H. ducreyi is an important causative agent of sexually transmit-ted disease called soft sore or chancroid in Asia and Africa. It is an obligate bacterium. H. ducreyi grows poorly on most media, requires X factor, but not V factor, and is biochemically inert. It produces very small colonies (0.5-mm diameter) on chocolate agar after prolonged incubation of 72 hours in the presence of CO2.
The chancroid is seen most commonly in men, and the condition remains asymptomatic in women. The incubation period varies from 5 to 7 days, after which a papule with an ery-thematous surface appears on the genitalia or on the perianal area. Subsequently, the lesion ulcerates and becomes painful. The condition is frequently associated with inguinal lymphade-nopathy. Gram-stain smears from the ulcerative lesions show pleomorphic, Gram-negative bacilli. Sulfonamide and tetracy-cline are the drugs of choice.
H. aphrophilus is present as a commensal in the mouth and inthe throat. On chocolate agar, it forms yellowish, high convex, large colonies (1.5-mm diameter) after 24 hours of incubation. It requires only X factor, but not V factor, and as the name sug-gests, it also requires high concentration of CO2 for growth. H.aphrophilus, which is present as a commensal in the mouth, can spread to the blood stream and infect previously infected heart valve, causing subacute endocarditis. It can also cause brain abscess, pneumonia, and sinusitis.
H. parainfluenzae is an opportunistic pathogen in the upperrespiratory tract. Occasionally, the bacteria may cause endocar-ditis, conjunctivitis, and bronchopulmonary infection. It forms relatively bigger colonies than those of H. influenzae on choco-late agar, which are opaque and yellowish white. It differs from H. influenzae in requiring the V factor only and not the X factor,fermenting sucrose but not D-xylose.
H. aegyptius was earlier known as Koch–Weeks bacillus. Thiswas observed by Koch in 1883 in a case of acute conjunctivi-tis and subsequently was first cultured by Weeks in 1887. The organism causes highly contagious form of acute conjunctivitis. It closely resembles H. influenzae; however, it differs from H. influenzae by being nutritionally more exacting, not ferment-ing xylose and by not being present as a commensal in the naso-pharynx of healthy people.
H. haemolyticus is present as a commensal in the upper respira-tory tract. It is a nonpathogen and does not cause any human disease. On blood agar, it produces a zone of hemolysis around the colonies and may be mistaken for hemolytic streptococci. It requires both X and V factors.