Gram Negative Rods Overview

GRAM NEGATIVE RODS

DR SIMPOKOLWE

MCB 2310

2025

LECTURE OBJECTIVES

  • Differentiate between the characteristics of common Gram-negative rods within the Enterobacteriaceae family, including their key structural features and motility.
  • Explain the pathogenesis and clinical manifestations of infections caused by Escherichia coli, focusing on various pathogenic strains and their associated diseases, like traveler's diarrhea and hemorrhagic colitis.
  • Analyze the primary virulence factors for each genus, such as the polysaccharide capsule of Klebsiella or the urease enzyme of Proteus, and their roles in evading host immunity.
  • Compare and contrast the epidemiology and disease outcomes of typhoidal versus non-typhoidal Salmonella strains and the different modes of transmission for the plague caused by Yersinia pestis.
  • Summarize the diagnostic and treatment strategies for infections caused by these bacteria, emphasizing the importance of supportive care for self-limiting illnesses and the challenges posed by antibiotic resistance.

ENTEROBACTERIACEAE

  • The Enterobacteriaceae are a large and diverse family of gram-negative, rod-shaped bacteria.
  • Often referred to as enterics because their natural habitat is the intestinal tract of humans and animals.
  • This family includes many well-known pathogens as well as harmless commensals that are part of the normal gut flora.
  • Commonly causes both community-acquired and nosocomial infections.
  • A defining characteristic of Enterobacteriaceae:
    • Ability to ferment glucose.
    • Negative oxidase test result.
  • They are facultative anaerobes, meaning they can grow in both the presence and absence of oxygen.
  • Important medical concern due to the increasing prevalence of antibiotic resistance, including resistance to last-resort antibiotics.

REGIONS OF THE SMALL INTESTINE

  • Major regions include:
    • Duodenum
    • Jejunum
    • Ileum
  • Other anatomical features indicated:
    • Esophagus
    • Large intestine
    • Mucosa
    • Submucosa
    • Plicae circulares (valves of Kerckring)
    • Villus Structure:
    • Epithelium
    • Absorptive Cells (Enterocytes)
    • Goblet Cells
    • Endocrine Cells
    • Crypt (Lieberkühn gland)
    • Villi
    • Blood Capillaries

IMPORTANT GENERA

  1. Escherichia
  2. Klebsiella
  3. Salmonella
  4. Shigella
  5. Proteus
  6. Yersinia

GENUS ESCHERICIA

1. Structure/Growth/Species

  • Gram-negative, rod-shaped bacteria.
  • Facultative anaerobes (capable of growing in the presence or absence of oxygen).
  • Motile with peritrichous flagella.
  • Other species of importance include: E. fergusonii, E. albertii, E. hermannii, and E. marmotae.

2. Extracellular Products/Toxins and Functions

  • Shiga toxins (Stx): Produced by enterohemorrhagic E. coli. These potent toxins inhibit protein synthesis in host cells, leading to cell death and bloody diarrhea.
  • Heat-labile (LT) & Heat-stable (ST) toxins: Produced by enterotoxigenic E. coli. These toxins disrupt intestinal water and ion balance, resulting in profuse watery diarrhea.

3. Epidemiology

  • Ubiquitous presence in the environment and gut flora.
  • Transmitted via the fecal-oral route.
  • Common sources include contaminated food (notably undercooked meat and produce), water, and direct contact.

4. Pathogenesis

  • Pathogenic strains overcome host defenses using fimbriae to adhere to intestinal mucosa.
  • Secretion of toxins or effector proteins disrupt host cell function.
  • Mechanisms vary by strain type: enterotoxigenic, enterohemorrhagic, and enteropathogenic E. coli.

5. Virulence Factors

  • Adhesins: Facilitate binding to host cells for colonization.
  • Toxins: Induce specific disease symptoms.
  • Type 3 Secretion Systems: Inject bacterial proteins directly into host cells.
  • Capsules: Polysaccharide layer protecting bacteria from host immune system.

6. Immunity

  • Primarily mucosal immunity, concentrated in the gut.
  • Innate immune responses involve macrophages and neutrophils.
  • Adaptive immunity includes secretory IgA antibodies that neutralize toxins.
  • Cell-mediated immunity helps in controlling invasive infections.

