Shaping microbiome function with a human milk-oligosaccharide synbiotic.

In a recent article, Button and colleagues demonstrate that human milk oligosaccharides create a nutrient niche that supports reversible colonization by Bifidobacterium infantis. Using this tunable system, they assessed the impact of B. infantis on microbiome recovery after antibiotic treatment. Overall, this work highlights synbiotics as a useful approach for developing live biotherapeutic products (LBPs).

Fiber-deficient diets reprogram the microbiota.

Exclusive enteral nutrition, a diet lacking fiber, is used to treat pediatric Crohn's disease. In this issue of Cell Host & Microbe, Kuffa et al. find that a fiber-deficient diet thins the mucus layer and alters microbial cross-feeding, causing pro-inflammatory Mucispirillum to move away from the epithelium, which ameliorates colitis.

A functional Wood-Ljungdahl pathway devoid of a formate dehydrogenase in the gut acetogens Blautia wexlerae, Blautia luti and beyond.

Species of the genus Blautia are typical inhabitants of the human gut and considered as beneficial gut microbes. However, their role in the gut microbiome and their metabolic features are poorly understood. Blautia schinkii was described as an acetogenic bacterium, characterized by a functional Wood-Ljungdahl pathway (WLP) of acetogenesis from H2  + CO2 . Here we report that two relatives, Blautia luti and Blautia wexlerae do not grow on H2  + CO2 . Inspection of the genome sequence revealed all genes of the WLP except genes encoding a formate dehydrogenase and an electron-bifurcating hydrogenase. Enzyme assays confirmed this prediction. Accordingly, resting cells neither converted H2  + CO2 nor H2  + HCOOH + CO2 to acetate. Carbon monoxide is an intermediate of the WLP and substrate for many acetogens. Blautia luti and B. wexlerae had an active CO dehydrogenase and resting cells performed acetogenesis from HCOOH + CO2  + CO, demonstrating a functional WLP. Bioinformatic analyses revealed that many Blautia strains as well as other gut acetogens lack formate dehydrogenases and hydrogenases. Thus, the use of formate instead of H2  + CO2 as an interspecies hydrogen and electron carrier seems to be more common in the gut microbiome.

Microbiome-based therapeutics.

Symbiotic microorganisms inhabiting the gastrointestinal tract promote health by decreasing susceptibility to infection and enhancing resistance to a range of diseases. In this Review, we discuss our increasing understanding of the impact of the microbiome on the mammalian host and recent efforts to culture and characterize intestinal symbiotic microorganisms that produce or modify metabolites that impact disease pathology. Manipulation of the intestinal microbiome has great potential to reduce the incidence and/or severity of a wide range of human conditions and diseases, and the biomedical research community now faces the challenge of translating our understanding of the microbiome into beneficial medical therapies. Our increasing understanding of symbiotic microbial species and the application of ecological principles and machine learning are providing exciting opportunities for microbiome-based therapeutics to progress from faecal microbiota transplantation to the administration of precisely defined and clinically validated symbiotic microbial consortia that optimize disease resistance.

TAM mediates adaptation of carbapenem-resistant Klebsiella pneumoniae to antimicrobial stress during host colonization and infection.

Gram-negative pathogens, such as Klebsiella pneumoniae, remodel their outer membrane (OM) in response to stress to maintain its integrity as an effective barrier and thus to promote their survival in the host. The emergence of carbapenem-resistant K. pneumoniae (CR-Kp) strains that are resistant to virtually all antibiotics is an increasing clinical problem and OM impermeability has limited development of antimicrobial agents because higher molecular weight antibiotics cannot access sites of activity. Here, we demonstrate that TAM (translocation and assembly module) deletion increases CR-Kp OM permeability under stress conditions and enhances sensitivity to high-molecular weight antimicrobials. SILAC-based proteomic analyses revealed mis-localization of membrane proteins in the TAM deficient strain. Stress-induced sensitization enhances clearance of TAM-deficient CR-Kp from the gut lumen following fecal microbiota transplantation and from infection sites following pulmonary or systemic infection. Our study suggests that TAM, as a regulator of OM permeability, represents a potential target for development of agents that enhance the effectiveness of existing antibiotics.

