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Despite the recognized capacity of the gut microbiota to regulate intestinal lipid metabolism, the role of specific commensal species remains undefined. Here, we aimed to understand the bacterial effectors and molecular mechanisms by which Lactobacillus paracasei and Escherichia coli regulate lipid metabolism in enterocytes. We show that L-lactate produced by L. paracasei inhibits chylomicron secretion from enterocytes and promotes lipid storage by a mechanism involving L-lactate absorption by enterocytes, its conversion to malonyl-CoA, and the subsequent inhibition of lipid beta-oxidation. In contrast, acetate produced by E. coli also inhibits chylomicron secretion by enterocytes but promotes lipid oxidation by a mechanism involving acetate absorption by enterocytes, its metabolism to acetyl-CoA and AMP, and the subsequent upregulation of the AMPK/PGC-1α/PPARα pathway. Our study opens perspectives for developing specific bacteria- and metabolite-based therapeutic interventions against obesity, atherosclerosis, and malnutrition by targeting lipid metabolism in enterocytes., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Shigella is a leading cause of dysentery worldwide, with the majority of infections caused by two subgroups, S. flexneri and S. sonnei. Although S. flexneri has been highly prevalent in low-income countries, global development has brought an increase in S. sonnei at the expense of S. flexneri. However, the mechanisms behind this shift are not understood. Here we report that S. sonnei, but not S. flexneri, encodes a type VI secretion system (T6SS) that provides a competitive advantage in the gut. S. sonnei competes against E. coli and S. flexneri in mixed cultures, but this advantage is reduced in T6SS mutant strains. In addition, S. sonnei can persist as well as outcompete E. coli and S. flexneri in mice in a T6SS-dependent manner. These findings suggest that S. sonnei has a competitive advantage over S. flexneri and potentially explain the increasing global prevalence of S. sonnei., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Salmonella invasion of intestinal epithelial cells requires extensive, though transient, actin modifications at the site of bacterial entry. The actin-modifying protein villin is present in the brush border where it participates in the constitution of microvilli and in epithelial restitution after damage through its actin-severing activity. We investigated a possible role for villin in Salmonella invasion. The absence of villin, which is normally located at the bacterial entry site, leads to a decrease in Salmonella invasion. Villin is necessary for early membrane-associated processes and for optimal ruffle assembly by balancing the steady-state level of actin. The severing activity of villin is important for Salmonella invasion in vivo. The bacterial phosphatase SptP tightly regulates villin phosphorylation, while the actin-binding effector SipA protects F-actin and counterbalances villin-severing activity. Thus, villin plays an important role in establishing the balance between actin polymerization and actin severing to facilitate the initial steps of Salmonella entry., (Copyright © 2015 Elsevier Inc. All rights reserved.)

