Gram Negative Bacilli

Gram Negative Bacilli
Overview of Enterics

  • Definition: Enterics are a group of enterobacteria that are commonly found in the intestinal tract as part of the normal flora but can also cause diseases.

  • Major Groups:

    • Enterobacteriaceae: includes organisms like Salmonellae, Shigellae, and E. coli.

    • Vibrionaceae: includes Vibrio and Campylobacter.

    • Pseudomonadaceae: includes Pseudomonas.

    • Bacteroidaceae.

  • Classification: Organisms are categorized based on biochemical and antigenic properties.

Biochemical Classifications

  • Lactose Fermentation

    • EMB Media:

    • Lactose fermenters appear dark purple or black.

    • Suppresses growth of Gram-positive bacteria.

    • MacConkey Media:

    • Lactose fermenters appear pink-purple.

    • Also inhibits Gram-positive bacteria.

    • Type of Media: Both EMB and MacConkey media are classified as selective and differential.

  • Additional Tests:

    • H2S production: can indicate specific enteric bacteria.

    • Hydrolysis of Urea: distinguishes organisms based on urea processing ability.

    • Gelatin Liquefaction: indicates proteolytic activity.

    • Amino Acid Decarboxylation:

    • Negative result for arginine.

    • Positive results for ornithine and lysine, with controls demonstrating expected outcomes.

Classification Using Surface Antigens

  • O-antigen: The outermost layer of lipopolysaccharides (LPS); variable and unique to different enterics.

  • K-antigen: Covers the O-antigen providing an additional protective layer.

  • H-antigen: The flagellar subunit, present only in motile bacteria.

Diseases Caused by Enterics

  • Enteric bacteria can cause a range of illnesses primarily characterized by diarrhea, which can occur with various complications:

    1. Diarrhea without systemic invasion:

    • Bacteria adhere to intestinal cells without invading.

    • Exotoxins lead to fluid and electrolyte loss, resulting in watery diarrhea and no fever.

    • Example: Vibrio cholerae.

    1. Diarrhea with intestinal cell invasion:

    • Pathogen virulence factors enable cell binding and invasion, leading to bloody stools.

    • Presents with fever due to immune response.

    • Example: Shigella.

    1. Diarrhea with invasion of lymph nodes and bloodstream:

    • Symptoms include abdominal pain, diarrhea with white and red blood cells, fever, and increased leukocyte counts.

    • Examples include Salmonella Typhi, Yersinia enterocolitica, and Campylobacter jejuni.

  • Other Infections:

    • Include urinary tract infections, pneumonia, bacteremia, and sepsis.

    • Nosocomial infections often involve:

      • E. coli.

      • Klebsiella pneumoniae.

      • Proteus mirabilis.

      • Enterobacter.

      • Serratia.

      • Pseudomonas aeruginosa, known as an opportunistic pathogen.

      • Common in burn patients.

Salmonellae

  • Belongs to the Enterobacteriaceae family.

  • Lactose Fermentation: Salmonella is unable to ferment lactose.

  • Animal Reservoirs: Most species have animal hosts, except S. enterica serovar Typhi, which only infects humans.

  • Types of infections: range from enterocolitis, enteric fever to opportunistic infections, septicemia, and osteomyelitis.

  • Species: Two main species include S. enterica and S. bongori.

  • Specific serovar infections:

    • Enterocolitis:

    • S. enterica serovar Enteritidis.

    • S. enterica serovar Typhimurium.

    • Typhoid fever:

    • S. enterica serovar Typhi.

    • S. enterica serovar Paratyphi.

Pathogenesis of S. enterica serovar Enteritidis

  • Factors influencing pathogenesis:

    • Dose of the organism (minimum of 10^5 needed).

    • Host immune status.

    • Virulence of the specific strain.

    • Incubation period: 6-48 hours; occurs predominantly in the small intestine.

Symptoms of Enterocolitis

  • Classic symptoms include:

    • Nausea.

    • Vomiting.

    • Profuse diarrhea.

    • Abdominal pain.

    • Associated fever, chills, headache, and myalgia.

  • Recovery typically in 2-3 days.

  • Septicemia is rare, diagnosis achieved through stool cultures on MacConkey media.

Transmission and Prevention for Enterocolitis

  • Common sources of S. enterica serovar Enteritidis include:

    • Contaminated poultry, eggs, meat, and milk.

    • Person-to-person transmission is also possible.

  • Most cases are under-reported and occur at home.

  • Antibiotics: Not recommended due to risk of complications.

Enteric Fever

  • Causative Agent: S. enterica serovar Typhi; includes paratyphoid fever from serotypes such as Paratyphi A.

  • Characteristics:

    • Generalized infection with bacteria multiplying in lymphoid tissue leading to potential ulceration.

    • Symptoms include necrosis of intestinal lymphoid tissue, hemorrhage, and perforation.

  • Untreated cases show: approximately 10% mortality rate.

  • Carriers: Convalescent carriers may excrete pathogens for 3 months; chronic carriers (1-2%) may excrete for at least 6 months, sometimes life-long.

Diagnosis and Sources

  • Diagnosis Methods:

    • Bacterial isolation from blood during the first week.

    • Stool and urine cultures obtained during the second and third weeks.

  • Infective Dose: Needs about 10^6 organisms.

  • Sources of Contamination:

    • Contaminated drinking water, shellfish, and dairy products.

  • Prevention Strategies: Include clean handling of food, water treatment, and safe sewage disposal. A vaccine is available but is effective only against smaller bacterial loads.

Nomenclature for Examinations

  • For exam purposes:

    • Stomach issues linked to Salmonella are referred to as:

    • Salmonella enteritidis or Salmonella typhimurium.

