Influence of serotype A capsulation on cell surface physiologic factors in Pasteurella multocida

Serotype A extracellular polysaccharide produced by Pasteurella multocida is composed largely of hyaluronic acid and is an important virulence factor in avian pasteurellosis. Hyaluronidase, mechanical shearing, and serial subculturing were employed to decapsulate strains exhibiting disparate degrees of capsulation, thereby allowing an investigation of the role serotype A capsulation plays in outer cell envelope physiology and virulence of avian isolates. Experimental loss of capsular material occurred concomitantly with an overall increase in cell surface hydrophobicity values, which approached those of naturally-occurring noncapsulated strains. The electronegativity of capsulated cell surfaces was found to be significantly greater than that of both noncapsulated and experimentally decapsulated cells. Naturally-occurring noncapsulated variants were readily phagocytized by mouse peritoneal macrophages, whereas capsulated strains were not. A moderately capsulated variant was phagocytized less effectively than noncapsulated strains. Experimental decapsulation of capsulated variants did not significantly hinder their ability to resist phagocytosis. Moreover, small amounts of residual capsular material appeared to remain after experimental decapsulation as detected with the aid of light and transmission electron microscopy. These data suggest that while cell surface hydrophobicity, charge, and susceptibility to phagocytosis are influenced by the degree to which cells are capsulated with serotype A extracellular polysaccharide, other factors which are unaffected by experimental decapsulation may also be involved in protection from phagocytosis in capsulated avian isolates of P. multocida.