Rapid transcriptional and metabolic adaptation of intestinal microbes to host immune activation.

The gut microbiota produces metabolites that regulate host immunity, thereby impacting disease resistance and susceptibility. The extent to which commensal bacteria reciprocally respond to immune activation, however, remains largely unexplored. Herein, we colonized mice with four anaerobic symbionts and show that acute immune responses result in dramatic transcriptional reprogramming of these commensals with minimal changes in their relative abundance. Transcriptomic changes include induction of stress-response mediators and downregulation of carbohydrate-degrading factors such as polysaccharide utilization loci (PULs). Flagellin and anti-CD3 antibody, two distinct immune stimuli, induced similar transcriptional profiles, suggesting that commensal bacteria detect common effectors or activate shared pathways when facing different host responses. Immune activation altered the intestinal metabolome within 6 hours, decreasing luminal short-chain fatty acid and increasing aromatic metabolite concentrations. Thus, intestinal bacteria, prior to detectable shifts in community composition, respond to acute host immune activation by rapidly changing gene transcription and immunomodulatory metabolite production.

Functional and Genomic Variation between Human-Derived Isolates of Lachnospiraceae Reveals Inter- and Intra-Species Diversity.

Bacteria belonging to the Lachnospiraceae family are abundant, obligate anaerobic members of the microbiota in healthy humans. Lachnospiraceae impact their hosts by producing short-chain fatty acids, converting primary to secondary bile acids, and facilitating colonization resistance against intestinal pathogens. To increase our understanding of genomic and functional diversity between members of this family, we cultured 273 Lachnospiraceae isolates representing 11 genera and 27 species from human donors and performed whole-genome sequencing assembly and annotation. This analysis revealed substantial inter- and intra-species diversity in pathways that likely influence an isolate's ability to impact host health. These differences are likely to impact colonization resistance through lantibiotic expression or intestinal acidification, influence host mucosal immune cells and enterocytes via butyrate production, or contribute to synergism within a consortium by heterogenous polysaccharide metabolism. Identification of these specific functions could facilitate development of probiotic bacterial consortia that drive and/or restore in vivo microbiome functions.

Impact of Antibiotic-Resistant Bacteria on Immune Activation and Clostridioides difficile Infection in the Mouse Intestine.

Antibiotic treatment of patients undergoing complex medical treatments can deplete commensal bacterial strains from the intestinal microbiota, thereby reducing colonization resistance against a wide range of antibiotic-resistant pathogens. Loss of colonization resistance can lead to marked expansion of vancomycin-resistant Enterococcus faecium (VRE), Klebsiella pneumoniae, and Escherichia coli in the intestinal lumen, predisposing patients to bloodstream invasion and sepsis. The impact of intestinal domination by these antibiotic-resistant pathogens on mucosal immune defenses and epithelial and mucin-mediated barrier integrity is unclear. We used a mouse model to study the impact of intestinal domination by antibiotic-resistant bacterial species and strains on the colonic mucosa. Intestinal colonization with K. pneumoniae, Proteus mirabilis, or Enterobacter cloacae promoted greater recruitment of neutrophils to the colonic mucosa. To test the hypothesis that the residual microbiota influences the severity of colitis caused by infection with Clostridioides difficile, we coinfected mice that were colonized with ampicillin-resistant bacteria with a virulent strain of C. difficile and monitored colonization and pathogenesis. Despite the compositional differences in the gut microbiota, the severity of C. difficile infection (CDI) and mortality did not differ significantly between mice colonized with different ampicillin-resistant bacterial species. Our results suggest that the virulence mechanisms enabling CDI and epithelial destruction outweigh the relatively minor impact of less-virulent antibiotic-resistant intestinal bacteria on the outcome of CDI.

Isoginkgetin, a Natural Biflavonoid Proteasome Inhibitor, Sensitizes Cancer Cells to Apoptosis via Disruption of Lysosomal Homeostasis and Impaired Protein Clearance.

