Pathogenic E. coli and Diarrhea Mechanisms

Economic Impact and Classification of Pathogenic E. coli

  • Economic impact of pathogenic E. coli in California:

    • In California alone, losses due to the 2005 outbreak reached up to _________.

    • In 2005, the total value of the California spinach crop was ___________.

  • Classification of Pathogenic E. coli:

    • There are many different types of pathogenic E. coli capable of causing disease.

    • They are categorized by the three types of disease they cause.

    • Broadly, there are five main types that cause intestinal disease:

      • Enteropathogenic E. coli (EPEC)

      • Enterotoxigenic E. coli (ETEC)

      • Enteroinvasive E. coli (EIEC)

      • Enteroaggregative E. coli (EAEC)

      • Enterohemorrhagic E. coli (EHEC)

Characteristics and Transmission of Enterohaemorrhagic E. coli (EHEC)

  • Transmission and Duration:

    • Mode of transmission: Occurs through contaminated food or person-to-person contact.

    • Typical duration of infection: Usually 77 days.

  • E. coli O157:H7:

    • This strain was first isolated in Argentina in 1971.

    • The infective dose is extremely low, requiring only 10−10010 - 100 organisms to cause infection.

    • Primary symptoms include bloody diarrhea and abdominal cramps.

    • Severe complications include:

      • Hemolytic-uremic syndrome (HUS).

      • The potential for kidney failure.

    • Infection statistics: Affects between 73,000−110,00073,000 - 110,000 people per year in the United States.

    • The condition is treatable.

Mechanisms of Gram-Negative Pathogen Infection

  • Use of Effectors:

    • Gram-negative (G-) pathogens infect cells using bacterially derived "effectors."

    • Pathogens utilize these effectors to control host cells.

  • Delivery Methods for Effectors:

    1. Location on the bacterial surface.

    2. Direct delivery into the host cell cytoplasm.

    3. Secreted toxins (exotoxins).

  • Targeted Host Functions for Modification:

    • Cytoskeleton structure.

    • Organellar functions.

    • Cellular structures, such as channels.

    • Signaling pathways.

The Type III Secretion System (T3SS)

  • Function and Structure:

    • The Type III Secretion System is a specialized needle apparatus used to inject proteins directly into the host cell.

    • It operates as a protein secretion system.

    • It requires energy in the form of ATP: ATP→ADP+Pi\text{ATP} \rightarrow \text{ADP} + P_i.

    • Components of the T3SS include:

      • The basal body, which spans the bacterial plasma membrane, the periplasm (peptidoglycan layer), and the bacterial outer membrane.

      • The needle, which extends from the bacterium to the eukaryotic cell membrane.

      • A pore that is formed in the eukaryotic cell membrane to allow effector entry.

      • Chaperone proteins that assist in the transport of effector proteins.

      • Specific proteins involved: Yop, YscJ, Sec, and the DEFGY complex.

  • Other Secretion Systems:

    • Pathogens may also utilize Type I, Type II, Type IV, and Type V secretion systems for various delivery needs.

E. coli Pedestals and Extra Cellular Pathogenesis

  • Mechanism of Pedestal Generation:

    • EHEC and EPEC are extracellular pathogens.

    • They inject effector proteins into host cells via the T3SS.

    • This injection causes the collapse of host cell microvilli.

    • The bacteria then generate "pedestals" upon which they sit.

  • Functional Significance of Pedestals:

    • The exact function of the pedestal is currently unknown.

    • Generation of pedestals is a hallmark of pathogenic E. coli disease; a functional secretion system leads to both disease and pedestal formation.

    • Pedestals allow the bacteria to "surf shop" on the surface of infected cells.

  • Impact of Mutation:

    • If the secretion system is mutated so it does not function properly, disease symptoms are greatly decreased or non-existent.

Shiga Toxin and Pathogenic Strategies of EHEC

  • Overview of Shiga Toxin:

    • In addition to injecting effector proteins, EHEC releases a potent toxin called "Shiga Toxin."

    • Shiga Toxin is a specific host-site exotoxin that targets the intestine.

    • It was first identified by Kiyoshi Shiga in the pathogen Shigella dysenteriae.

    • Because it produces this toxin, E. coli O157:H7 is classified as a Shiga-toxigenic E. coli (STEC).

  • Effects of Shiga Toxin:

    • Targets both endothelial and epithelial cells.

    • Coats blood vessels (BV).

    • Causes diarrhea and lesions through the release of blood.

  • Dual-Diarrhea Strategy of E. coli O157:H7:

    1. Injected effectors through the T3SS.

    2. The release of Shiga toxin.

The Physiological Role and Impact of Diarrhea

  • Evolutionary Perspectives:

    • Pathogen perspective: Diarrhea allows the pathogen to disseminate into the environment. The goal is to come to a host, replicate, and leave, rather than simply killing the host.

    • Host perspective: Diarrhea may be a defense mechanism used to rid the system of bacteria. Flushing out the intestinal tract removes much of the E. coli population.

  • Health Consequences:

    • Dehydration.

    • Loss of vital ions.

    • Hemorrhage (specifically associated with O157:H7).

    • Potential for death.

Molecular Mechanisms of Diarrhea Generation

  • General Physiological Changes:

    • Healthy tissue movement is characterized by standard gastrointestinal (GI) contractions, and the water content is < 85\% \, H_2O.

    • Bacterial-induced diarrhea involves increased GI motility, and the water content rises to > 85\% \, H_2O.

  • Specific Mechanisms by Pathogenic E. coli:

    1. Breach of Tight Junctions: Pathogens use effectors such as EspF, EspG, and Map to disrupt the junctions between cells, leading to intestinal leakage. EspF specifically interacts with claudin and occludin, leading to their removal via vesicles.

    2. Alteration of Water Channels (Aquaporins [AQP]): Bacterial effectors EspF and EspG cause the removal ("ripping out") of AQP2 and AQP3 from the cell membrane. These aquaporins are then sequestered into vesicles within the cell (e.g., AQP2-containing vesicles, AQP3-containing vesicles), decreasing the cell's ability to manage water.

    3. Alteration of Serotonin Function: The intestine acts as a "second brain" due to its small nerves. The pathogen affects the Enteric Nervous System (ENS) to produce diarrhea.

    4. Alteration of Ion Channels:

      • Shiga toxins attach to receptors like GM1.

      • This attachment triggers intracellular signaling cascades that alter ion channel functions.

      • Signaling involves increases in cyclic AMP (cAMP) and cyclic GMP (cGMP).

      • Activates Protein Kinase A (PKA) and Protein Kinase G (PKG/CGKII).

      • Targets include the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) and sodium (Na+Na^+) channels.

      • The net result is the massive release of ions and H2OH_2O from enterocytes into the intestinal lumen.