Mycobacterium tuberculosis can gain access to adipose depots of mice infected via the intra-nasal route and to lungs of mice with an infected subcutaneous fat implant.

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis has the remarkable ability to persist as non-replicating forms in the host. These persisters are tolerant to drugs targeting actively replicating bacilli and hence are responsible for the need of an extended duration of anti-tubercular therapy. The anatomical locations and cell types housing Mtb persisters are being investigated in the recent times. Adipose tissue and the adipocytes are proposed niches of Mtb persisters. In the present study, we carried out experiments in the immunocompetent Swiss mice to see the dissemination of Mtb from lungs to adipose tissue and vice versa. Mice infected intra-nasally with ∼ 10(6), 10(4) or 10(2) bacilli harboured Mtb in various adipose depots distal to the lungs such as the visceral, subcutaneous and peri-renal depots. The dissemination was minimal at two weeks post-infection, as evident from culture negative adipose tissue samples. But at seven weeks post-infection, viable Mtb could be detected in 78%, 66% and 66% of the samples from high, moderate and low dose-infection groups respectively. In a separate experiment, Mtb-infected pre-adipocytes were implanted subcutaneously to un-infected mice. At five weeks post-implantation, the intact implants had a mean 7 ± 0.53 log10 CFUs/100 mg tissue, while the lungs had a mean 3.25 ± 0.32 log10 CFUs/100 mg tissue. In conclusion, the study shows that Mtb can disseminate from lungs to distant adipose depots and vice versa.

Fluorescence resonance energy transfer (FRET)-based subcellular visualization of pathogen-induced host receptor signaling.

BACKGROUND: Bacteria-triggered signaling events in infected host cells are key elements in shaping the host response to pathogens. Within the eukaryotic cell, signaling complexes are spatially organized. However, the investigation of protein-protein interactions triggered by bacterial infection in the cellular context is technically challenging. Here, we provide a methodological approach to exploit fluorescence resonance energy transfer (FRET) to visualize pathogen-initiated signaling events in human cells. RESULTS: Live-cell microscopy revealed the transient recruitment of the Src family tyrosine kinase Hck upon bacterial engagement of the receptor carcinoembryonic antigen-related cell adhesion molecule 3 (CEACAM3). In cells expressing a CEACAM3 variant lacking the cytoplasmic domain, the Src homology 2 (SH2) domain of Hck (Hck-SH2) was not recruited, even though bacteria still bound to the receptor. FRET measurements on the basis of whole cell lysates revealed intimate binding between Hck-SH2 (using enhanced yellow fluorescent protein (YPet)-Hck-SH2) and the tyrosine-phosphorylated enhanced cyan fluorescent protein-labeled cytoplasmic domain of wild-type CEACAM3 (CEACAM3 WT-CyPet) and a flow cytometry-based FRET approach verified this association in intact cells. Using confocal microscopy and acceptor photobleaching, FRET between Hck-SH2 and CEACAM3 was localized to the sites of bacteria-host cell contact. CONCLUSION: These data demonstrate not only the intimate binding of the SH2 domain of Hck to the tyrosine-phosphorylated cytoplasmic domain of CEACAM3 in intact cells, but furthermore, FRET measurements allow the subcellular localization of this process during bacterial infection. FRET-based assays are valuable tools to resolve bacteria-induced protein-protein interactions in the context of the intact host cell.

Deciphering interplay between Salmonella invasion effectors.

Bacterial pathogens have evolved a specialized type III secretion system (T3SS) to translocate virulence effector proteins directly into eukaryotic target cells. Salmonellae deploy effectors that trigger localized actin reorganization to force their own entry into non-phagocytic host cells. Six effectors (SipC, SipA, SopE/2, SopB, SptP) can individually manipulate actin dynamics at the plasma membrane, which acts as a 'signaling hub' during Salmonella invasion. The extent of crosstalk between these spatially coincident effectors remains unknown. Here we describe trans and cisbinary entry effector interplay (BENEFIT) screens that systematically examine functional associations between effectors following their delivery into the host cell. The results reveal extensive ordered synergistic and antagonistic relationships and their relative potency, and illuminate an unexpectedly sophisticated signaling network evolved through longstanding pathogen-host interaction.