Protein degradation pathways are critical for maintaining proper protein dynamics within the cell, and considerable efforts have been made toward the development of therapeutics targeting these catabolic processes. We report here that isoginkgetin, a naturally derived biflavonoid, sensitized cells undergoing nutrient starvation to apoptosis, induced lysosomal stress, and activated the lysosome biogenesis gene TFEB Isoginkgetin treatment led to the accumulation of aggregates of polyubiquitinated proteins that colocalized strongly with the adaptor protein p62, the 20S proteasome, and the endoplasmic reticulum-associated degradation (ERAD) protein UFD1L. Isoginkgetin directly inhibited the chymotrypsin-like, trypsin-like, and caspase-like activities of the 20S proteasome and impaired NF-κB signaling, suggesting that the molecule may display its biological activity in part through proteasome inhibition. Importantly, isoginkgetin was effective at killing multiple myeloma (MM) cell lines in vitro and displayed a higher rate of cell death induction than the clinically approved proteasome inhibitor bortezomib. We propose that isoginkgetin disturbs protein homeostasis, leading to an excess of protein cargo that places a burden on the lysosomes/autophagic machinery, eventually leading to cancer cell death.

Correction: Interbacterial mechanisms of colonization resistance and the strategies pathogens use to overcome them.

The original version of this article contained an error in the published figures, where they appeared in black and white. These have now been corrected to display in colour.

Interbacterial mechanisms of colonization resistance and the strategies pathogens use to overcome them.

The communities of bacteria that reside in the intestinal tract are in constant competition within this dynamic and densely colonized environment. At homeostasis, the equilibrium that exists between these species and strains is shaped by their metabolism and also by pathways of active antagonism, which drive competition with related and unrelated strains. Importantly, these normal activities contribute to colonization resistance by the healthy microbiota, which includes the ability to prevent the expansion of potential pathogens. Disruption of the microbiota, resulting from, for example, inflammation or antibiotic use, can reduce colonization resistance. Pathogens that engraft following disruption of the microbiota are often adapted to expand into newly created niches and compete in an altered gut environment. In this review, we examine both the interbacterial mechanisms of colonization resistance and the strategies of pathogenic strains to exploit gaps in colonization resistance.

Inhibiting antibiotic-resistant Enterobacteriaceae by microbiota-mediated intracellular acidification.

Klebsiella pneumoniae, Escherichia coli, and other members of the Enterobacteriaceae family are common human pathogens that have acquired broad antibiotic resistance, rendering infection by some strains virtually untreatable. Enterobacteriaceae are intestinal residents, but generally represent <1% of the adult colonic microbiota. Antibiotic-mediated destruction of the microbiota enables Enterobacteriaceae to expand to high densities in the colon, markedly increasing the risk of bloodstream invasion, sepsis, and death. Here, we demonstrate that an antibiotic-naive microbiota suppresses growth of antibiotic-resistant clinical isolates of Klebsiella pneumoniae, Escherichia coli, and Proteus mirabilis by acidifying the proximal colon and triggering short chain fatty acid (SCFA)-mediated intracellular acidification. High concentrations of SCFAs and the acidic environment counter the competitive edge that O2 and NO3 respiration confer upon Enterobacteriaceae during expansion. Reestablishment of a microbiota that produces SCFAs enhances clearance of Klebsiella pneumoniae, Escherichia coli, and Proteus mirabilis from the intestinal lumen and represents a potential therapeutic approach to enhance clearance of antibiotic-resistant pathogens.

Complement C3 Drives Autophagy-Dependent Restriction of Cyto-invasive Bacteria.