The intestinal crypt is a site of potential interactions between microbiota products, stem cells, and other cell types found in this niche, including Paneth cells, and thus offers a potential for commensal microbes to influence the host epithelium. However, the complexity of this microenvironment has been a challenge to deciphering the underlying mechanisms. We used in vitro cultured organoids of intestinal crypts from mice, reinforced with in vivo experiments, to examine the crypt-microbiota interface. We find that within the intestinal crypt, Lgr5(+) stem cells constitutively express the cytosolic innate immune sensor Nod2 at levels much higher than in Paneth cells. Nod2 stimulation by its bona fide agonist, muramyl-dipeptide (MDP), a peptidoglycan motif common to all bacteria, triggers stem cell survival, which leads to a strong cytoprotection against oxidative stress-mediated cell death. Thus, gut epithelial restitution is Nod2 dependent and triggered by the presence of microbiota-derived molecules., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Numerous pathogenic Gram-negative bacteria use a type three secretion apparatus (T3SA) to translocate effector proteins into host cells. Detecting and monitoring the T3SA of intracellular bacteria within intact host cells has been challenging. Taking advantage of the tight coupling between T3S effector-gene transcription and T3SA activity in Shigella flexneri, together with a fast-maturing green fluorescent protein, we developed reporters to monitor T3SA activity in real time. These reporters reveal a dynamic temporal regulation of the T3SA during the course of infection. T3SA is activated initially during bacterial entry and downregulated subsequently when bacteria gain access to the host cell cytoplasm, allowing replenishment of the bacterial stores of T3S substrates necessary for invading neighboring cells. Reactivation of the T3SA was strictly dependent on actin-based motility and formation of plasma membrane protrusions during cell-to-cell spread. Thus, the T3SA is subject to a tight on/off regulation within the bacterial intracellular niche., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Shigella infection causes destruction of the human colonic epithelial barrier. The Golgi network and recycling endosomes are essential for maintaining epithelial barrier function. Here we show that Shigella epithelial invasion induces fragmentation of the Golgi complex with consequent inhibition of both secretion and retrograde transport in the infected host cell. Shigella induces tubulation of the Rab11-positive compartment, thereby affecting cell surface receptor recycling. The molecular process underlying the observed damage to the Golgi complex and receptor recycling is a massive redistribution of plasma membrane cholesterol to the sites of Shigella entry. IpaB, a virulence factor of Shigella that is known to bind cholesterol, is necessary and sufficient to induce Golgi fragmentation and reorganization of the recycling compartment. Shigella infection-induced Golgi disorganization was also observed in vivo, suggesting that this mechanism affecting the sorting of cell surface molecules likely contributes to host epithelial barrier disruption associated with Shigella pathogenesis., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Intracellular pathogens like Shigella flexneri enter host cells by phagocytosis. Once inside, the pathogen breaks the vacuolar membrane for cytosolic access. The fate and function of the vacuolar membrane remnants are not clear. Examining Shigella-infected nonmyeloid cells, we observed that proteins associated with vacuolar membrane remnants are polyubiquinated, recruit the autophagy marker LC3 and adaptor p62, and are targeted to autophagic degradation. Further, inflammasome components and caspase-1 were localized to these membranes and correlated with dampened inflammatory response and necrotic cell death. In Atg4B mutant cells in which autophagosome maturation is blocked, polyubiquitinated proteins and P62 accumulated on membrane remnants, and as in autophagy-deficient Atg5(-/-) cells, the early inflammatory and cytokine response was exacerbated. Our results suggest that host membranes, after rupture by an invading cytoplasm-targeted bacterium, contribute to the cellular responses to infection by acting as a signaling node, with autophagy playing a central role in regulating these responses.

Shigella rapidly kills myeloid cells via a caspase-1 inflammasome-dependent cell death mechanism. However, despite a critical role for nonmyeloid cells in the physiopathology of Shigella infection, the mechanism by which Shigella kills nonmyeloid cells remains uncharacterized. Here we demonstrate that, in nonmyeloid cells, Shigella infection induces loss of mitochondrial inner membrane potential, mitochondrial damage, and necrotic cell death through a pathway dependent on Bnip3 and cyclophilin D, two molecules implicated in the host oxidative stress responses. This mitochondrial cell death mechanism was potently counterbalanced by a Nod1-dependent Rip2/IKKbeta/NF-kappaB signaling pathway activated by the pathogen in the first hours of infection. Our results suggest that in nonmyeloid cells, oxidative stress pathways and signaling triggered by an intracellular bacterial pathogen are tightly linked and demonstrate the existence of specific Shigella-induced prodeath and prosurvival pathways converging at the mitochondria to control a necrotic cell death program.

Tolerance to lipopolysaccharide (LPS) is part of the host's strategy to avoid harmful excessive inflammation in the presence of Gram-negative commensals and pathogens. Macrophages are the major cells endowed with this property, allowing them to limit expression of proinflammatory genes while maintaining efficient antimicrobial functions. In a recent paper, Foster et al. (2007) demonstrate that this subtle balance is established and imprinted via epigenetic regulation.

Many bacteria pathogenic for plants or animals, including Shigella spp., which is responsible for shigellosis in humans, use a type III secretion apparatus to inject effector proteins into host cells. Effectors alter cell signaling and host responses induced upon infection; however, their precise biochemical activities have been elucidated in very few cases. Utilizing Saccharomyces cerevisiae as a surrogate host, we show that the Shigella effector IpaH9.8 interrupts pheromone response signaling by promoting the proteasome-dependent destruction of the MAPKK Ste7. In vitro, IpaH9.8 displayed ubiquitin ligase activity toward ubiquitin and Ste7. Replacement of a Cys residue that is invariant among IpaH homologs of plant and animal pathogens abolished the ubiquitin ligase activity of IpaH9.8. We also present evidence that the IpaH homolog SspH1 from Salmonella enterica can ubiquitinate ubiquitin and PKN1, a previously identified SspH1 interaction partner. This study assigns a function for IpaH family members as E3 ubiquitin ligases.