Capsules and Virulence in the HAP Group of Bacteria

Many species in the genera Haemophilus, Actinobacillus, and Pasteurella produce serotype-specific, negatively-charged, polysaccharide capsules. In pure form, these capsules are nontoxic, relatively inert biologically, and some are poorly immunogenic. The capsule forms the outer-most surface of the bacterium, and prior to the development of specific antibody, inhibits the bactericidal and opsonic activity of normal serum. In contrast, noncapsulated isogenic mutants are generally avirulent, and are quickly cleared by host defenses. In conjunction with the bacterial cell, or through covalent conjugation to a protein carrier, relatively high titers of antibody to capsule can be produced in immunocompetent hosts. For some pathogens (such as H. influenzae type b) antibody to capsule alone is bactericidal, opsonic, and adequate for complete protection against disease. For pathogens that produce potent exotoxins, however, (such as P. haemolytica and A. pleuropneumoniae) antibody to capsule alone may provide only partial protection. The composition or structure of the capsule may also influence the relative virulence of a particular organism. Molecular analysis of the interaction between capsules and host defenses will be required to understand more fully how capsules influence bacterial virulence.