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.

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