Hemophilic bacteria are so designated because they require growth factors contained in blood. The most important human pathogen in this genus is H. influenzae. Other Haemophilus species either infect only animals or are found in the normal human mucosal flora. 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 (length: 1.0-1.5 im, width: 0.3 im), often encapsulated, nonmotile, Gram-negative rods (Fig. 4.21a). The encapsulated strains are subclassified in serovars a-f based on the fine structure of their capsule polysaccharides. Serovar b (Hib) causes most Haemophi-lus infections in humans.
— Haemophilus influenzae -
Fig. 4.21 a Gram-stained cerebrospinal fluid sediment preparation. Fine, Gram-negative rods surrounded by a capsule (serovar b). Clinical diagnosis: purulent meningitis.
b Satellite colonies of Haemophilus influenzae surrounding the Staphylococcus aureus streak. S. aureus provides small amounts of V factor. The blood agar contains free X factor.
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 (cytochromes, catalase, 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. This is known as the satellite phenomenon (Fig. 4.21b). 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. Others include the affinity of H. influenzae to respiratory tract mucosa and meninges and production of an lgA1 protease (see p. 15).
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. ln 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.
Epidemiology and prevention. H. influenzae is found only in humans. The incidence of severe invasive infections (meningitis, sepsis, epiglottitis) in children has been reduced drastically—to about one in 10 of the numbers seen previously—since a vaccination program was started, and will continue to fall assuming the vaccinations are continued (see vaccination schedule, p. 33).
Immunization is achieved with the conjugate vaccine Hib in which the capsule polysaccharide epitope "b" conferring immunity is conjugated to protein. Such a conjugate vaccine can be administered as early as the first month of life. The immune system does not respond to pure polysaccharide vaccines until about the age of two, since polysaccharides are T-independent antigens against which hardly any antibodies are produced in the first two years of life. There is also no booster response. A four-day regimen of rifam-picin has proved to be an effective chemoprophylactic treatment for nonvac-cinated small children who have been exposed to the organism.
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