Efferocytosis, the process by which dying/dead cells are removed by phagocytosis, plays an important role in development, tissue homeostasis and innate immunity1. Efferocytosis is mediated, in part, by receptors that bind to exofacial phosphatidylserine (PS) on cells or cellular debris after loss of plasma membrane asymmetry. Here we show that a bacterial pathogen, Listeria monocytogenes (Lm), can exploit efferocytosis to promote cell-to-cell spread during infection. These bacteria can escape the phagosome in host cells using the pore-forming toxin Listeriolysin O (LLO) and two phospholipases C2. Expression of the cell surface protein ActA allows Lm to activate host actin regulatory factors and undergo actin-based motility in the cytosol, eventually leading to formation of actin-rich protrusions at the cell surface. We show that protrusion formation is associated with plasma membrane damage due to LLO’s pore-forming activity. LLO also promotes the release of bacteria-containing protrusions from the host cell, generating membrane-derived vesicles with exofacial PS. The PS-binding receptor TIM-4 contributes to efficient cell-to-cell spread by Lm in macrophages in vitro and growth of these bacteria is impaired in TIM-4−/− mice. Thus, Lm promotes its dissemination in a host by exploiting efferocytosis. Our study suggests that PS-targeted therapeutics may be useful in the fight against infections by Lm and other bacteria that utilize similar strategies of cell-to-cell spread during infection.

Bacterial pathogens use horizontal gene transfer to acquire virulence factors that influence host colonization, alter virulence traits, and ultimately shape the outcome of disease following infection. One hallmark of the host-pathogen interaction is the prokaryotic type III secretion system that translocates virulence factors into host cells during infection. Salmonella enterica possesses two type III secretion systems that are utilized during host colonization and intracellular replication. Salmonella pathogenicity island 2 (SPI2) is a genomic island containing approximately 30 contiguous genes required to assemble a functional secretion system including the two-component regulatory system called SsrA-SsrB that positively regulates transcription of the secretion apparatus. We used transcriptional profiling with DNA microarrays to search for genes that coregulate with the SPI2 type III secretion machinery in an SsrB-dependent manner. Here we report the identification of a Salmonella-specific translocated effector called SseL that is required for full virulence during murine typhoid-like disease. Analysis of infected macrophages using fluorescence-activated cell sorting revealed that sseL is induced inside cells and requires SsrB for expression. SseL is retained predominantly in the cytoplasm of infected cells following translocation by the type III system encoded in SPI2. Animal infection experiments with sseL mutant bacteria indicate that integration of SseL into the SsrB response regulatory system contributes to systemic virulence of this pathogen.The gram-negative bacillus Salmonella enterica serovar Typhimurium infects multiple hosts by using virulence factors that promote invasiveness and intracellular survival. The genetic diversity of this pathogen has been shaped prominently by phage lysogeny and by acquisition of mobile genetic elements, many of which harbor genes that have become important for virulence during the diverse stages of host infection (3,13,23,24). In the S. enterica species, two major virulence determinants are encoded on large pathogenicity islands called Salmonella pathogenicity islands 1 and 2 (SPI1 and SPI2, respectively), both of which encode a functional type III secretion system (T3SS) to translocate protein effectors into host cells during infection. The SPI1 T3SS is common to all Salmonella lineages and secretes effectors responsible for entry into nonphagocytic cells (16). Serotypes belonging to S. enterica additionally possess SPI2, which is absent from the other recognized Salmonella species, called S. bongori. SPI2 injects proteins required for intracellular survival and systemic infections of mammalian cells (19,21,25) and likely represents a major second phase in the evolution of Salmonella virulence (1). A more pervasive role for SPI2 in intestinal inflammation and early colonization events is indicated by recent data showing that SPI2 is activated in the lumen of the intestine in a manner independent of SPI1-mediated invasion (2) and contributes to disease (5,8,18). SPI2 c...

The acquisition of DNA by horizontal gene transfer enables bacteria to adapt to previously unexploited ecological niches. Although horizontal gene transfer and mutation of protein-coding sequences are well-recognized forms of pathogen evolution, the evolutionary significance of cis-regulatory mutations in creating phenotypic diversity through altered transcriptional outputs is not known. We show the significance of regulatory mutation for pathogen evolution by mapping and then rewiring a cis-regulatory module controlling a gene required for murine typhoid. Acquisition of a binding site for the Salmonella pathogenicity island-2 regulator, SsrB, enabled the srfN gene, ancestral to the Salmonella genus, to play a role in pathoadaptation of S. typhimurium to a host animal. We identified the evolved cis-regulatory module and quantified the fitness gain that this regulatory output accrues for the bacterium using competitive infections of host animals. Our findings highlight a mechanism of pathogen evolution involving regulatory mutation that is selected because of the fitness advantage the new regulatory output provides the incipient clones.bacterial pathogenesis ͉ cis-regulatory mutation ͉ evo-devo ͉ pathoadaptation ͉ regulatory evolution

() is a Gram-positive facultative intracellular pathogen. Infections in humans can lead to listeriosis, a systemic disease with a high mortality rate. One important mechanism of dissemination involves cell-to-cell spread after bacteria have entered the cytosol of host cells. Listeriolysin O (LLO; encoded by the gene) is a virulence factor present in that plays a central role in the cell-to-cell spread process. LLO is a member of the cholesterol-dependent cytolysin (CDC) family of toxins that were initially thought to promote disease largely by inducing cell death and tissue destruction-essentially acting like a 'bazooka'. This view was supported by structural studies showing CDCs can form large pores in membranes. However, it is now appreciated that LLO has many subtle activities during infection of host cells, and many of these likely do not involve large pores, but rather small membrane perforations. It is also appreciated that membrane repair pathways of host cells play a major role in limiting membrane damage by LLO and other toxins. LLO is now thought to represent a 'Swiss army knife', a versatile tool that allows to induce many membrane alterations and cellular responses that promote bacterial dissemination during infection.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

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