Comprehensive Study Notes on Escherichia Coli: Biology, Pathotypes, and Pathogenesis
Introduction and Course Context
The lecture is part of the second semester curriculum, following topics like anthrax and salmonella.
This session focuses on bacterial diseases, specifically Escherichia coli (E. coli), before moving into virology.
Learning Outcomes: - Explain the basics of the biology of fisheraceae koai (as transcribed) in the soul as a commensal and pathogen. - Describe the variation between strains or pathotypes of E. coli and their impact on host infection. - Describe the basis by which E. coli can cause internal disease. - Describe the importance of the E. coli life cycle in relation to urinary tract infections (UTI) and reproductive tract infections across species (e.g., chickens and humans). - Recognize the importance of E. coli as an opportunistic and systemic infection, specifically Avian Pathogenic E. coli (APEC) in poultry.
Biological Characteristics of E. Coli
Taxonomy and Morphology: - It is a Gram-negative bacterium. - Gram-negative definition: Characterized by a thin peptidoglycan layer and an outer membrane containing Lipopolysaccharides (LPS). - Under a microscope, they appear pink/faintly colored due to the Gram stain. - Shape: Rod-shaped (bacillus). - Size: Very small, ranging from to in diameter.
Metabolism: - It is a facultative anaerobe, meaning it can replicate and survive in both the presence and absence of oxygen.
Classification: Commensal, Pathogen, and Opportunist
E. coli is found naturally in the intestines and environment.
Commensal Bacteria: Normally present in the body (especially the Gastrointestinal Tract or GIT) without causing disease. It is part of the normal microflora (microbiome).
Pathogenic Bacteria: Strains that possess virulence factors enabling them to secrete toxins or damage cells to cause disease.
Opportunistic Bacteria: Strains that normally live as commensals but transition to pathogens when provided an opportunity, such as a depressed immune system, stress, or environmental changes.
The Microbiome: The number of bacterial cells in the human body is approximately times the number of human cells. E. coli within the microflora can be beneficial, specifically aiding in the biosynthesis of Vitamin K within the body.
Pathogenicity vs. Virulence: - Pathogenic: The ability to cause disease. - Virulence: Indicates the degree or level of the disease's severity (e.g., low virulent vs. severely virulent).
Classification of E. Coli Infections
Infections are generally divided into two main categories: - Intestinal (Enteric): Primarily causes diarrhea. Strains are referred to as enterogenic E. coli. - Extra-intestinal: Infection occurs outside the intestine (e.g., brain, urinary tract, bloodstream).
Key Distinction: In many E. coli infections, the bacteria remain in the intestine while their toxins travel through the blood or lymphatic system to other organs. This is a contrast to Salmonella, where the bacteria themselves often travel.
Pathotypes (Basal Types)
Intestinal Pathotypes: - ETEC (Enterotoxigenic E. coli): Secretes toxins that damage enterocytes, leading to water leakage and diarrhea. - EHEC (Enterohemorrhagic E. coli): Also known as STEC (Shiga toxin-producing E. coli). Causes bloody diarrhea. - EPEC (Enteropathogenic E. coli): Causes general diarrhea through specific attachment mechanisms. - EAEC (Enteroaggregative E. coli): Bacteria aggregate together to cause infection. - DAEC (Diffusely Adherent E. coli): Adheres to enterocytes. - EIEC (Enteroinvasive E. coli): A rare type that can invade cells and travel to the blood.
Extra-intestinal Pathotypes: - NMEC (Neonatal Meningitis Associated E. coli): Causes meningitis in newborn human infants. - APEC (Avian Pathogenic E. coli): Affects immune systems in poultry (chicken and turkey). - UPEC (Uropathogenic E. coli): Associated with urinary tract infections. - Other types cause cases of mastitis in cattle or metritis/endometritis in dogs and cats.