    • Typhoid fever is associated with:

    • Salmonella typhi or Salmonella paratyphi.

Escherichia coli

  • Characteristics: Most numerous aerobic bacteria in the normal gut flora and known for lactose fermentation.

  • Pathological Role: Causes disease in other body systems and is responsible for 85% of urinary tract infections.

  • Cultural Marker: Diagonal appearance on MacConkey agar.

E. coli Gastroenteritis

  • Different pathogenic categories:

    • Enterotoxigenic E. coli: Associated with infant diarrhea in developing countries and travel-related diarrhea due to enterotoxins.

    • Enteroinvasive E. coli: Mimics symptoms of Shigellosis.

    • Enteropathogenic E. coli: Now considered an older term for specific serotypes linked to infant diarrhea.

    • E. coli O157:H7: Known for causing hemorrhagic colitis, referred to as hamburger disease which can lead to severe health complications. Emphasizes the importance of proper food handling, preparation, and cooking.

    • Other implications include urinary tract infections and neonatal meningitis.

Shigellae

  • Characteristics: Associated with acute diarrhea that features mucus, pus, and blood; generally characterized as non-lactose fermenters.

  • Specific species include:

    • Shigella sonnei: prevalent in Europe and North America.

    • S. dysenteriae: associated with severe illness characterized by watery diarrhea, cramps, and fever.

  • Transmission Mode: Infection occurs via very small numbers of organisms, typical in children with poor sanitation.

  • Prevention: Key focus on safe food handling, water treatment, and proper sewage disposal. No existing vaccine for active prevention.

Vibrio cholerae

  • Disease Association: Causes cholera, an acute gastrointestinal illness that results in profuse watery diarrhea.

  • Mechanism: The enterotoxin interacts with cells in the small intestine, eliciting chloride secretion, which leads to excessive water accumulation in the gut.

  • Symptoms: Characterized by severe dehydration, which can result in death if left untreated. Mass loss can total 10-15 liters per day.

  • Geographical Presence: Endemic to South East Asia and certain regions of Africa, primarily water-borne due to the lack of clean drinking water.

Campylobacter

  • Types: Includes C. jejuni and C. coli.

  • Notable as a major cause of human enteritis, commonly found as normal flora in birds and domestic animals.

  • Pathogenic Variants: Some strains exhibit invasiveness while others are toxic.

  • Symptoms: Can cause fever, abdominal pain, and bloody diarrhea, with potential implications for traveler's diarrhea.

Pseudomonas

  • Nature: Known as an opportunistic pathogen, thrives in moist habitats such as humidifiers.

  • Challenges in Treatment: The species is resistant to many antibiotics, complicating therapeutic strategies, particularly in serious infections.

Pseudomonas aeruginosa

  • Clinical Relevance: Recognized as a respiratory pathogen particularly concerning cystic fibrosis patients and can cause infections in burn patients.

Pseudomonas cepacia

  • Habitat and Infection: Commonly contaminates saline solutions and water, also acts as a respiratory pathogen in cystic fibrosis patients due to its ability to thrive in low nutrient conditions.

Haemophilus influenzae

  • Habitat: Part of the typical nasopharyngeal flora for many young children and adults.

  • Diseases Caused: Can induce serious infections in young children, including meningitis, pneumonia, and joint infections.

  • Vaccination Impact: The introduction of vaccinations has led to a significant decrease in cases in Canada.

  • Chronic Conditions: Can worsen bronchial inflammation in patients with existing chronic bronchitis.

Cronobacter spp.

  • Clinical Significance: Associated with nosocomial infections, including wound infections, pneumonia, and bacteremia.

  • Specific Concerns: C. sakazakii has been linked to illnesses in infants resulting from contaminated powdered infant formula.

Helicobacter pylori

  • Characteristics: Microaerophilic, spiral-shaped bacilli, recognized as a predominant cause of stomach ulcers.

  • Historical Context: Initially attributed to stress and dietary factors, its link to ulcers was discovered in 1982 by Dr. Robin Warren and Dr. Barry Marshall, with broad acceptance in 1994 by the National Institute of Health.

  • Mechanism of Pathology: Produced urease which aids in neutralizing stomach acidity.

  • Treatment: Typically involves a triple therapy approach incorporating antibiotics and H+ pump inhibitors.

Bordetella pertussis

  • Disease: Causes whooping cough, characterized by violent coughing fits.

  • Virulence Factors:

    • Pertussis toxin: an A-B toxin disrupting host immune responses.

    • Extracytoplasmic adenylate cyclase: decreases host defenses.

    • Filamentous hemagglutinin: binds bronchial tissues and helps in exotoxin release.

    • Tracheal cytotoxin: damages ciliated cells, impairing mucus clearance.

  • Prevention: Vaccination with heat-killed organisms is recommended to reduce incidence.

Legionella pneumophilia

  • Characteristics: A fastidious Gram-negative rod that is typically found in aquatic environments.

  • Disease: Causes Legionnaires' disease, a severe form of pneumonia, and Pontiac fever, a milder flu-like illness.

  • Transmission: Primarily transmitted through inhalation of aerosols containing the bacteria from contaminated water systems (e.g., air conditioning cooling towers, hot water tanks, whirlpool spas).

  • Risk Factors: Individuals at higher risk include the elderly, smokers, and those with weakened immune systems or chronic lung disease.

  • Pathogenesis: Invades and replicates within alveolar macrophages.

  • Treatment: Requires specific antibiotics, often macrolides or fluoroquinolones, as it is resistant to many common antibiotics effective against other Gram-negative bacteria.