The members of the genus Haemophilus are small, sometimes pleomorphic, Gram-negative, nonmotile, and nonsporing organisms. They require one or both the accessory growth fac-tors, namely, X and V present in the blood. Hence, the genus derived its name from its essential growth requirement of cer-tain factors, such as X and V present in the blood (haemophilus: haem, blood; philus, loving). Koch in the year 1883 isolated Haemophilus aegyptius, first member of the genus from a case ofconjunctivitis in Egypt.
Haemophilus spp. are obligate bacteria present in the mucousmembranes of the humans and certain species of animals.
Haemophilus influenzae and Haemophilus ducreyi are two majorspecies associated with disease in humans.
H. influenzae causes meningitis, pneumonia, epiglottitis,bronchitis, and otitis media. H. ducreyi is the causative agent of sexually transmitted disease, soft chancre, or chancroid. Haemophilus aphrophilus is less frequent but an important causeof endocarditis. OtherHaemophilus species are rarely patho-genic. They are responsible primarily for opportunistic infec-tions (Table 38-1).
H. influenzae is the species most commonly associated withhuman disease. It is an important cause of meningitis in chil-dren and also of respiratory tract infection in children as well as in adults.