epithelial damage and bloody diarrhea

Enteropathogenic E. coli (EPEC)

• Uses bundle-forming pili (BFP) to attach to intestinal epithelial cells
• Injects effector proteins via a Type III secretion system, causing actin cytoskeleton rearrangement
• Leads to pedestal formation and microvilli effacement
• Impairs absorption, causing watery diarrhea, primarily in children under two years of age

Uropathogenic E. coli (UPEC)

• Colonizes the periurethral area and ascends the urinary tract
• Uses type 1 fimbriae and P fimbriae to adhere to uroepithelial cells
• Invades and replicates within bladder cells
• Produces alpha and beta hemolysins, causing lysis of urinary tract cells
• Can cause cystitis, infection of the bladder, leading to dysuria and frequent urination
• Can ascend to the kidneys, causing flank pain and more severe symptoms (pyelonephritis)

Symptoms and Complications

• General symptoms: diarrhea, abdominal cramps, vomiting
• STEC: bloody diarrhea, low-grade fever, potential HUS with symptoms like body swelling, confusion, and jaundice
• ETEC: watery diarrhea, possible fever, and bloating
• EIEC: bloody diarrhea and chills
• EPEC: severe dehydration in children, prolonged diarrhea leading to malnutrition
• UPEC: UTI symptoms like dysuria, urinary frequency, and possible flank pain if kidneys are affected
• Risk of infection from cows (unpasteurized milk) and infected pets (notably puppies)

Pathogenesis and Virulence Factors

• Attachment: uses fimbri-like filaments and cell surface proteins like PEV-1 and CADF to attach to the mucosa of the small intestine and colon
• Invasion: drills into mucosa using its spiral shape and long flagella
• Releases cytotoxins like cytolethal distending toxins (CDT), causing cell damage and inflammation
• Complications: toxic megacolon, bacteremia, Guillain-Barré Syndrome, and reactive arthritis

Pathogenesis of Each Pathotype

Shiga-like Toxin-Producing E. coli (STEC)

• Attaches to host's intestinal cells using fimbriae
• Produces Shiga-like toxins (Stx1 and Stx2)
• Toxins cause damage to intestinal epithelium and blood vessels, resulting in inflammation
• Inflammation and vessel damage lead to fluid and blood leaking into the intestinal lumen, causing bloody diarrhea
• Systemic toxin release affects the kidneys, leading to hemolytic uremic syndrome (HUS)

Enterotoxigenic E. coli (ETEC)

• Uses fimbriae to adhere to intestinal mucosa
• Produces heat-labile enterotoxin (LT) and heat-stable enterotoxin (ST)
• LT activates adenylate cyclase, increasing cAMP, leading to chloride and water secretion
• ST activates guanylate cyclase, increasing cGMP, leading to electrolyte and water secretion
• Causes watery diarrhea without damaging the intestinal wall

Enteroinvasive E. coli (EIEC)

• Attaches to and invades intestinal epithelial cells
• Causes bloody diarrhea and chills

Escherichia coli (E. coli) Characteristics

• Rod-shaped bacteria
• Catalase-positive: Produces catalase enzyme
• Lactose fermenter: Produces beta-galactosidase and cleaves lactose into glucose and galactose
• Encapsulation: Covered by a polysaccharide layer called a capsule
• Motility: Has flagella for movement
• Facultative Anaerobe: Can live with or without oxygen

Tests and Cultivation

• Catalase Test: Adds hydrogen peroxide to a colony; the presence of catalase results in foaming
• Lactose Fermentation Test: Cultivation on lactose-containing media, resulting in acid production and color change from red to yellow
• Eosin Methylene Blue (EMB) Agar: Grows into black colonies with a greenish-black metallic sheen

Pathogenic Strains

• Attachment: Uses fimbriae to attach to host cells
• Serotype Classification: Based on antigens:
  • Somatic (O) antigens: On the cell membrane
  • Capsular (K) antigens: On the capsule
  • Fimbrial (F) antigens: On the fimbriae
  • Flagellar (H) antigens: On the flagella

Pathotype Classification

• Shiga-like toxin-producing E. coli (STEC):
  • Produces Shiga toxin, causes bloody diarrhea, and can lead to hemolytic uremic syndrome (HUS)
• Enterotoxigenic E. coli (ETEC):
  • Produces heat-labile and heat-stable enterotoxins, causing watery diarrhea
• Enteroinvasive E. coli (EIEC):
  • Invades and destroys intestinal epithelial cells, causing bloody diarrhea
• Enteropathogenic E. coli (EPEC):
  • Causes disease in children under two years old, leading to watery diarrhea
• Uropathogenic E. coli (UPEC):
  • Causes urinary tract infections (UTIs), including cystitis and pyelonephritis

Pathogenesis of Each Pathotype

Shiga-like Toxin-Producing E. coli (STEC)

• Attachment: STEC attaches to the host's intestinal cells using fimbriae
• Toxin Production: Produces Shiga-like toxins (Stx1 and Stx2)
• Intestinal Injury: Toxins cause damage to the intestinal epithelium and blood vessels, resulting in inflammation
• Bloody Diarrhea: Inflammation and vessel damage lead to fluid and blood leaking into the intestinal lumen
• Hemolytic Uremic Syndrome (HUS):
  • Systemic Toxin Release: Toxins enter the bloodstream, affecting the kidneys
  • Kidney Damage: Toxins bind to endothelial cells in the glomeruli, causing apoptosis and gaps in the capillary walls, leading to proteinuria
  • Inflammatory Response: Cytokines and chemokines are released, activating platelets and causing clot formation
  • Thrombocytopenia: Decreased platelet count due to clot formation
  • Hemolytic Anemia: Red blood cells are fragmented (schistocytes) as they pass through obstructed microvessels
  • Ischemic Kidney Damage: Clots obstruct arterioles, leading to kidney ischemia, reduced filtration, and uremia

Enterotoxigenic E. coli (ETEC)

• Attachment: ETEC uses fimbriae to adhere to the intestinal mucosa
• Toxin Production:
  • Heat-labile Enterotoxin (LT):
    • Mechanism: Activates adenylate cyclase, increasing cAMP, leading to chloride and water secretion
  • Heat-stable Enterotoxin (ST):
    • Mechanism: Activates guanylate cyclase, increasing cGMP, leading to electrolyte and water secretion
• Outcome: Causes watery diarrhea without damaging the intestinal wall

Enteroinvasive E. coli (EIEC)

• Attachment and Invasion: EIEC attaches to and invades intestinal epithelial cells