Phagocytic uptake and killing of virulent and avirulent strains of Pasteurella multocida of capsular serotype A by chicken macrophages

The susceptibility of two highly virulent (VP161 and VP138) and two less virulent (VP17 and VP21) strains of Pasteurella multocida to phagocytic uptake and killing by chicken macrophages was compared using in vitro phagocytosis and bactericidal assays. When compared with VP17 and VP21, particularly after they were preopsonised with specific immune serum, VP161 and VP138 were more resistant to phagocytosis by chicken macrophages. The uptake of these bacteria increased following the removal of the bacterial capsules with hyaluronidase. All strains preopsonised with specific immune serum were killed to some extent by chicken macrophages. However, the percentages of killing for VP17 and VP21 were higher than those of VP161 and VP138. When the capsules of VP161 and VP138 were removed, the susceptibility of the bacteria to bactericidal activity of chicken macrophages increased. It can be concluded that the virulent strains of P. multocida were more resistant to phagocytosis and phagocytic killing by chicken macrophages compared with the less virulent strains. The hyaluronic acid capsule was considered to be important in the resistance, but might not be the only factor contributing to the resistance since the less virulent strains of P. multocida also possess capsules.