Haemophilus Influenzae Infections

Pathogenesis and Spectrum of Disease

• Production of a capsule and factors that mediate bacterial attachment to human epithelial cells are the primary virulence factors associated with Haemophilus spp.
• Infections caused by H. influenzae are often systemic and life-threatening, whereas infections caused by nontypeable (do not have a capsule) strains are usually localized.
• Most serious infections caused by H. influenzae type b are biotypes I and II.
• Most H. influenzae infections are now caused by nontypeable strains (NTHi).
• Transmission is often via respiratory secretions.
• The organism is able to gain access to sterile sites from colonization in the upper respiratory tract.
• Clinical infections include otitis media, sinusitis, bronchitis, pneumonia, and conjunctivitis.
• Immunodeficiencies and chronic respiratory problems may predispose an individual to infection with NTHi.
• Chancroid is the sexually transmitted disease caused by H. ducreyi.

General Characteristics

• The genus Haemophilus contains significant genetic diversity.
• Members of the genus are small, nonmotile, pleomorphic gram-negative bacilli.
• The cells are typically coccobacillary or short rods.
• Species of the genus Haemophilus require protoporphyrin IX (X factor) and V factor (NAD or NADP) for in vitro growth.
• Haemophilus spp. are facultative anaerobes enhanced in a 5% to 7% CO2-enriched atmosphere.

Epidemiology

• Haemophilus spp. normally inhabit the upper respiratory tract of humans.
• Asymptomatic colonization with H. influenzae type b is rare.
• H. ducreyi is only found in humans, and its presence in clinical specimens indicates infection.

Laboratory Diagnosis

• Specimens consist of expectorated sputum and other types of respiratory specimens, pus, blood, and spinal fluid.
• Direct Gram stain examination of body fluid specimens may be performed after centrifugation.
• Gram stains of the smears from clinical specimens must be examined carefully.
• Haemophilus spp. stain a pale pink and may be difficult to detect in the pink background of proteinaceous material often found in clinical specimens.
• Antigen detection can be performed using commercially available particle agglutination assays.
• Molecular methods such as PCR can be used for rapid detection.

Cultivation / Media of Choice

• Haemophilus spp. typically grow on chocolate agar as smooth, flat or convex, buff or slightly yellow colonies.
• Chocolate agar provides hemin (X factor) and NAD (V factor), necessary for the growth of Haemophilus spp.
• The satellite phenomenon may be observed on sheep blood agar, where tiny colonies of Haemophilus spp. grow near colonies of bacteria capable of producing V factor.

Treatment

• Invasive H. influenzae infection often requires hospitalization.
• The current recommended treatment of life-threatening illness caused by H. influenzae is cefotaxime or ceftriaxone.
• Alternative drugs include trimethoprim-sulfamethoxazole, imipenem, and ciprofloxacin.