Escherichia coli

Learning Objectives for the Module on Gram-Negative Enteric Bacilli

• Describe the nature, characteristics, and classification of Escherichia.

• Identify morbidity and mortality rates as well as the distribution of Escherichia-related diseases in the Philippines.

• Indicate the age, sex, and racial predilection of the infectious diseases caused by these bacteria.

• Discuss the microbiological pathogenesis and the corresponding immunologic response.

• List clinical manifestations based on microbiological etiopathogenesis.

• Appraise the presence of complications and/or medical emergencies.

• Correlate diagnostic markers with corresponding diagnostic examinations.

• Identify therapeutic interventions and correlate the mechanism of action of drugs.

• Explain epidemiological characteristics and preventive interventions, including the role of passive and active immunization.

• Identify appropriate preventive health protocols and wellness programs to educate patients, families, and caregivers.

Medical Importance of Escherichia

• Escherichia serves as both a beneficial component of the normal human gut flora and a major pathogen.

• It is the leading cause of community-acquired and hospital-acquired urinary tract infections (UTIs).

• It is a primary driver of life-threatening conditions, including sepsis, pneumonia, and neonatal meningitis.

• Pathogenic strains, such as Shiga toxin-producing E. coli (STEC), cause severe foodborne gastrointestinal illness and acute kidney complications like Hemolytic Uremic Syndrome (HUS).

Classification and Cellular Structure

• Microscopic Morphology:

  • Short, straight, rod-like bacilli.

  • Gram-negative staining: The thin cell wall takes up the safranin counterstain, appearing pink to red.

  • Arrangement: Found as single cells or in random small structures.

• The Cell Envelope (Gram-Negative Structure):

  • Outer Membrane: An asymmetrical lipid bilayer. The inner leaflet consists of phospholipids, while the outer leaflet is composed of Lipopolysaccharides (LPS). LPS acts as a barrier to toxic molecules (including certain antibiotics) and functions as an endotoxin (O antigen) that triggers strong immune responses in humans.

  • Periplasmic Space: The region located between the outer and inner membranes. It contains a thin layer of peptidoglycan, which provides structural integrity and shape, as well as various enzymes involved in nutrient transport and antibiotic degradation.

  • Inner (Plasma) Membrane: A symmetric phospholipid bilayer that regulates nutrient transport, houses respiratory chain machinery, and generates cellular energy in the form of ATP.

• Surface Appendages:

  • Flagella: Long, helical protein filaments arranged peritrichously (distributed over the entire cell surface). They rotate like propellers to enable swimming motility and chemotaxis.

  • Fimbriae (Pili): Short, hair-like projections covering the surface. These are critical virulence factors. Type 1 pili allow the bacteria to bind to bladder cells (causing UTIs), while sex pili enable the transfer of genetic material, such as antibiotic-resistant plasmids, via conjugation.

• Internal Components:

  • Nucleoid: Lacks a membrane-bound nucleus. Genetic material consists of a single, circular, double-stranded DNA chromosome condensed into the nucleoid region.

  • Plasmids: Small, circular, extrachromosomal DNA molecules that replicate independently. They often carry advantageous genes such as those encoding for toxins or antibiotic resistance.

  • Ribosomes ($70S$): Scattered throughout the cytoplasm, these molecular machines translate genetic code into proteins and are targets for several antibiotic classes.

Cultural, Physiologic, and Biochemical Properties

• Cultural Growth Characteristics:

  • Blood Agar: Large, circular, grey, and moist colonies.

  • MacConkey Agar: Pink/red colonies (indicates rapid lactose fermentation).

  • Eosin Methylene Blue (EMB) Agar: Produces colonies with a distinct metallic green sheen due to intense acid production from rapid lactose fermentation.

  • Sorbitol MacConkey (SMAC) Agar: Used to identify E. coli O157:H7, which produces colorless or pale colonies, whereas regular E. coli turns pink.

• Biochemical Identification (IMViC Series):

  • Indole Test: Positive ($+$).

