Question

What organisms that could invade during phagocytosis and how?

Answer 1

Intracellular pathogens and obligate intracellular parasites can infect phagocytes but prevent formation of the phagolysosomes. These mechanisms allow the bacteria to live in a protected environment in early stages of infection until they can produce virulence factors that help in evasion of immune responses of host. The organism are prevented from killing by extracellular bactericides, antibodies, drugs etc. A few organisms are obligate parasites that require the host macrophages for reproduction. Some of the organisms are:

a) Leishmania are obligate intracellular protozoa infecting mononuclear phagocytes. Infective metacyclic promastigotes are introduced into the host blood through bites of infected sand fly[3]. Neutrophils are recruited to site of injury and engulf the promastigotes. Infected neutrophils or free parasites are taken up by professional phagocytes (dendritic cells and macrophages). Leishmania promastigotes survive in the vacuoles and transform into amastigotes and replicate. The cell rupture the extracellular amastigotes re-infect local phagocytes.

Leishmania parasites are intracellular parasites that require host cell for survival and to reproduction. They alter the β1-Integrin Function and Chemokine Receptor in the phagocytes. The macrophage/ phagocyte is required for reproduction of promastigotes to amastigotes. Amastigotes complete their division to mastigotes in the sand fly. The amsatigotes infect the skin and the lymph nodes via the phagocytes. Hence, the phagocyte is a important vehicle for disease transmission.

b) Legionella pneumophila enters mononuclear phagocytes by depositing complement C3b on its surfaces. This protein is used as a ligand for binding to macrophage cell surfaces. The bacteria in vacuoles do not fuse to form phagolysosomes. Thus, they can evade the immune responses and evade killing by the macrophages.

c) Salmonella produce an invasin operon (inv A - H) that encodes for factors that regulate their entry into host cells.

d) Mycobacterium tuberculosis has bacterial cell wall components (sulfatides) which are released in the phagosome. These components modify lysosomal membranes to avoid fusion. Their waxy, hydrophobic cell wall has mycolic acids and other lipids, which are not easily attacked by lysosomal enzymes.

e) Brucella abortus and Staphylococcus aureus are vigorous catalase and superoxide dismutase producers. Catalase and superoxide dismutase neutralize the toxic oxygen radicals generated in phagocytes.