The effect of Pseudomonas alginate on rat alveolar macrophage phagocytosis and bacterial opsonization

Alginate obtained from a mucoid strain of Pseudomonas aeruginosa was shown to inhibit the phagocytosis of an isogenic non-mucoid revertant by rat alveolar macrophages. Phagocytosis of Staphylococcus albus, binding of sensitized sheep erythrocytes to Fc receptors and uptake of latex particles were also inhibited. These results suggest that the alginate acts as a barrier, surrounding the macrophage preventing the attachment of bacteria or other particles to the plasma membrane. This conclusion was supported by showing that alginic acid, a polysaccharide from seaweed structurally similar to alginate also inhibited the phagocytosis of non-mucoid Ps. aeruginosa. The alginate also inhibited opsonisation of the non-mucoid revertant by a non-agglutinating hyperimmune serum. It is proposed that alginate confers a selective advantage on mucoid producing forms of Ps. aeruginosa by impairing the host immune response by its action on alveolar macrophages and opsonization of bacteria.