HAEMOPHILUS INFLUENZAE

Microbiology

Haemophilus is a genus of aerobic non-motile, non-sporeforming, small Gram-negative rods with rather special growth requirements, including haem (X factor, protoporphyrin IX containing iron), for which there seems to be a special receptor. Haemophilus influenzae is the Haemophilus species with the most significant impact on human health, but Haemophilus parainfluenzae and Haemophilus aphrophilus are responsible for rare cases of both invasive diseases such as endocarditis and for occasional cases of respiratory tract infection.

The cell wall of H. influenzae has the same structure as in other Gram-negative bacteria including an inner, cytoplasmic, phospholipid bilayer, a thin peptidoglycan layer, a periplasmic space and an outer membrane (Fig. 7.1). The outer membrane is an asymmetric bilayer with proteins inserted. The outer membrane of Haemophilus is different from most other Gram-negative bacteria in so far as it is devoid of the long polysaccharide side chains of the lipopolysaccharide. These O-chains have distinct antigenic properties in other Gram-negative bacteria and confer some protection against the effect of antibiotics and the killing action of serum complement, actions to which H. influenzae is therefore more susceptible. The lipopolysaccharide of Haemophilus, most often designated lipooligosaccharide (LOS), is nonetheless, as other endotoxins, a structure with potent biological effects.