The type III secretion system apparatus determines the intracellular niche of bacterial pathogens

Abstract

Upon entry into host cells, intracellular bacterial pathogens establish a variety of replicative niches. Although some remodel phagosomes, others rapidly escape into the cytosol of infected cells. Little is currently known regarding how professional intracytoplasmic pathogens, including Shigella, mediate phagosomal escape. Shigella, like many other Gram-negative bacterial pathogens, uses a type III secretion system to deliver multiple proteins, referred to as effectors, into host cells. Here, using an innovative reductionist-based approach, we demonstrate that the introduction of a functional Shigella type III secretion system, but none of its effectors, into a laboratory strain of Escherichia coli is sufficient to promote the efficient vacuole lysis and escape of the modified bacteria into the cytosol of epithelial cells. This establishes for the first time, to our knowledge, a direct physiologic role for the Shigella type III secretion apparatus (T3SA) in mediating phagosomal escape. Furthermore, although protein components of the T3SA share a moderate degree of structural and functional conservation across bacterial species, we show that vacuole lysis is not a common feature of T3SA, as an effectorless strain of Yersinia remains confined to phagosomes. Additionally, by exploiting the functional interchangeability of the translocator components of the T3SA of Shigella, Salmonella, and Chromobacterium, we demonstrate that a single protein component of the T3SA translocon—Shigella IpaC, Salmonella SipC, or Chromobacterium CipC—determines the fate of intracellular pathogens within both epithelial cells and macrophages. Thus, these findings have identified a likely paradigm by which the replicative niche of many intracellular bacterial pathogens is established.

How Do the Virulence Factors of Shigella Work Together to Cause Disease?

“…This creates intrinsic limitations in studying their role in intracellular pathogenesis (Guichon et al, 2001). Nonetheless, an “entry region” encoding only the T3SS and IpaD, IpaB, and IpaC has been demonstrated to be sufficient for vacuolar lysis (Figure 1, step 8) (Sansonetti et al, 1986; Du et al, 2016). High et al (1992) used macrophages, which are naturally phagocytic, to overcome the non-invasive ipaB − phenotype, and described how IpaB plays a role in lysis of the phagocytic vacuole.…”

“…Moreover, when within the naturally inserted translocon IpaB is connected to the T3SS via the needle tip, which - along with low osmolarity in the vacuole - would prevent the influx of water into the vacuole to cause lysis. Furthermore, by exploring the functional interchangeability of translocon components from Shigella and Salmonella , which remains in its invasion vacuole, IpaC was shown to be directly involved in lysis of the single membrane vacuole (Osiecki et al, 2001; Du et al, 2016). Yet, any environmental cue for the translocon to switch between possible invasion, translocation, and lysis conformations remains unknown.…”

Listeria monocytogenes and Shigella flexneri Activate the NLRP1B Inflammasome

“…Instead, a type three secretion system translocon protein mediates escape (28); this process is facilitated by an injected effector protein that recruits Rab11 vesicles to the vacuole (29,30). Membrane damage or scavenging of host nutrients might cause the observed reduction of ATP.…”

Genetic Determinants of Salmonella enterica Serovar Typhimurium Proliferation in the Cytosol of Epithelial Cells

“…In comparison, only 10 to 20% of internalized S. Typhimurium bacteria lyse their nascent SCV in epithelial cells (1 to 1.5 h p.i.) (3,14,15), and even fewer escape the SCV in macrophages (14,39). Once S. Typhimurium reaches the cytosol of epithelial cells, and after a lag period of 2 to 3 h, it replicates very quickly, and up to half of the total population occupies the cytosol late in infection (3).…”

“…In epithelial cells, a proportion of S. Typhimurium bacteria (ϳ10 to 20%) damage their nascent vacuole via T3SS1-dependent mechanisms (3,14,15). Bacteria in compromised vacuoles that are not cleared by autophagic engulfment, called xenophagy (16), eventually hyperreplicate in the epithelial cell cytosol (2).…”