Direct Delivery Of Bacterial Toxins

Some bacteria are able to directly deliver their toxins into the cytoplasm of eukaryotic cells through a contact-dependent secretion system direct injection is both efficient and selective The potency of intracellularly acting toxins is derived, in part, from their mode of action –most are enzymes that catalyze the covalent modification of specific molecular targets. To be successful intracellularly acting toxins must access their substrates inside target cells. To overcome the membrane barrier, intracellularly acting toxins are either injected directly into host cells by pathogenic microbes or, alternatively, enter cells in a manner that is microbe-independent

The eukaryotic plasma membrane is a formidable gatekeeper that effectively restricts macromolecules such as toxins from passing freely into cells.

Some gram-negative pathogens inject their toxins into the cytosol of host cells through bacterial transport machines that function as macromolecular syringes Some gram-negative pathogens inject their toxins into the cytosol of host cells through bacterial transport machines that function as macromolecular syringes The syringes, which resemble either bacterial flagella or conjugative pili, facilitate the direct passage of toxin effectors from bacterial cells into eukaryotic cells by processes referred to as Type III or Type IV secretion mechanisms The syringes, which resemble either bacterial flagella or conjugative pili, facilitate the direct passage of toxin effectors from bacterial cells into eukaryotic cells by processes referred to as Type III or Type IV secretion mechanisms Type III and Type IV secretion machines do not allow the free exchange of proteins between prokaryotic and eukaryotic cells Type III and Type IV secretion machines do not allow the free exchange of proteins between prokaryotic and eukaryotic cells –They appear to selectively regulate which toxin effectors pass through the syringe needle

 Injection mechanisms limit the extent to which a pathogen can remodel the host, because a bacterium can directly affect only a single target cell at any given time.  Pathogenic microbes exert a broader sphere of influence by releasing a bolus of toxin that can act upon many cells within a given tissue and/or diffuse away to modulate one or more types of cells at multiple locations within the host.

How do the toxins affect eukaryotic cells?

Toxins can affect signal transduction such as phosphorylation Yersinia uses a protein kinase YpkA and a protein tyrosine phosphatase YopH YpkA and YopH are injected into the cell through Type III secretion system YpkA and YopH are able to paralyze macrophages before they can kill the bacteria YopH dephosphorylates cytoskeletal proteins disrupting phagocytosis

 EPEC Tir (translocated intimin receptor) is a protien found in enteropathogenic E. coli  EPEC Tir is injected via Type III secretion system  EPEC Tir becomes part of the eukaryotic cell membrane and functions as a receptor for a bacterial adhsin called intimin  EPEC infection is characterized by destruction of host-cell intestinal microvilli, act in rearrangement within epithelial cells, and the formation of a raised platform or pedestal at the site of bacterial attachment

Toxins can act on small G proteins and affect the cytoskeleton of the host cell SopE is injected into eukaryotes from Salmonella via type III secretion system It binds and activates small G proteins (Rac and Dcd42) Causes membrane ruffling and actin cytoskeleton reorganization This mediates the uptake of the bacteria into the eukaryotic cell

 P. aeruginosa Exoenzyme S has ADP- ribosylating activity that ADP-ribosylates the small G protein Ras  The toxin is injected via a type III secretion  It causes the cytoskeleton to collapse and the cell morphology to change  The result is the rounding of the cell

► YopE is injected via type III secretion system from Yersinia ► It is able to paralyze macrophage phagocytosis by causing actin depolymerization  Resulting in the disruption of cell stress fibers; hence rounding of cells/loss of cell shape

What to take note of Bacteria can use structures similar to conjugative pili to insert their toxins directly to the cell The bacteria inject their toxins through Type III or Type IV secretion systems This process is more efficent, but it can only affect one cell at a time The toxins can act on signal transduction pathways, small G proteins and the cytoskeleton of the eukaryotic host cell