  • Methyl Red (MR) Test: Positive ($+$).

  • Voges-Proskauer (VP) Test: Negative ($-$).

  • Citrate Utilization Test: Negative ($-$).

  • Triple Sugar Iron (TSI): Results in Acid/Acid ($A/A$) with Gas production; produces no $H_2S$ (no blackening).

  • Other Markers: Motile, catalase-positive, and oxidase-negative.

Antigenic Structures and Classification

• O Antigen (Somatic): The outermost part of the LPS layer in the outer membrane. It is heat-stable and consists of repeating sugar units. It helps bacteria evade the host immune system.

• H Antigen (Flagellar): Located on the flagella. It is a heat-labile protein that elicits strong immune responses. Classification is based on peritrichous flagella.

• K Antigen (Capsular): An acidic polysaccharide layer forming a capsule. It is heat-labile and prevents phagocytosis. In Salmonella Typhi, this is referred to as the Vi antigen.

• F Antigens (Fimbrial): Proteinaceous, hair-like appendages (fimbriae/pili) that allow tight adherence to host tissues like intestinal walls or the urinary tract.

• Antigenic Formula Example: O55:K5:H21.

• Comparative Data: E. coli has more than $180$ O strains, $53$ H strains, and more than $80$ K strains.

Epidemiology in the Philippines

• Morbidity:

  • Diarrheal diseases (Acute Gastroenteritis) are in the Top 10 causes of morbidity, especially during rainy seasons due to contaminated deep wells or community water systems.

  • Bloodstream infections: E. coli accounts for $75\%$ of UTIs and over $20\%$ of bacteremia cases in tertiary hospitals.

• Mortality:

  • Diarrhea and gastroenteritis are leading causes of death for children aged $1$ to $4$ years old due to severe dehydration.

  • E. coli bacteremia has a mortality rate of $37.0\%$ arising from untreated UTIs and abdominal infections.

• High-Risk Populations:

  • Children under $5$ years old.

  • Elderly over $60$ years old (risk of sepsis from UTIs).

  • Informal settlers with poor WASH (Water, Sanitation, and Hygiene) infrastructure.

  • Patients with chronic medical conditions or those using medical catheters.

Pathogenesis of Escherichia coli

• 1. Adherence:

  • Uropathogenic E. coli (UPEC) use P fimbriae to lock onto kidney cells.

  • Diarrheagenic strains (EPEC/EHEC) use intimin (an anchoring protein) to glue to the gut wall.

• 2. Invasion and Evasion:

  • Uses the Type III Secretion System to inject bacterial proteins into human cells. This forces the host cell's actin skeleton to morph, creating "attaching and effacing" (A/E) lesions, leading to an inability to absorb water and triggering diarrhea.

• 3. Toxin Production:

  • Endotoxin (LPS/O Antigen): In bacteremia, massive amounts spill into the blood, leading to septic shock (dangerous drop in blood pressure).

  • Shiga Toxins (Stx): Secreted by EHEC (e.g., O157:H7). Shuts down protein production in human cells, causing colonic hemorrhage (bloody diarrhea) and Hemolytic Uremic Syndrome (HUS).

  • Enterotoxins (LT and ST): Secreted by Enterotoxigenic E. coli (ETEC). Causes intestinal cells to pump out water and salt, leading to acute watery diarrhea.

Host Immunologic Response

• Mechanical Defense: The body uses direct physical flushing, such as hyper-motility (diarrhea) in the GIT and increased frequency of urination in the GUT.

• Innate Immune System: TLR4 recognizes the O antigen/LPS. Cytokines like TNF-alpha, IL-1, and IL-6 are released, causing vasodilation, inflammation, and fever. Neutrophils phagocytose the bacteria, resulting in pyuria (pus in urine).

• Adaptive Immune System: Plasma cells create specific antibodies against O, H, and K antigens. These antibodies neutralize toxins, block motility by binding flagella, and opsonize the capsule to facilitate phagocytosis.

