Bacteria exploit many mechanisms to communicate with each other and their surroundings. Mechanisms using small diffusible signals to coordinate behaviour with cell density (quorum sensing) frequently contribute to pathogenicity. However, pathogens must also be able to acquire nutrients and replicate to successfully invade their host. One quorum-sensing system, based on the possession of LuxS, bears the unique feature of contributing directly to metabolism, and therefore has the potential to influence both gene regulation and bacterial fitness. Here, we discuss the influence that LuxS and its product, autoinducer-2, have on virulence, relating the current evidence to the preferred niche of the pathogen and the underlying mechanisms involved.

The HIV-1 Tat protein is secreted by infected cells. Extracellular Tat can affect bystander uninfected T cells and induce numerous biological responses such as apoptosis and cytokine secretion. Tat is likely involved in several immune disorders during AIDS. Nevertheless, it is not known whether Tat triggers cell responses directly upon binding to signaling receptors at the plasma membrane or after delivery to the cytosol. The pathway that enables Tat to reach the cytosol is also unclear. Here we visualized Tat within T-cell-coated pits and endosomes. Moreover, inhibitors of clathrin/AP-2-mediated uptake such as chlorpromazine, activated RhoA, or dominant-negative mutants of Eps15, intersectin, dynamin, or rab5 impaired Tat delivery to the cytosol by preventing its endocytosis. Molecules neutralizing low endosomal pH or Hsp90 inhibitors abolished Tat entry at a later stage by blocking its endosomal translocation, as directly shown using a cell-free translocation assay. Finally, endosomal pH neutralization prevented Tat from inducing T-cell responses such as NF-B activation, apoptosis, and interleukin secretion, indicating that cytosolic delivery is required for Tat signaling. Hence, Tat enters T cells essentially like diphtheria toxin, using clathrin-mediated endocytosis before low-pH-induced and Hsp90-assisted endosomal translocation. Cell responses are then induced from the cytosol. INTRODUCTIONTat is a strong trans-activator that enables productive transcription from the HIV-1 long terminal repeat (LTR) and is required for HIV-1 replication (Rubartelli et al., 1998;Watson and Edwards, 1999;Noonan and Albini, 2000). Albeit devoid of signal sequence, it is released by infected cells and nanomolar Tat concentrations were measured in the sera of HIV-1-infected patients (Xiao et al., 2000). Exogenous Tat can affect monocytes, endothelial cells and neurons, but one of its main targets is the T cell (Rubartelli et al., 1998;Watson and Edwards, 1999;Noonan and Albini, 2000). Indeed, Tat induces IL-2 and IL-8 hypersecretion by T cells (Ott et al., 1997(Ott et al., , 1998 and can also trigger their apoptosis (Li et al., 1995;Chen et al., 2002). Circulating Tat is thus thought to be involved in AIDS development (Rubartelli et al., 1998;Watson and Edwards, 1999). Consistently, evaluations of Tat-containing vaccines have yielded encouraging results (Voss et al., 2003).Tat has the capacity to enter the cytosol from the outside medium, like several bacterial toxins such as diphtheria and cholera toxin catalytic subunits (Falnes and Sandvig, 2000). This property was demonstrated in pioneer studies by showing that extracellular Tat could trans-activate reporter genes placed under the control of HIV-1 LTR Mann and Frankel, 1991). This finding was later confirmed using several Tat fusion proteins and different readouts for monitoring Tat cytosolic delivery (Fawell et al., 1994). Nevertheless, contrary to bacterial toxins, the overall pathway enabling extracellular Tat to access the cytosol remains elusive, although endocytosis s...

Formerly regarded as small 'bags' of nucleic acids with randomly diffusing enzymes, bacteria are organized by a sophisticated and tightly regulated molecular machinery. Here, we review qualitative and quantitative data on the intracellular organization of bacteria and provide a detailed inventory of macromolecular structures such as the divisome, the degradosome and the bacterial 'nucleolus'. We discuss how these metabolically active structures manage the spatial organization of the cell and how macromolecular crowding influences them. We present for the first time a visualization program, lifeexplorer, that can be used to study the interplay between metabolism and spatial organization of a prokaryotic cell.

Junonia coenia densovirus (JcDNV) is an ambisense insect parvovirus highly pathogenic for lepidopteran pests at larval stages. The potential use of DNVs as biological control agents prompted us to reinvestigate the host range and cellular mechanisms of infection. In order to understand the early events of infection, we set up a functional infection assay in a cell line of the pest Lymantria dispar to determine the intracellular pathway undertaken by JcDNV to infect a permissive lepidopteran cell line. Our results show that JcDNV particles are rapidly internalized into clathrin-coated vesicles and slowly traffic within early and late endocytic compartments. Blocking late-endocytic trafficking or neutralizing the pH with drugs inhibited infection. During internalization, disruption of the cytoskeleton, and inhibition of phosphatidylinositol 3-kinase blocked the movement of vesicles containing the virus to the nucleus and impaired infection. In summary, our results define for the first time the early endocytic steps required for a productive DNV infection.