Structure and Antigenic Typing (Serotyping)
Serotyping is used to group E. coli strains based on surface structures that act as antigens: - O Antigen (Somatic Antigen): Located on the Lipopolysaccharide (LPS). There are more than different O antigens. - K Antigen: Located on the capsule. - H Antigen: Located on the flagella (used for movement). - F Antigen: Located on the fimbriae (e.g., K99 fimbriae in pigs are associated with increased virulence).
Virulence Factors and Genetic Acquisition
Adhesins: Structures like pili and fimbriae that allow the bacteria to stick to the intestinal wall against the flow of food and peristaltic movement.
Nutrient Scavenging Systems: Proteins secreted to collect essential nutrients like iron from the environment when the bacterium faces deficiency.
Toxins: Damage host cells and induce diarrhea.
LPS (Endotoxin): Stimulates the innate immune system, leading to inflammation, fever, and pyrexia.
Horizontal Gene Transfer (HGT): The primary way commensal E. coli becomes pathogenic by acquiring new DNA: - Transformation: Taking up DNA fragments from the environment. - Transduction: DNA transfer mediated by bacteriophages (viruses that infect bacteria). - Conjugation: Transfer of plasmids between bacteria via a pilus.
Type III Secretion System (T3SS)
Mechanism: Encoded by a specific region of DNA called the Pathogenicity Island, specifically the LEE locus (Locus of Enterocyte Effacement).
Function: The T3SS acts as a protein "needle." The bacteria create a receptor on the host cell (enterocyte), attach to it, and then use the needle to inject toxins, enzymes, and proteins directly into the cell.
Structural Changes: The host cell membrane undergoes rearrangement, forming a pedestal that keeps the E. coli fixed to the surface.
Note: E. coli generally does not invade the cell via this mechanism; it remains attached to the surface while injecting its factors.
Shiga Toxin (STX)
Some strains of E. coli produce a toxin resembling that of the microbe Shigella. This is called Shiga-like toxin.
The gene for this toxin is typically acquired through a bacteriophage.
Types: Shiga toxin 1 (STX1) and Shiga toxin 2 (STX2).
Human Impact: The serotype O157:H7 is a famous STEC that causes bloody diarrhea and can lead to kidney disease (nephropathy). It is often transmitted via raw or undercooked beef and contaminated vegetables.
Porcine Impact: Serotypes O138, O139, and O141 cause "Edema Disease" in pigs. Symptoms include edema in the eyelids, face, intestine, and brain due to the toxin traveling through the bloodstream.
Clinical Presentations in Animals
Avian Pathogenic E. coli (APEC): - Causes Colibacillosis, a systemic infection in chickens and turkeys. - High mortality in young birds ( or higher). - Signs include Airsaculitis (inflamed, cloudy air sacs with whitish exudate), Pericarditis (inflammation around the heart), Perihepatitis (fibrin deposits on the liver), and Salpingo-peritonitis (inflammation of the oviduct and ovary).
Cattle: - Causes Coliform Mastitis. It begins as local inflammation in the udder (hard, painful, inflamed) but can progress to systemic infection (fever, toxemia, septic shock) as toxins enter the blood.
Dogs: - Associated with Pyometra (pus in the uterus) and Urinary Tract Infections (UTI).
Antimicrobial Resistance (AMR)
E. coli has a high capacity for resisting antibiotics.
It is significant because it can produce resistance and transfer these resistance genes to other bacteria via horizontal gene transfer.
Questions & Discussion
Q: What determines the difference between pathogenic and virulent?
A: Pathogenic refers to the ability to cause disease. Virulence refers to the degree of severity of that disease.
Q: Does E. coli invade the enterocyte using the Type III Secretion System?
A: No. Unlike Salmonella, E. coli uses the T3SS to attach firmly to the host and inject toxins, but it remains on the surface (extracellular) rather than entering the cell.
Q: What causes the blood in bloody diarrhea associated with STEC?
A: The Shiga-like toxins cause significant damage to the blood vessels and cavities within the intestinal lining, leading to hemorrhage.
Q: How does Vitamin K relate to E. coli?
A: E. coli in the normal microflora assists in the biosynthesis of Vitamin K inside the body, which is distinct from dietary sources like kale.