Janus kinase 2 inhibition by pacritinib as potential therapeutic target for liver fibrosis.

BACKGROUND AND AIMS: Janus kinase 2 (JAK2) signaling is increased in human and experimental liver fibrosis with portal hypertension. JAK2 inhibitors, such as pacritinib, are already in advanced clinical development for other indications and might also be effective in liver fibrosis. Here, we investigated the antifibrotic role of the JAK2 inhibitor pacritinib on activated hepatic stellate cells (HSCs) in vitro and in two animal models of liver fibrosis in vivo . APPROACH AND RESULTS: Transcriptome analyses of JAK2 in human livers and other targets of pacritinib have been shown to correlate with profibrotic factors. Although transcription of JAK2 correlated significantly with type I collagen expression and other profibrotic genes, no correlation was observed for interleukin-1 receptor-associated kinase and colony-stimulating factor 1 receptor. Pacritinib decreased gene expression of fibrosis markers in mouse primary and human-derived HSCs in vitro . Moreover, pacritinib decreased the proliferation, contraction, and migration of HSCs. C 57 BL/6J mice received ethanol in drinking water (16%) or Western diet in combination with carbon tetrachloride intoxication for 7 weeks to induce alcoholic or nonalcoholic fatty liver disease. Pacritinib significantly reduced liver fibrosis assessed by gene expression and Sirius red staining, as well as HSC activation assessed by alpha-smooth muscle actin immunostaining in fibrotic mice. Furthermore, pacritinib decreased the gene expression of hepatic steatosis markers in experimental alcoholic liver disease. Additionally, pacritinib protected against liver injury as assessed by aminotransferase levels. CONCLUSIONS: This study demonstrates that the JAK2 inhibitor pacritinib may be promising for the treatment of alcoholic and nonalcoholic liver fibrosis and may be therefore relevant for human pathology.

Colon Cancer Microbiome Landscaping: Differences in Right- and Left-Sided Colon Cancer and a Tumor Microbiome-Ileal Microbiome Association.

In the current era of precision oncology, it is widely acknowledged that CRC is a heterogeneous disease entity. Tumor location (right- or left-sided colon cancer or rectal cancer) is a crucial factor in determining disease progression as well as prognosis and influences disease management. In the last decade, numerous works have reported that the microbiome is an important element of CRC carcinogenesis, progression and therapy response. Owing to the heterogeneous nature of microbiomes, the findings of these studies were inconsistent. The majority of the studies combined colon cancer (CC) and rectal cancer (RC) samples as CRC for analysis. Furthermore, the small intestine, as the major site for immune surveillance in the gut, is understudied compared to the colon. Thus, the CRC heterogeneity puzzle is far from being solved, and more research is necessary for prospective trials that separately investigate CC and RC. Our prospective study aimed to map the colon cancer landscape using 16S rRNA amplicon sequencing in biopsy samples from the terminal ileum, healthy colon tissue, healthy rectal tissue and tumor tissue as well as in preoperative and postoperative stool samples of 41 patients. While fecal samples provide a good approximation of the average gut microbiome composition, mucosal biopsies allow for detecting subtle variations in local microbial communities. In particular, the small bowel microbiome has remained poorly characterized, mainly because of sampling difficulties. Our analysis revealed the following: (i) right- and left-sided colon cancers harbor distinct and diverse microbiomes, (ii) the tumor microbiome leads to a more consistent cancer-defined microbiome between locations and reveals a tumor microbiome-ileal microbiome association, (iii) the stool only partly reflects the microbiome landscape in patients with CC, and (iv) mechanical bowel preparation and perioperative antibiotics together with surgery result in major changes in the stool microbiome, characterized by a significant increase in the abundance of potentially pathogenic bacteria, such as Enterococcus. Collectively, our results provide new and valuable insights into the complex microbiome landscape in patients with colon cancer.

Short-Term Western Diet Aggravates Non-Alcoholic Fatty Liver Disease (NAFLD) With Portal Hypertension in TGR(mREN2)27 Rats.

