Clinical Pathology: Microbiology

• Severe Haemophilus influenzae infections, such as meningitis, epiglottitis, and bacteremia have become rare since the introduction of childhood vaccination programs in developed countries. Less life-threatening infections that are common today include conjunctivitis, otitis media, sinusitis, and pneumonia.

• Rapid antigen detection assays directly from cerebrospinal fluid are available for H. influenzae, Streptococcus pneumonia, and Neisseria meningitidis; however, their sensitivity and specificity are lower than that of the Gram stain. Therefore, this test is discouraged and is not used by most microbiology laboratories in the United States.

• H. influenzae is a catalase-positive, gram-negative coccobacillary organism. Biochemical tests useful for differentiating the eight biotypes of H. influenzae include indole, ornithine decarboxylase (OCD), and urease. A rapid porphyrin test can help distinguish H. influenzae from non-H. influenzae species.

• The requirement of X (protoporphyrin IX) and V (nicotinamide) factors for H. influenzae growth is commonly used for identification. The presumptive isolate is inoculated onto nonnutrient agar, X and V strips are applied, and the plate is incubated overnight. Haemophilus will only grow along the periphery of the strips containing both factors (also called satellite growth).

• For systemic infections, treatment with a third-generation cephalosporin is often warranted. For less severe infections, amoxicillin was previously commonly used; however, treatment failure due to the production of a plasmid-mediated β-lactamase became common. Therefore, the current treatment of choice is amoxicillin-clavulanate.