7. Clinical Manifestations

  • Enterotoxigenic E. coli (ETEC): Main cause of "traveler's diarrhea".
    • Pathogenesis: Adheres to the small intestine, secreting LT and ST toxins.
  • Enteropathogenic E. coli (EPEC): Frequent cause of infantile diarrhea.
    • Pathogenesis: Utilizes a Type III secretion system to inject effector proteins, leading to destruction and flattening of intestinal microvilli.
  • Enterohemorrhagic E. coli (EHEC): Results in hemorrhagic colitis characterized by severe abdominal cramps and bloody, non-febrile diarrhea.
    • Pathogenesis: Colonizes the large intestine, secreting potent Shiga toxins that travel via bloodstream causing tissue damage.
  • Enteroinvasive E. coli (EIEC): Invades and destroys epithelial cells of the colon leading to a dysentery-like illness.
    • Pathogenesis: Multiplies within colonic cells, utilizing the host cell's actin to facilitate intercellular movement.
  • Enteroaggregative E. coli (EAEC): Associated with persistent watery diarrhea, especially in children and travelers.
    • Pathogenesis: Adheres in a "stacked-brick" pattern, producing biofilms that prolong mucosal colonization and induce fluid secretion.

8. Hemolytic-Uremic Syndrome (HUS)

  • Life-threatening complication mainly from Shiga toxin-producing E. coli (EHEC).
  • HUS features include:
    • Hemolytic Anemia: Destruction of red blood cells.
    • Thrombocytopenia: Low platelet count.
    • Acute Renal Failure: Rapid kidney function decline.
  • Most prevalent in young children and the elderly.

9. Diagnosis

  • Culture: From stool, urine, or blood utilizing specific media like sorbitol-MacConkey agar for EHEC.
  • Polymerase Chain Reaction (PCR): Detects specific virulence genes.

10. Treatment

  • Most E. coli infections are self-limiting requiring only supportive care (fluid replacement).
  • Antibiotics typically avoided in EHEC due to HUS risk; other strains may be treated with Macrolides and Fluoroquinolones.

11. Prevention

  • Public health measures and personal hygiene critical;
    • Key practices: proper handwashing, thorough cooking, and access to safe water.

GENUS KLEBSIELLA

1. Structure/Growth/Species

  • Gram-negative, rod-shaped and non-motile bacteria.
  • Known for having a prominent polysaccharide capsule.
  • Facultative anaerobes; significant species include K. pneumoniae, K. oxytoca, and K. granulomatis.

2. Toxins

  • Endotoxin (Lipopolysaccharide or LPS): Major component of the outer membrane; released upon cell lysis, causing fever, inflammation, and septic shock.

3. Epidemiology

  • Found in normal flora of the human gastrointestinal tract, skin, and nasopharynx.
  • Common in environment: soil, water, plants.
  • Leading cause of nosocomial infections, notably in intensive care units with risk factors including weakened immune systems and the use of medical devices.

4. Pathogenesis

  • Klebsiella evades host defenses primarily via its large polysaccharide capsule that inhibits phagocytosis.
  • Adheres to surfaces using adhesins, allowing it to colonize and form biofilms.
  • Can cause both localized and systemic infections.

5. Virulence Factors

  • Polysaccharide Capsule: Main virulence factor providing evasion from phagocytosis and immune system killing.
  • Adhesins: Facilitate attachment to host tissues.
  • Siderophores: Iron-scavenging molecules essential for bacterial growth.

6. Host Immunity

  • Host defense primarily targets against the capsule.
  • Specific antibodies can lead to efficient phagocytosis; however, the capsule inhibits the complement activation pathway.

7. Clinical Manifestations

  • Pneumonia: Inflammation of air sacs in lungs characterized by thick, gelatinous, blood-tinged sputum.
  • Urinary Tract Infections (UTIs): Frequent in catheterized or immunocompromised patients, symptoms include dysuria, hematuria, pyelonephritis.
  • Liver Abscess: Potentially spreading to the brain and eyes.
  • Sepsis: May develop from other infections, with significant systemic inflammatory response.

8. Diagnosis

  • Culture: From clinical samples (blood, urine, sputum) showing mucoid colonies.
  • Biochemical Tests: Differentiation from other Enterobacteriaceae members.
  • Molecular Methods: PCR for antibiotic resistance detection.

9. Treatment

  • Dependent on infection site and antibiotic susceptibility; widespread resistance, particularly to cephalosporins and carbapenems, poses a treatment challenge.
  • Carbapenem-Resistant Klebsiella pneumoniae (CRKP): Significant health concern requiring combination antibiotic therapy.

10. Prevention

  • Strict hospital infection control measures; proper hand hygiene and isolation.
  • Proper care of medical devices; careful antibiotic use.

GENUS SALMONELLA

1. Structure/Growth

  • Gram-negative, rod-shaped and motile bacteria with peritrichous flagella.
  • Divided into two main species: S. enterica and S. bongori, with S. enterica having the majority of pathogenic serotypes.