Non-alcoholic fatty liver disease (NAFLD) is gaining in importance and is linked to obesity. Especially, the development of fibrosis and portal hypertension in NAFLD patients requires treatment. Transgenic TGR(mREN2)27 rats overexpressing mouse renin spontaneously develop NAFLD with portal hypertension but without obesity. This study investigated the additional role of obesity in this model on the development of portal hypertension and fibrosis. Obesity was induced in twelve-week old TGR(mREN2)27 rats after receiving Western diet (WD) for two or four weeks. Liver fibrosis was assessed using standard techniques. Hepatic expression of transforming growth factor-ß1 (TGF-ß1), collagen type Iα1, α-smooth muscle actin, and the macrophage markers Emr1, as well as the chemoattractant Ccl2, interleukin-1ß (IL1ß) and tumor necrosis factor-α (TNFα) were analyzed. Assessment of portal and systemic hemodynamics was performed using the colored microsphere technique. As expected, WD induced obesity and liver fibrosis as confirmed by Sirius Red and Oil Red O staining. The expression of the monocyte-macrophage markers, Emr1, Ccl2, IL1ß and TNFα were increased during feeding of WD, indicating infiltration of macrophages into the liver, even though this increase was statistically not significant for the EGF module-containing mucin-like receptor (Emr1) mRNA expression levels. Of note, portal pressure increased with the duration of WD compared to animals that received a normal chow. Besides obesity, WD feeding increased systemic vascular resistance reflecting systemic endothelial and splanchnic vascular dysfunction. We conclude that transgenic TGR(mREN2)27 rats are a suitable model to investigate NAFLD development with liver fibrosis and portal hypertension. Tendency towards elevated expression of Emr1 is associated with macrophage activity point to a significant role of macrophages in NAFLD pathogenesis, probably due to a shift of the renin-angiotensin system towards a higher activation of the classical pathway. The hepatic injury induced by WD in TGR(mREN2)27 rats is suitable to evaluate different stages of fibrosis and portal hypertension in NAFLD with obesity.

Fungus-associated bacteriome in charge of their host behavior.

Bacterial-fungal interactions are widespread in nature and there is a growing number of studies reporting distinct fungus-associated bacteria. However, little is known so far about how shifts in the fungus-associated bacteriome will affect the fungal host's lifestyle. In the present study, we describe for the first time the bacterial community associated with the saprotrophic fungus Mucor hiemalis, commonly found in soil and rhizosphere. Two broad-spectrum antibiotics that strongly altered the bacterial community associated with the fungus were applied. Our results revealed that the antibiotic treatment did not significantly reduce the amount of bacteria associated to the fungus but rather changed the community composition by shifting from initially dominating Alpha-Proteobacteria to dominance of Gamma-Proteobacteria. A novel approach was applied for the isolation of fungal-associated bacteria which also revealed differences between bacterial isolates obtained from the original and the antibiotic-treated M. hiemalis. The shift in the composition of the fungal-associated bacterial community led to significantly reduced fungal growth, changes in fungal morphology, behavior and secondary-metabolites production. Furthermore, our results showed that the antibiotic-treated isolate was more attractive and susceptible to mycophagous bacteria as compared to the original isolate. Overall, our study highlights the importance of the fungus-associated bacteriome for the host's lifestyle and interactions and indicate that isolation with antibacterials is not sufficient to eradicate the associated bacteria.

Enhanced surface colonisation and competition during bacterial adaptation to a fungus.

Bacterial-fungal interactions influence microbial community performance of most ecosystems and elicit specific microbial behaviours, including stimulating specialised metabolite production. Here, we use a co-culture experimental evolution approach to investigate bacterial adaptation to the presence of a fungus, using a simple model of bacterial-fungal interactions encompassing the bacterium Bacillus subtilis and the fungus Aspergillus niger. We find in one evolving population that B. subtilis was selected for enhanced production of the lipopeptide surfactin and accelerated surface spreading ability, leading to inhibition of fungal expansion and acidification of the environment. These phenotypes were explained by specific mutations in the DegS-DegU two-component system. In the presence of surfactin, fungal hyphae exhibited bulging cells with delocalised secretory vesicles possibly provoking an RlmA-dependent cell wall stress. Thus, our results indicate that the presence of the fungus selects for increased surfactin production, which inhibits fungal growth and facilitates the competitive success of the bacterium.

The non-selective Rho-kinase inhibitors Y-27632 and Y-33075 decrease contraction but increase migration in murine and human hepatic stellate cells.

BACKGROUND: The Rho-kinase ROCK II plays a major role in the activation of hepatic stellate cells (HSC), which are the key profibrotic and contractile cells contributing to the development of chronic liver disease. Inhibition of ROCK II ultimately blocks the phosphorylation of the myosin light chain (MLC) and thus inhibits stress fibre assembly and cell contraction. We investigated the effects of the ROCK inhibitors Y-33075 as well as Y-27632 in murine and human hepatic stellate cells. METHODS: Primary isolated HSC from FVB/NJ mice and the immortalized human HSC line TWNT-4 were culture-activated and incubated with Y-27632 and Y-33075 (10nM to 10µM) for 24h. Protein expression levels were analyzed by Western Blots and transcriptional levels of pro-fibrotic markers and proliferative markers were evaluated using real-time qPCR. Migration was investigated by wound-healing assay. Proliferation was assessed by BrdU assay. Contraction of HSC was measured using 3D collagen matrices after incubation with Y-27632 or Y-33075 in different doses. RESULTS: Both Rho-kinase inhibitors, Y-27632 and Y-33075, reduced contraction, fibrogenesis and proliferation in activated primary mouse HSC (FVB/NJ) and human HSC line (TWNT-4) significantly. Y-33075 demonstrated a 10-times increased potency compared to Y-27632. Surprisingly, both inhibitors mediated a substantial and unexpected increase in migration of HSC in FVB/NJ. CONCLUSION: ROCK inhibition by the tested compounds decreased contraction but increased migration. Y-33075 proved more potent than Y27632 in the inhibition of contraction of HSCs and should be further evaluated in chronic liver disease.

