Haemophilus

General: There are strain-specific and species-specific proteins in the outer membrane.

Clinical: Nonencapsulated species/strains do not usually cause sepsis, but encapsulated species/strains do. Some also produce IgA1 protease, facilitating mucosal colonization. Use of a conjugated vaccine to capsular antigens (polyribitol phosphate (PRP)) has dramatically reduced the incidence of significant infection (esp. Haemophilus influenzae).

• Normal: Usually found on mucous membranes of humans; typically colonize the upper respiratory tract within the first months of life.
• Abnormal: Commonly cause otitis media, sinusitis, conjunctivitis, bronchitis, and pneumonia; less commonly but significantly cause meningitis, endocarditis, epiglottitis, arthritis, or cellulitis.

Resistance: About 30% of H. influenzae are resistant to ampicillin. Most H. ducreyi are susceptible to erythromycin, its recommended treatment.

Morphology: Typically are small Gram- rods, but may be pleomorphic and appear filamentous or coccobacillary.

Growth characteristics: Most species require growth media supplemented with either/both: 1) Hemin (X factor), 2) Nicotinamide adenine dinucleotide (NAD, V factor). Usually chocolate agar is used, as blood agar contains NAD inhibitors. CSF specimens should be >=1mL for microscopy, culture, and antigen detection.

Common/important pathogens:

• Actinobacillus actinomycetemcomitans (strictly speaking it is in genus Haemophilus, but remains traditionally in Actinobacillus)
• Haemophilus aegyptius (not to be confused with H. influenzae biogroup aegyptius)
• Haemophilus aphrophilus
• Haemophilus ducreyi
• Haemophilus influenzae
• Haemophilus parainfluenzae