2. Typhoidal vs Non-Typhoidal

  • Typhoidal Serotypes: Cause systemic disease known as typhoid fever.
    • S. Typhi: The cause of typhoid fever, which can be severe and life-threatening.
    • S. Paratyphi: Causes a milder illness called paratyphoid fever.
  • Non-Typhoidal Salmonella (NTS): Broad host range; most common foodborne illness cause.
    • S. Typhimurium: Common cause of gastroenteritis, often linked to contaminated poultry.
    • S. Enteritidis: Frequently associated with contaminated eggs.

3. Extracellular Products/Toxins

  • Endotoxin (LPS): Released upon cell death, causing fever and shock.
  • Enterotoxins: Can lead to fluid secretion and diarrhea but not the main mechanism.
  • Cytotoxins: Cause cellular toxicity particularly in the intestines.

4. Epidemiology

  • Non-typhoidal strains mainly found in animals; common cause of human food poisoning through contaminated food and water.
  • S. Typhi: Human-specific pathogen without an animal reservoir; transmitted person-to-person via fecal-oral route.

5. Pathogenesis

  • Initial Invasion: Surviving stomach acidity, bacteria use Type III secretion systems to engender host cell engulfment.
  • Non-Typhoidal: Multiply within intestinal cells, causing inflammation.
  • Typhoidal Strains: Taken up by macrophages, which helps the bacteria to disseminate systemically.
  • Carrier State: Around 3% of typhoid cases can lead to asymptomatic carriers post-infection.

6. Virulence Factors

  • Adhesins (Fimbriae): Aid attachment and colonization in host cells.
  • Type III Secretion System: Essential for host cell invasion.
  • Capsule: Specific to some strains such as S. Typhi, aiding in evading phagocytosis.

7. Clinical Manifestations

  • Gastroenteritis: Characterized by nausea, vomiting, non-bloody diarrhea, fever, and abdominal cramps; usually self-limiting.
  • Enteric Fever (Typhoid Fever): Severe systemic infection with prolonged fever and characteristic rose spot rash.

8. Diagnosis

  • Culture: Isolated from stool for gastroenteritis and blood for enteric fever.
  • Biochemical Tests: Identify the genus.
  • Serology: Widal test detects antibodies against S. Typhi.

9. Treatment

  • Gastroenteritis: Supportive care like fluid and electrolyte replacement.
  • Enteric Fever: Requires antibiotics like fluoroquinolones or macrolides.

10. Prevention

  • Food Hygiene: Proper food handling and cooking.
  • Water Safety: Ensure access to clean drinking water.
  • Vaccination: Available for typhoid fever, particularly for travelers.

GENUS SHIGELLA

1. Structure/Growth

  • Gram-negative, rod-shaped, and non-motile bacteria.
  • Four main species: Shigella dysenteriae, Shigella flexneri, Shigella boydii, Shigella sonnei.

2. Extracellular Products/Toxins

  • Shiga Toxin: Key virulence factor produced by S. dysenteriae causing cell death and bloody diarrhea, linked to HUS.
  • Endotoxin (LPS): Contributes to fever associated with infections.

3. Epidemiology

  • Human-exclusive reservoir; primarily transmitted via fecal-oral route with a low infectious dose making it highly contagious.
  • Outbreaks frequent in areas with poor sanitation such as daycares and nursing homes.

4. Pathogenesis

  • Enters epithelial cells of the colon using Type III secretion systems and invasion proteins.
  • Propagation through actin-based motility causes inflammation and ulceration leading to dysentery.
  • More severe outcome from S. dysenteriae due to the production of Shiga toxin leading to systemic complications like HUS.

5. Virulence Factors

  • Invasion Plasmid Antigens (Ipa): Help induce host cell engulfment.
  • Type III Secretion System: Critical for injecting virulence proteins for invasion.
  • Shiga Toxin: Enhances disease severity and complications.

6. Host Immunity

  • Cell-mediated immunity: Crucial for clearing infection.
  • Mucosal Immunity: Local secretory IgA plays a role in preventing colonization.

7. Clinical Manifestations

  • Bacillary Dysentery: Initial watery diarrhea progressing to severe abdominal cramps and bloody, mucoid stools.
  • Systemic Complications: May include HUS, toxic megacolon, and reactive arthritis.

8. Diagnosis

  • Stool Culture: Primary diagnostic method.
  • PCR-based tests: Rapid detection of Shigella DNA.

9. Treatment

  • Antibiotics: Recommended to reduce symptoms duration and shedding.
  • Supportive Care: Essential for fluid and electrolyte replacement to prevent dehydration.