In physiological settings, the complement protein C3 is deposited on all bacteria, including invasive pathogens. However, because experimental host-bacteria systems typically use decomplemented serum to avoid the lytic action of complement, the impact of C3 coating on epithelial cell responses to invasive bacteria remains unexplored. Here, we demonstrate that following invasion, intracellular C3-positive Listeria monocytogenes is targeted by autophagy through a direct C3/ATG16L1 interaction, resulting in autophagy-dependent bacterial growth restriction. In contrast, Shigella flexneri and Salmonella Typhimurium escape autophagy-mediated growth restriction in part through the action of bacterial outer membrane proteases that cleave bound C3. Upon oral infection with Listeria, C3-deficient mice displayed defective clearance at the intestinal mucosa. Together, these results demonstrate an intracellular role of complement in triggering antibacterial autophagy and immunity against intracellular pathogens. Since C3 indiscriminately associates with foreign surfaces, the C3-ATG16L1 interaction may provide a universal mechanism of xenophagy initiation.

Cellular Aspects of Shigella Pathogenesis: Focus on the Manipulation of Host Cell Processes.

Shigella is a Gram-negative bacterium that is responsible for shigellosis. Over the years, the study of Shigella has provided a greater understanding of how the host responds to bacterial infection, and how bacteria have evolved to effectively counter the host defenses. In this review, we provide an update on some of the most recent advances in our understanding of pivotal processes associated with Shigella infection, including the invasion into host cells, the metabolic changes that occur within the bacterium and the infected cell, cell-to-cell spread mechanisms, autophagy and membrane trafficking, inflammatory signaling and cell death. This recent progress sheds a new light into the mechanisms underlying Shigella pathogenesis, and also more generally provides deeper understanding of the complex interplay between host cells and bacterial pathogens in general.

Intracellular Bacterial Pathogens Trigger the Formation of U Small Nuclear RNA Bodies (U Bodies) through Metabolic Stress Induction.

Invasive bacterial pathogens induce an amino acid starvation (AAS) response in infected host cells that controls host defense in part by promoting autophagy. However, whether AAS has additional significant effects on the host response to intracellular bacteria remains poorly characterized. Here we showed that Shigella, Salmonella, and Listeria interfere with spliceosomal U snRNA maturation in the cytosol. Bacterial infection resulted in the rerouting of U snRNAs and their cytoplasmic escort, the survival motor neuron (SMN) complex, to processing bodies, thus forming U snRNA bodies (U bodies). This process likely contributes to the decline in the cytosolic levels of U snRNAs and of the SMN complex proteins SMN and DDX20 that we observed in infected cells. U body formation was triggered by membrane damage in infected cells and was associated with the induction of metabolic stresses, such as AAS or endoplasmic reticulum stress. Mechanistically, targeting of U snRNAs to U bodies was regulated by translation initiation inhibition and the ATF4/ATF3 pathway, and U bodies rapidly disappeared upon removal of the stress, suggesting that their accumulation represented an adaptive response to metabolic stress. Importantly, this process likely contributed to shape the host response to invasive bacteria because down-regulation of DDX20 expression using short hairpin RNA (shRNA) amplified ATF3- and NF-κB-dependent signaling. Together, these results identify a critical role for metabolic stress and invasive bacterial pathogens in U body formation and suggest that this process contributes to host defense.

Emerging themes in bacterial autophagy.

The role of autophagy in the control of intracellular bacterial pathogens, also known as xenophagy, is well documented. Here, we highlight recent advances in the field of xenophagy. We review the importance of bacterial targeting by ubiquitination, diacylglycerol (DAG) or proteins such as Nod1, Nod2, NDP52, p62, NBR1, optineurin, LRSAM1 and parkin in the process of xenophagy. The importance of metabolic sensors, such as mTOR and AMPK, in xenophagy induction is also discussed. We also review the in vitro and in vivo evidence that demonstrate a global role for xenophagy in the control of bacterial growth. Finally, the mechanisms evolved by bacteria to escape xenophagy are presented.

NOD proteins: regulators of inflammation in health and disease.

