Shape of microbes matter in phagocytosis

Phagocytosis is an actin-polymerization-dependent process for the uptake of particles larger than 0.5 μm into cells. Macrophages are cells forming a part of the innate immune system, and phagocytosis is central to their immune function . Phagocytosis is an extraordinarily complex process involving both structural rearrangement (cytoskeletal and membrane) and a complex network of signaling events. Phagocytosis plays a critical role in the clearance of infectious agents or senescent cells and is central to regulating immune responses, inflammation, and tissue remodeling.

Phagocytosis also plays a role in clearing inorganic particulate material from body surfaces such as inhaled carbon or mineral particles. The biochemical mechanisms of phagocytosis have been extensively studied and described in the literature. The phagocytic process essentially involves three steps: recognition by receptors of either an opsonized or nonopsonized particle; receptor-mediated actin polymerization leading to internalization and cleavage of the phagosome from the cytoplasmic surface; and intracellular trafficking for phagosomal maturation. Maturation of the phagosome ultimately leads to the degradation of phagosomal contents and the induction of the appropriate immune responses.

The data shows an increase in the average engulfment time for increased target size, for spherical particles. The uptake time data on nonspherical particles confirms that target shape plays a more dominant role than target size for phagocytosis: Ellipsoids with an eccentricity of 0.954 and much smaller surface areas than spheres were taken up five times more slowly than spherical targets.

Hydrophobic targets are much more susceptible to phagocytosis than the hydrophilic targets (9,18,19). The presence of surface charge (i.e., targets with higher net ζ-potential) also leads to increased uptake: cationic and anionic particles with comparable absolute ζ-potential values have similar levels of ingestion by macrophages (9,14,18–21).

Nonpolar molecules that repel the water molecules are said to be hydrophobic; molecules forming ionic or a hydrogen bond with the water molecule are said to behydrophilic. This property of water was important for the evolution of life.

Water is repelled more by a surface when the hydrophobicity of the surface is increased. The contact angle of a water droplet is larger on a more hydrophobic surface.