Clinical Manifestations

• Gastroenteritis:

  • Watery (Non-Bloody) Diarrhea: Caused by ETEC (Traveler’s diarrhea) and EPEC (infantile diarrhea). Symptoms include loose stools, abdominal cramps, nausea, vomiting, and signs of dehydration (dry mouth, dark urine).

  • Bloody Diarrhea (Hemorrhagic Colitis): Caused by EHEC. Characterized by excruciating stomach cramps and grossly bloody stools. High fever is usually absent, which helps differentiate it from Shigella.

• Urinary Tract Infections (UTI): E. coli causes over $80\%$ of community-acquired UTIs. Symptoms include dysuria, urgency, frequency, and cloudy urine. May progress to pyelonephritis.

• Urosepsis/Bacteremia: Results from untreated kidney or abdominal infections. Signs include high fever or hypothermia, tachycardia, tachypnea, and septic shock leading to multi-organ failure.

• Neonatal Meningitis: Common in premature infants; strains with protective capsules cross the blood-brain barrier. Signs include poor feeding, irritability, high-pitched crying, and a bulging fontanelle.

Complications and Emergencies: Hemolytic Uremic Syndrome (HUS)

• Develops in $5\%$ to $10\%$ of those infected with Shiga-toxin E. coli (STEC).

• Occurs $5$ to $10$ days after diarrheal symptoms seem to improve.

• Triad of Symptoms (Medical Emergency):

  • Hemolytic Anemia: Extreme fatigue and pallor.

  • Thrombocytopenia: Petechiae and low platelet count.

  • Acute Kidney Failure: Oliguria (low urine output) or Anuria (no urine output), leading to edema in the face and extremities.

Diagnostic Laboratory Methods

• Specimen Collection: Midstream clean-catch urine, stool samples, or blood culture bottles.

• Microscopy: Gram stain showing Gram-negative short rods. Urinalysis dipstick showing positive ($+$) nitrites and positive ($+$) leukocyte esterase.

• Media: Isolation on MacConkey (bright pink colonies with bile salt precipitation), EMB (metallic green sheen), and SMAC (colorless for O157:H7).

• Serologic/Molecular Identification:

  • Latex Agglutination: Uses antibody-coated beads for specific O or H antigens.

  • ELISA: Detects Shiga toxins directly in stool.

  • PCR (Modern Gold Standard): Detects specific virulence genes: $stx1$ and $stx2$ (STEC/EHEC), $lt$ and $st$ (ETEC), and $eae$ (EPEC).

• Automated Systems: VITEK 2 or MicroScan for identification and antibiotic resistance profiling within $8$ to $24$ hours. MALDI-TOF MS identifies bacteria via "protein fingerprinting" of ribosomal proteins within minutes.

Therapeutic Interventions

• Intestinal Infections:

  • Mild/Moderate: Oral Rehydration Salts (ORS).

  • Severe: IV Lactated Ringer's solution.

  • Contraindications: Loperamide (anti-diarrheal) is strictly contraindicated in suspected EHEC. Antibiotics are generally restricted in EHEC/STEC as they can trigger HUS.

• Extraintestinal Infections:

  • Uncomplicated UTI: Nitrofurantoin, Fosfomycin, or Trimethoprim-Sulfamethoxazole.

  • Complicated UTI/Pyelonephritis: Ceftriaxone or Ciprofloxacin.

  • Sepsis (ESBL-producing strains): Carbapenems or Piperacillin-Tazobactam.

  • Neonatal Meningitis: High-dose IV combination of Cefotaxime or Ceftriaxone paired with Ampicillin or Gentamicin.

Prevention and Health Promotion

• Personal Hygiene: Rigorous handwashing and avoidance of cross-contamination.

• Food Safety: Thorough cooking (E. coli is heat-labile) and produce washing.

• Water Safety: Infrastructure improvements, water treatment (boiling/chlorination), and well maintenance.

• Institutional Controls: Antimicrobial stewardship and robust hospital infection control protocols.