Entry of bacteria into host cells is an important virulence mechanism. Through peptidoglycan recognition, the nucleotide-binding oligomerization domain (NOD) proteins NOD1 and NOD2 enable detection of intracellular bacteria and promote their clearance through initiation of a pro-inflammatory transcriptional programme and other host defence pathways, including autophagy. Recent findings have expanded the scope of the cellular compartments monitored by NOD1 and NOD2 and have elucidated the signalling pathways that are triggered downstream of NOD activation. In vivo, NOD1 and NOD2 have complex roles, both during bacterial infection and at homeostasis. The association of alleles that encode constitutively active or constitutively inactive forms of NOD2 with different diseases highlights this complexity and indicates that a balanced level of NOD signalling is crucial for the maintenance of immune homeostasis.

Stalling autophagy: a new function for Listeria phospholipases.

Listeria monocytogenes is a Gram-positive bacterial pathogen that induces its own uptake in non-phagocytic cells. Following invasion, Listeria escapes from the entry vacuole through the secretion of a pore-forming toxin, listeriolysin O (LLO) that acts to damage and disrupt the vacuole membrane. Listeria then replicates in the cytosol and is able to spread from cell-to-cell using actin-based motility. In addition to LLO, Listeria produces two phospholipase toxins, a phosphatidylinositol-specific phospholipase C (PI-PLC, encoded by plcB) and a broad-range phospholipase C (PC-PLC, encoded by plcA), which contribute to bacterial virulence. It has long been recognized that secretion of PI- and PC-PLC enables the disruption of the double membrane vacuole during cell-to-cell spread, and those phospholipases have also been shown to augment LLO-dependent escape from the entry endosome. However, a specific role for Listeria phospholipases during the cytosolic stage of infection has not been previously reported. In a recent study, we demonstrated that Listeria PI-PLC and PC-PLC contribute to the bacterial escape from autophagy through a mechanism that involves direct inhibition of the autophagic flux in the infected cells [Tattoli et al. EMBO J (2013), 32, 3066-3078].

The protein ATG16L1 suppresses inflammatory cytokines induced by the intracellular sensors Nod1 and Nod2 in an autophagy-independent manner.

The peptidoglycan sensor Nod2 and the autophagy protein ATG16L1 have been linked to Crohn's disease (CD). Although Nod2 and the related sensor, Nod1, direct ATG16L1 to initiate anti-bacterial autophagy, whether ATG16L1 affects Nod-driven inflammation has not been examined. Here, we uncover an unanticipated autophagy-independent role for ATG16L1 in negatively regulating Nod-driven inflammatory responses. Knockdown of ATG16L1 expression, but not that of ATG5 or ATG9a, specifically enhanced Nod-driven cytokine production. In addition, autophagy-incompetent truncated forms of ATG16L1 regulated Nod-driven cytokine responses. Mechanistically, we demonstrated that ATG16L1 interfered with poly-ubiquitination of the Rip2 adaptor and recruitment of Rip2 into large signaling complexes. The CD-associated allele of ATG16L1 was impaired in its ability to regulate Nod-driven inflammatory responses. Overall, these results suggest that ATG16L1 is critical for Nod-dependent regulation of cytokine responses and that disruption of this Nod1- or Nod2-ATG16L1 signaling axis could contribute to the chronic inflammation associated with CD.

Listeria phospholipases subvert host autophagic defenses by stalling pre-autophagosomal structures.

Listeria can escape host autophagy defense pathways through mechanisms that remain poorly understood. We show here that in epithelial cells, Listeriolysin (LLO)-dependent cytosolic escape of Listeria triggered a transient amino-acid starvation host response characterized by GCN2 phosphorylation, ATF3 induction and mTOR inhibition, the latter favouring a pro-autophagic cellular environment. Surprisingly, rapid recovery of mTOR signalling was neither sufficient nor necessary for Listeria avoidance of autophagic targeting. Instead, we observed that Listeria phospholipases PlcA and PlcB reduced autophagic flux and phosphatidylinositol 3-phosphate (PI3P) levels, causing pre-autophagosomal structure stalling and preventing efficient targeting of cytosolic bacteria. In co-infection experiments, wild-type Listeria protected PlcA/B-deficient bacteria from autophagy-mediated clearance. Thus, our results uncover a critical role for Listeria phospholipases C in the inhibition of autophagic flux, favouring bacterial escape from host autophagic defense.