H. influenzae

The organism is misnamed: it was wrongly implicated in the 1890 influenza pandemic and is bacterial, not viral. Growth requires both X and V factors, which is why it grows on chocolate agar, where lysed red cells release them, and why it satellites around Staphylococcus aureus colonies on blood agar, the staph providing NAD. Encapsulated type b strains carry a polyribosylribitol phosphate capsule and were the leading cause of paediatric meningitis, epiglottitis with the thumbprint sign and a drooling, tripoding child, septic arthritis, and periorbital cellulitis. The Hib conjugate vaccine, given from two months, links the polysaccharide to a protein carrier to produce T-cell-dependent memory in infants and has nearly eliminated invasive disease; rifampin is used for close-contact prophylaxis in meningitis. Non-typeable, unencapsulated strains remain common and cause otitis media, sinusitis, conjunctivitis, and COPD exacerbations, and are unaffected by the vaccine. Beta-lactamase production is frequent, so amoxicillin-clavulanate or a cephalosporin is used.