GRAM NEGATIVE COCCOBACILI

I. Haemophilus spp.

A. Haemophilus influenzae

  1. Cultures

    • Requirements: Cultures require enriched media containing blood or its derivatives for isolation.

    • Characteristics: Small, gram-negative, pleomorphic bacteria, often seen as coccobacilli in acute infections.

    • Morphology: In specimens from acute infections, H. influenzae appears as short (1.5 μm) coccoid bacilli that can exist in pairs or short chains.

  2. Growth Characteristics

    • Media: Grows best on chocolate agar, forming flat, grayish-brown colonies (1-2 mm in diameter) after 24 hours of incubation.

    • Enhancers: IsoVitaleX enhances growth and helps in differentiating species.

    • Satellite Phenomenon: One unique growth feature is the so-called “satellite phenomenon,” where H. influenzae colonies grow around staphylococci on sheep blood agar due to the release of NAD.

  3. Variation

    • Exhibits significant morphological variation depending on culture age and media conditions.

    • Young cultures (6-18 hrs) display encapsulation which is critical for typing H. influenzae.

    • Nonencapsulated variants lack iridescence and demonstrate a marked tendency to lose capsular type specificity.

  4. Transformation

    • H. influenzae can transfer its type specificity to other cells via DNA transformation under specific experimental conditions.

  5. Antigenic Structure

    • Capsule: Encapsulated H. influenzae possesses antigenic polysaccharides of types a-f. Type B's capsule is polyribose ribitol phosphate (PRP), crucial for virulence and antiphagocytic resistance.

    • Typing Methods: Typing can be done via slide agglutination, coagglutination, or immunofluorescence.

  6. Clinical Findings

    • Caused diseases include meningitis, pneumonia, epiglottitis, cellulitis, septic arthritis, and invasive infections, particularly from encapsulated serotype b (Hib).

    • Non-typeable strains often lead to chronic conditions like otitis media and sinusitis.

  7. Pathogenesis

    • Enters through respiratory tracts; invasive disease can follow, often in children under five. Local extensions may affect sinuses and middle ear.

    • H. influenzae can reach the bloodstream, then the meninges.

    • Meningitis from Hib was common before widespread vaccination, presenting with symptoms similar to other forms in children.

  8. Specimen

    • Samples include nasopharyngeal swabs, blood, pus, and spinal fluid. Commercial kits exist for immunological detection.

  9. Direct Identification

    • Gram stain is commonly used for initial identification but antigen detection tests in fluids often yield comparable results.

  10. Immunity

    • Infants possess maternal IgG antibodies; later, children acquire immunity through exposure or vaccination. Anti-PRP antibodies enhance bactericidal activity.

  11. Treatment

    • Resistance seen with ampicillin up to 25% due to plasmid-mediated β-lactamase. Newer cephalosporins are typically effective.

    • Prompt treatment can reduce complications like neurological impairment and fluid buildup.

  12. Control

    • Vaccination: Hib conjugate vaccines significantly lower disease rates and carry-over rates under 1% post-vaccination; hence, preventative medicine aims at maintaining immunization schedules.

B. Haemophilus Aegyptius

  1. Clinical associations with Brazilian Purpuric Fever in children, presenting with fever and purpura.

C. Haemophilus Aphrophilus

  1. Normal flora and can cause infective endocarditis and pneumonia.

D. Haemophilus Ducreyi

  1. Treatment: Caused by H. ducreyi leading to chancroid—a sexually transmitted infection marked by painful ulcers on genitalia.

E. Other Haemophilus Species

  1. Haemophilus haemoglobinophilus

  2. Haemophilus haemolyticus

  3. Haemophilus parainfluenzae

  4. Haemophilus suis

II. Bordetellae

A. Bordetella Pertussis

  1. Morphology and Identification: Typical Organisms

    • Minute, gram-negative coccobacilli resembling H. influenzae.

    • Stained with toluidine blue to detect bipolar metachromatic granules.

  2. Culture

    • Requires specialized media such as Bordet-Gengou or charcoal-containing media for optimal growth at 35-37°C for 3-7 days in a moist environment.

  3. Growth Characteristics

    • Non-motile and strictly aerobic, capable of acid formation from glucose and lactose but not gas. Oxidase and catalase positive.

    • Hemolysis indicates virulent strains. Production of pertussis toxin that leads to disease development.

  4. Variation

    • Exhibits reversible Phenotypic modulation based on environmental conditions, e.g., temperature shifts.

  5. Antigenic Structure, Pathogenesis, and Pathology

    • Pertussis toxin with similar mechanisms as cholera toxin—leads to lymphocytosis and other severe symptoms.

    • Adheres to ciliated cells, irritating surface cells and promoting severe coughing; blood is not invaded.

  6. Clinical Findings

    • Stages of whooping cough: Catarrhal (subclinical cough), then paroxysmal (intense coughing with “whoop”) leading to exhaustion, and possible complications.

    • High white blood cell counts (16,000-30,000/L).

  7. Specimen

    • Preferred specimens: nasopharyngeal swabs and aspirates. Use Dacron or rayon versus cotton to prevent organism death.

  8. Diagnostic Laboratory Tests

    • Techniques include culture, PCR, serology, and direct fluorescent antibody tests.

    • PCR is vital for early diagnosis as it is sensitive and specific.

  9. Immunity

    • Previous infection and vaccination induce immunity, but it does not guarantee lifelong protection.

  10. Treatment

    • Erythromycin can help during the catarrhal phase but is less effective in later stages. Prophylactic treatment may apply to significant contacts.

  11. Prevention

    • Vaccination with acellular pertussis vaccines is standard; multiple doses are necessary, often combined with DTP (diphtheria-tetanus-pertussis).

  12. Epidemiology and Control

    • Endemic with high transmission rates. Most infections occur in children under five years, necessitating proactive immunization efforts.

III. Pasteurella spp.

  1. Clinical Findings

    • Pasteurella Multocida

      • Common cause of infections from animal bites, particularly from cats and dogs, leading to redness, swelling, and pain.

      • Produces bacteremia or chronic respiratory infections with limited connections to animals.

      • DOC: Penicillin G; alternatives include tetracyclines and fluoroquinolones.

    • Pasteurella Pneumotropica: A normal inhabitant that can cause disease under certain conditions.

    • Pasteurella Haemolytica: Prominent in epizoonotic pneumonia in livestock.

    • Pasteurella Ureae: Rare in animals but part of chronic human respiratory infections.

IV. References

  • Consult Jawetz’s Medical Microbiology and relevant scientific literature.

V. Practice Questions

  • Evaluate understanding through questions regarding species associated with specific infections, treatment options, and diagnostic tests.