MIID - MICRO - VIDEO LECT Gram Negative Bacilli: Salmonella, Shigella (Nash)

  • Salmonella

    • Gram negative bacilli

    • Lactose non-fermenter (Does not utilize lactose as a source of energy, meaning it cannot ferment lactose into acids or gases)

    • Motile

    • Produces H2S (hydrogen sulfide gas)

    • It can live inside a macrophage and outside of the cell

    • Non-Typhoidal Salmonella

      • Zoonotic transmission: poultry and reptiles

      • It is the causative agent of gastroenteritis: Potential to cause sepsis, meningitis in immunocompromised patients

    • Salmonella Typhi & Paratyphi (Only found in humans)

      • Found in developing countries especially in India or South-East Asia

      • Chronic asymptomatic carriage in the gallbladder

      • Causative agent of Typhoid fever (systemic)

    • TYPE III Secretion systems T3SS

     

    • A mechanism used by Gram-negative pathogenic bacteria to deliver effector proteins directly into host cells.

    • System resembles a needle-like apparatus

    • Key processes:

      1. Invasion: The bacteria attach to and invade the host cell.

      2. Effector injection: The T3SS injects effector proteins into the host cell.

      3. Cytoskeletal rearrangements: These effectors manipulate the host's cytoskeleton (actin filaments), helping the bacteria to invade or replicate inside the host.

      4. Intracellular replication: Once inside, the bacteria replicate in a protected environment.

    Components of the T3SS:

    • Basal body: Anchors the structure in the bacterial membrane (extends from the inner membrane (IM) to the outer membrane (OM)).

    • Needle: A long, hollow tube that crosses the host cell's membrane, allowing the injection of effectors.

    • Translocon: A complex at the tip of the needle that facilitates the delivery of effectors into the host cell.

    • Effector proteins: These proteins are the "payload" injected into the host cell, causing cellular changes like actin polymerization.

    Lists examples of bacteria that use T3SS:

    • Shiga-toxin-producing E. coli (causes severe intestinal diseases).

    • Enteropathogenic E. coli (EPEC, a major cause of diarrhea).

    • Enteroinvasive E. coli (EIEC, invades intestinal cells).

    • Salmonella (causes typhoid fever, food poisoning).

    • Shigella (causes dysentery).

    Salmonella Pathogenesis

    • Ingestion:

      • Acid-labile: Salmonella is sensitive to stomach acid, meaning a high infectious dose (>10,000 bacteria) is required to cause infection.

      • It needs to survive the acidic environment of the stomach before reaching the intestines.

    • Attachment:

      • The bacteria attach to small intestinal epithelial cells, which is the first step in establishing infection.

    • Internalization by Type III Secretion System (SPI-1)

      • SPI-1 (Salmonella Pathogenicity Island-1) encodes T3SS effectors that trigger actin polymerization, which facilitates the invasion of epithelial cells.

      • This results in the phagocytosis of Salmonella by the intestinal epithelial cells.

      • Essentially, the bacteria manipulate the host cell cytoskeleton to gain entry into the cells.

    • Replication in a protected vacuole

      • After being internalized, Salmonella replicates within a protected vacuole inside the host cells.

      • Once replication is sufficient, they are released into the lamina propria, a layer of the intestinal wall.

    • Recruitment of Immune Cells

    Resident macrophages:

    • These immune cells phagocytose (engulf) Salmonella.

    • The macrophages release IL-8 (interleukin-8) and TNF-α (tumor necrosis factor-alpha), which are inflammatory signals.

    • These signals recruit neutrophils to the site of infection.

    • Neutrophils are responsible for most of the localized gastrointestinal (GI) cellular damage, leading to inflammation and diarrhea.

    • Intracellular Survival by T3SS (SPI-2)

      • SPI-2 effectors allow intracellular survival by interfering with the host’s immune system.

      • They coat the phagosomal membrane, which prevents the phago-lysosomal fusion:

        • Normally, phagolysosomal fusion destroys bacteria by exposing them to digestive enzymes.

        • By blocking this fusion, Salmonella can survive and replicate inside macrophages.

