Bacterial Entry into the Host Cells

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Bacterial Entry into the Host Cells

  • Invasion of the host’s epithelium is essential for initiation of the infectious process. Some bacteria (Salmonella spp., Shigella spp., Yersinia spp. and others) invade specific types of host epithelial cells by flagella-like structures, known as “needle complex” or injectisome.
  • Bacteria inject special III type secretion proteins into the host cells. This action induces the cytoskeleton actin remodelling with subsequent formation of the vacuole.
  • Bacteria are captured further by cell membrane protrusions and transferred into the cell. For shigellae, at least three proteins (invasion plasmid antigens, Ipa), IpaB, IpaC, and IpaD contribute to this process. Shigellae bind to integrin receptors upon the surface of M cells in Peyer’s patches of the human intestine.
  • L. monocytogenes is also able to stimulate its own engulfment by the host cells. Protein internalin plays a primary role in this process.
  • A high variety of tissue degrading enzymes helps bacteria to invade. Hyaluronidases are the enzymes that hydrolyze hyaluronic acid, a major constituent of the ground substance of connective tissue. These enzymes are produced by many bacteria (e.g., staphylococci, streptococci, multiple anaerobic species, etc.). Hyaluronidase production spurs the microbial spread through the tissues.
  • Many bacteria (Clostridia spp., Pseudomonas spp., and others) synthesize proteolytic enzymes collagenase and elastase, which degrade collagen and elastin, the major proteins of fibrous connective tissue.
  • A great number of bacterial virulence factors inhibits host immune defence. For instance, multiple effector proteins of Salmonellae or Yersiniae delivered by injectisome of type III secretion system block the enzymes of respiratory burst in phagocytes.
  • Capsule production also promotes bacterial escape from microbicidal phagocyte actions thus maintaining microbial spread by phagocytes.
  • Legionella pneumonia infects pulmonary macrophages resulting in severe pneumonia. Macrophages capture the legionellae, but phagolysosome fusion is inhibited, and the bacteria propagate within the phagocyte vesicle.
  • Neisseria produces IgA proteases that degrade human IgA thereby subverting host mucosal immunity.
  • Similarly, Streptococcus pyogenes synthesizes C5a peptidase that destroys C5a fragment of complement preventing chemotaxis of phagocytes to bacterial cells.

Bacterial Entry into the Host Cells