These latter include H. parainfluenzae, H. hemolyticus, H. segnis, H. aphrophilus, and H. paraphrophilus. These species can cause infections on occasion.
MORPHOLOGY AND CULTURE: Haemophilus are small compared to other bacteria, often encapsulated, non-motile, Gram-negative rods. H. influenzae is a facultative anaerobe requiring growth factors X and V in its culture medium. The X factor is hemin, required by the bacteria to synthesize enzymes containing heme including; cytochromes, catalase, and oxidases. The X factor requirement is greatly reduced in anaerobic culturing. The V factor was identified as NAD or NADP. A standard blood agar plate does not contain sufficient free V factor. Some bacteria, in particular Staphylococcus aureus, produce excess NAD and even secrete this coenzyme into the medium. That is why H. influenzae can proliferate in the immediate vicinity of S. aureus colonies. In medical microbiology, this is known as the satellite phenomenon. The medium normally used to culture H. influenzae is chocolate agar containing sufficient amounts of the X and V factors.
PATHOGENESIS AND CLINICAL PICTURES: H. influenzae is a mucosal parasite of the upper respiratory tract present in 30–50% of healthy persons. The strains usually found are nonencapsulated and therefore hardly virulent. The capsule protects the cells from phagocytosis and is thus the primary determinant of pathogenicity. Pathogenicityof microbes
Others include the affinity of H. influenzae to respiratory tract mucosa and meninges and production of an IgA1 protease. H. influenzae infections are seen frequently in children aged from six months to four years of age due to the low levels of anticapsule antibodies in this age group. Maternal antibodies still protect children during the first months of life. The body has built up a sufficient store of antibodies by the age of four. Any list of potential clinical developments must begin with meningitis, followed by epiglottitis, pneumonia, empyema, septic arthritis, osteomyelitis, pericarditis, cellulitis, otitis media, and sinusitis.
Haemophilus infections in adults are usually secondary complications of severe primary illnesses or the result of compromised immune defenses. The most frequent complication is an acute exacerbation of chronic bronchitis. Pneumonias caused by H. influenzae are also observed, often as superinfections following viral influenza. In immunocompromised adults, even the nonencapsulated strains can cause infections of the upper and lower respiratory tract.
DIAGNOSIS: The method of choice is identification of the pathogen in cerebrospinal fluid, blood, pus, or purulent sputum using microscopy and culture assays. Satelliting on blood agar is an indication of a V factor requirement. An X factor requirement is confirmed most readily by the porphyrin test, with a negative result in the presence of H. influenzae.
THERAPY: In view of the increasing number of betalactamase-producing H. influenzae strains observed in recent years, penicillinase-stable betalactam antibiotics should be used to treat these infections. The likelihood that a strain produces betalactamase is 5–30% in most countries. 4-quinolones are an alternative to betalactams that should not, however, be used in children. The agent of choice in meningitis is ceftriaxone.
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