    • Dissemination of systemic disease:

      • Systemic Salmonella infection is more likely in individuals with decreased cell-mediated immunity (e.g., those with low CD4+ T-cell activity, like patients with HIV).

      • These patients are at highest risk for severe, disseminated infection.

    • Risk factors for severe disease: Low CD4+ activity, common in immunocompromised individuals.

    • Symptoms of Enteric Fever (Typhoid Fever)

      • BUZZWORD FOR TYPHOIDAL SALMONELLA is "STEP-WISE FEVER" which comes with bradycardia.

      • Rose spots (colored macules) on the trunk and abdomen can be seen but it is rare.

      • Most sever develop in the 3rd week of the illness: Hepatosplenomegaly, Intestinal bleeding and perforation, sepsis.

      • CBC would show anemia and leukopenia

      • Late leukocytosis with sever abdominal pain = abdominal perforation

    • Treatment : Fluoroquinolones, Azithromycin

    • Vaccines:

      • Weakened form of bacteria (capsule) taken orally

      • Injectable IV (contains Vi capsular polysaccharide)

      • Tetanus toxoid conjugate = Vi capsular polysaccharide + tetanus toxoid protein

     

     

     

     

     

     

     

     

     

    SHIGELLA

    • Gram negative enteric (Enterobacteriaceae) bacilli

    • Causes gastroenteritis that leads to bloody diarrhea

    • Facultative Anaerobe

    • Lactose non-fermenter

    • Non-motile

    • Acid-stable (needs fewer microorganisms to cause infection)

    • The bacterium invades the human intestinal epithelium, particularly M cells in Peyer's patches, where it uses the host cells' actin cytoskeleton to propel itself from one cell to another

    • Facultative intracellular bacterium - can live independently inside and outside of its own cell

     

    Epidemiology

    • Comparison of Shigella Species:

      • Shigella sonnei: More prevalent in the U.S.

      • Shigella dysenteriae: Associated with high mortality due to its Shiga toxin.

      • Epidemiology: Often found in daycare outbreaks; person-to-person transmission

     

    Pathogenesis

    • M cells sample things in the lumen and bring back antigens to the immune cells on the other side of the epithelium

    • Shigella induces M cells so it can eat them up (phagocytize them) but they escape from the phagolysosome before they are degraded.

    • Once they are in the cytoplasm, shigella would use the cell's actin cytoskeleton to create a tail that it would use to propel itself.

    • After Shigella has invaded the lymphoid and enterocytes surrounding the M cells, it damages the tissue and releases cytokines.

    • You will see fecal blood and leukocytes

     

    Inflammatory Pathways and Symptoms

    • Mechanism of Invasion:

      • Actin polymerization leads to intracellular movement and increased inflammation.

    • Signs of Shigella Infection:

      • Severe colitis, bloody diarrhea, fever, significant abdominal pain.

    • AB-type Shiga toxin: Triggers extensive endothelial damage, leading to hemorrhage and possible hemolytic uremic syndrome.

    Reactive Arthritis Association

    • Post-infection complications: patients can develop arthritis weeks after, especially in HLA-B27 individuals.

    Summary and Conclusions

    • Differential Diagnosis:

      • Start with E. coli in the U.S.; Salmonella as a second guess, followed by Shigella for non-lactose fermenting infections.

     

    Shigella Dysenteriae(Similar to Shiga-Toxin E. coli STEC)

     

    A child with shigellosis, particularly caused by a strain of Shigella dysenteriae, is at risk of developing hemolytic uremic syndrome (HUS).

    HUS is common in pts below the ages of 10.

    In a clinical scenario, if a child presents with prodromal diarrhea, begins to recover after about a week, and then develops signs of acute renal failure, consider HUS as a potential diagnosis.

    • Once Shigella invades, it releases Shiga-toxin

    • Secretion of A/B type Shiga-toxin (phage-encoded)

    • The toxin can induce endothelial damage once it is in our bloodstream

    • Toxin's Mechanism of Action

      • The toxin binds to the 60S (hourglass that last 60seconds) subunit of ribosomes to inhibit translation

      • The toxin cleaves 28S subunit of the host 60S ribosome, which would stop translation.

      • Shigella use a TYPE III system to secrete inflammatory cytokines.

    • There is a drop in platelet count