Escape from the phagosome

Although the induction of phagocytosis and entrance into the eukaryotic cell seems to be simple, this is not the final step of the bacteria's invasion. Majority of the enteropathogenic bacteria are incapable of dividing within the phagosome, but subsequently to leaving the phagosome they can proliferate rapidly within the cytoplasm.

Theoretical basis

Endosome escape.png

Escape from the phagosome is possible due to the hemolysin, which causes membrane permeabilisation and phagosome disruption [16]. It was previously demonstrated that transfection with hemolysin-encoding gene is sufficient to enable the escape from phagosomal vesicle of both Bacillus subtilis and Escherichia coli[1].
The process of escaping from the phagosome is usually strictly regulated. An interesting example is the PhoP/PhoQ regulon, which exists in Salmonella enterica ser. typhimurium LT2 (and some other bacteria). It's a two-component regulatory system which replies to intramacrophage conditions like low pH or low metal ions concentration, leading to the activation of PhoP-dependent genes responsible for virulence of bacteria, like the MgtC gene.

Endosome detection operon

Fig 3. Overview of endosomal detection operon.
Fig 3. Overview of endosomal detection operon. It is composed of cro – antirepressor from phage λ, tetR – repressor of tetracycline resistance operon, cI – repressor from phage λ and cfp – Cyan Fluorescent Protein.

Low endosomal pH and low concentration of metal ions leads to the decreasing likelihood of the transmembrane receptor kinase phoQ binding its ligands – divalent cations. This enables the phosphorylation of phoP by phoQ. Phosphorylated phoP activates the gene expression from MgtC promoter leading to the production of proteins encoded by the endosome detection operon. The cI protein inactivates the invasion operon and the TetR protein activates cytoplasmic operon. Not all genes encoded by cytoplasmic operon are expressed in the endosome, due to the presence of the cro box sequence, which binds the cro protein thus stopping the trancription. The cro protein is encoded by the endosome detection operon. After the bacterium escapes into the cytoplasm the endosome detection operon genes are no longer expressed since phoP is not phosphorylated in cytoplasmic conditions. The process can be monitored by observing the CFP fluorescence protein.