Gram-Negative Rods: Pseudomonas aeruginosa Overview

Gram-Negative Rods

• Pseudomonas aeruginosa:
  • Diseases: Wound infection, UTI, pneumonia, sepsis, nosocomial infections (especially in burn patients), endocarditis in intravenous drug users.
  • Characteristics: Aerobic gram-negative rods, non-lactose-fermenting, pyocyanin (blue-green) pigment producer, oxidase-positive.
  • Habitat and Transmission: Environmental water sources, inhabits the skin, upper respiratory tract and colon. Transmission via water aerosols, aspiration, and fecal contamination.
  • Pathogenesis: Endotoxin responsible for fever and shock associated with sepsis. Exotoxin A inactivates EF-2. Pili and capsule mediate attachment and inhibit phagocytosis, respectively. Glycocalyx-producing strains predominate in chronic infections in cystic fibrosis patients. Severe burns and neutropenia are predisposing factors.
  • Prevention: Disinfection of water-related equipment in hospitals, handwashing, prompt removal of urinary and intravenous catheters. No vaccine available.

Haemophilus influenzae

• Diseases: Sinusitis, otitis media, pneumonia are common. Epiglottitis is uncommon, meningitis was once a leading cause but vaccines have greatly reduced cases.
• Characteristics: Small gram-negative (coccobacillary) rods, requires factors X (hemin) and V (NAD) for growth. Six capsular polysaccharide types, type b causes 95% of invasive disease. Type b capsule is polyribitol phosphate.
• Habitat and Transmission: Upper respiratory tract. Transmission via respiratory droplets.
• Pathogenesis: Polysaccharide capsule is the most important determinant of virulence typically. Unencapsulated strains cause mucosal infections but not invasive infections. IgA protease is produced. Most cases of meningitis occur in children under two years of age, due to waning maternal antibodies and inadequate immune response to capsular polysaccharides.
• Treatment: Ceftriaxone is the treatment of choice for meningitis. Approximately 25% of strains produce β-lactamase.
• Prevention: Vaccine containing type b capsule polysaccharide conjugated to diphtheria toxoid or other protein given between 2 and 18 months of age. Rifampin can prevent meningitis in close contacts.

Bordetella pertussis

• Diseases: Whooping cough (pertussis).
• Characteristics: Small gram-negative rods.
• Habitat and Transmission: Human respiratory tract. Transmission via respiratory droplets.
• Pathogenesis: Pertussis toxin stimulates adenylate cyclase by adding ADP-ribose to the inhibitory G protein. Pertussis toxin has two subunits: A (ADP-ribosylating activity) and B (binds toxin to cell surface receptors). Pertussis toxin causes lymphocytosis in the blood by preventing lymphocytes from entering tissues, leading to a high number of lymphocytes in the blood. Tracheal cytotoxin damages ciliated epithelium of the respiratory tract. Whooping cough is acute tracheobronchitis that begins with mild upper respiratory tract symptoms, followed by severe paroxysmal cough lasting 1 to 4 weeks. Paroxysmal cough is characterized by hacking coughs accompanied by mucus production, ending with an inspiratory whoop as air rushes past a narrowed glottis. In adults, B. pertussis infection often manifests as paroxysmal cough of varying severity lasting several weeks. The characteristic whoop is often absent.
• Treatment: Erythromycin.
• Prevention: Acellular vaccine containing pertussis toxoid (recommended) and killed vaccine containing whole organisms. Given in combination with diphtheria and tetanus toxoids (DTaP) to children.

Legionella pneumophila

• Diseases: Legionnaires' disease ("atypical" pneumonia). Symptoms vary from mild influenzalike illness to severe pneumonia accompanied by confusion, nonbloody diarrhea, and microscopic hematuria. Cough with scanty, nonpurulent sputum. Most cases resolve spontaneously in 7 to 10 days but can be fatal in older or immunocompromised patients. Pontiac fever is a mild, flu-like form that doesn't result in pneumonia.
• Characteristics: Gram-negative rods (stain poorly with standard Gram stain), require iron and cysteine for growth in culture.
• Habitat and Transmission: Environmental water sources. Transmission via aerosol from the water source. No person-to-person transmission.
• Pathogenesis: Endotoxin production. Predisposing factors include age over 55, smoking (damaging alveolar macrophages), and high alcohol intake. Immunocompromised patients, like those with renal transplants, are at increased risk. Organism replicates intracellularly, so cell-mediated immunity is important for host defense.