10. Prevention

  • Hand Hygiene: Key preventive measure due to low infectious dose.
  • Sanitation: Improvement in human waste disposal critical for community prevention.

GENUS PROTEUS

1. Structure/Growth

  • Gram-negative, rod-shaped and highly motile bacteria.
  • Characteristic swarming motility on solid agar media.
  • Clinically significant species: Proteus mirabilis and Proteus vulgaris.

2. Extracellular Products/Toxins

  • Urease: Enzyme hydrolyzing urea, elevating urine pH crucial for urinary tract infection pathogenesis.
  • Endotoxin (LPS): Contributes to fever and symptoms during infection.

3. Epidemiology

  • Found in soil, water, and normal human intestinal flora.
  • Infections generally opportunistic, often endogenous or hospital-acquired, especially in catheterized patients.

4. Pathogenesis

  • Bacteria enter through urethra, ascend to bladder leading to pyelonephritis.
  • Urease production leads to an alkaline environment toxic to bladder epithelium.
  • Swarming motility aids rapid colonization and persistent infections.

5. Virulence Factors

  • Urease: Main virulence factor impacting urinary pH.
  • Fimbriae: Aid adherence to urinary tract epithelial cells.
  • Hemolysins: Damage host cells.

6. Host Immunity

  • Host Response: Inflammatory response and urine flow help clear infection.
  • Evasion Mechanisms: Urease-induced stones and motility allow Proteus to evade defenses, causing persistent infections.

7. Clinical Manifestations

  • UTIs: Commonly present as cystitis or pyelonephritis.
  • Struvite Stones: Formation of stones during infection can lead to chronic issues.
  • Systemic Complications: If infection spreads, it may lead to bacteremia and sepsis.

8. Diagnosis

  • Urine Culture: Identify bacteria from urine samples.
  • Swarming Motility: Visible on agar cultures as a key identifier.
  • Urease Test: Positive test confirms urease presence.

9. Treatment

  • Antibiotics: Include Fluoroquinolones, Cephalosporins, and Aminoglycosides, subject to resistance testing.
  • Surgical Removal: Required for struvite stones to clear infections.

10. Prevention

  • Hygiene: Essential in clinical settings for preventing infections.
  • Catheter Care: Meticulous care minimizes colonization risks.
  • Early Detection: Crucial to prevent chronic problems arising from UTIs.

GENUS YERSINIA

1. Structure/Growth

  • Gram-negative, rod-shaped bacteria, non-motile at body temperature but motile at lower temperatures (Y. enterocolitica).
  • Major pathogenic species include:
    • Yersinia enterocolitica (causes gastroenteritis).
    • Yersinia pseudotuberculosis (causes similar but rarer gastroenteritis).
    • Yersinia pestis (the causative agent of plague).

2. Extracellular Products/Toxins and Virulence Factors

  • Type III Secretion System: Injects proteins into host cells, disrupting immune signaling.
  • Yersinia Outer Proteins (Yops): Inhibit phagocytosis and apoptosis in immune cells.
  • Fraction 1 (F1) Capsular Antigen: Protects Y. pestis from immune engulfment.
  • Plasminogen Activator (Pla): Facilitates bacterial spread from initial site by degrading fibrin.

3. Epidemiology

  • Reservoir varies by species:
    • Y. enterocolitica found in pigs, cattle, and rodents.
    • Y. pestis mainly a zoonotic pathogen in wild rodents, transmitted through fleas and human contact.

4. Pathogenesis

  • Y. enterocolitica invades intestinal mucosa causing severe inflammation and mimicking appendicitis.
  • Y. pestis survives inside phagocytes, spreads to lymph nodes forming “buboes,” can lead to septic plague.

5. Clinical Manifestations

  • Gastroenteritis caused by Y. enterocolitica characterized by fever and diarrhea.
  • Bubonic Plague symptoms include fever, chills, and painful lymph node swelling.
  • Pneumonic Plague: Severe lung infection, highly contagious leading to rapid deterioration.

6. Diagnosis

  • Culture: Isolation from stool (Y. enterocolitica) or blood/bubo aspirates (Y. pestis).
  • PCR: Rapid and specific diagnosis.
  • Serology: For retrospective diagnosis via antibody detection.

7. Treatment

  • Antibiotics: Essential for plague, typical treatment includes streptomycin or gentamicin.
  • Supportive Care: Critical for managing gastroenteritis symptoms.

8. Prevention

  • Food Safety: Proper cooking and handling of food, especially pork, prevent Y. enterocolitica infections.
  • Vector and Rodent Control: Essential for preventing Y. pestis outbreaks.
  • Vaccination: Available but not routinely used due to side effects and limited efficacy.