Exploring the Interspecific Interactions and the Metabolome of the Soil Isolate Hylemonella gracilis.

Microbial community analysis of aquatic environments showed that an important component of its microbial diversity consists of bacteria with cell sizes of ~0.1 µm. Such small bacteria can show genomic reductions and metabolic dependencies with other bacteria. However, so far, no study has investigated if such bacteria exist in terrestrial environments like soil. Here, we isolated soil bacteria that passed through a 0.1-µm filter. The complete genome of one of the isolates was sequenced and the bacterium was identified as Hylemonella gracilis. A set of coculture assays with phylogenetically distant soil bacteria with different cell and genome sizes was performed. The coculture assays revealed that H. gracilis grows better when interacting with other soil bacteria like Paenibacillus sp. AD87 and Serratia plymuthica. Transcriptomics and metabolomics showed that H. gracilis was able to change gene expression, behavior, and biochemistry of the interacting bacteria without direct cell-cell contact. Our study indicates that in soil there are bacteria that can pass through a 0.1-µm filter. These bacteria may have been overlooked in previous research on soil microbial communities. Such small bacteria, exemplified here by H. gracilis, can induce transcriptional and metabolomic changes in other bacteria upon their interactions in soil. In vitro, the studied interspecific interactions allowed utilization of growth substrates that could not be utilized by monocultures, suggesting that biochemical interactions between substantially different sized soil bacteria may contribute to the symbiosis of soil bacterial communities. IMPORTANCE Analysis of aquatic microbial communities revealed that parts of its diversity consist of bacteria with cell sizes of ~0.1 µm. Such bacteria can show genomic reductions and metabolic dependencies with other bacteria. So far, no study investigated if such bacteria exist in terrestrial environments such as soil. Here, we show that such bacteria also exist in soil. The isolated bacteria were identified as Hylemonella gracilis. Coculture assays with phylogenetically different soil bacteria revealed that H. gracilis grows better when cocultured with other soil bacteria. Transcriptomics and metabolomics showed that H. gracilis was able to change gene expression, behavior, and biochemistry of the interacting bacteria without direct contact. Our study revealed that bacteria are present in soil that can pass through 0.1-µm filters. Such bacteria may have been overlooked in previous research on soil microbial communities and may contribute to the symbiosis of soil bacterial communities.

Neprilysin-dependent neuropeptide Y cleavage in the liver promotes fibrosis by blocking NPY-receptor 1.

Development of liver fibrosis is paralleled by contraction of hepatic stellate cells (HSCs), the main profibrotic hepatic cells. Yet, little is known about the interplay of neprilysin (NEP) and its substrate neuropeptide Y (NPY), a potent enhancer of contraction, in liver fibrosis. We demonstrate that HSCs are the source of NEP. Importantly, NPY originates majorly from the splanchnic region and is cleaved by NEP in order to terminate contraction. Interestingly, NEP deficiency (Nep-/-) showed less fibrosis but portal hypertension upon liver injury in two different fibrosis models in mice. We demonstrate the incremental benefit of Nep-/- in addition to AT1R blocker (ARB) or ACE inhibitors for fibrosis and portal hypertension. Finally, oral administration of Entresto, a combination of ARB and NEP inhibitor, decreased hepatic fibrosis and portal pressure in mice. These results provide a mechanistic rationale for translation of NEP-AT1R-blockade in human liver fibrosis and portal hypertension.

The Specific NLRP3 Antagonist IFM-514 Decreases Fibrosis and Inflammation in Experimental Murine Non-Alcoholic Steatohepatitis.

Background and Aims: Activation of the inflammasome NLRP3 (NOD-, LRR- and pyrin domain containing 3) contributes to the development of non-alcoholic fatty liver disease (NAFLD) and progression to non-alcoholic steatohepatitis (NASH). Therefore, this study explored the therapeutic effects of a novel and selective NLRP3 antagonist in a murine dietary model of NASH. Methods: Groups of 12-week-old ApoE -/- mice were fed ad lib for 7 weeks with a methionine/choline deficient (MCD) and western diet (WD). After 3 weeks of diet-induced injury, mice were injected i. p. with the NLRP3 antagonist IFM-514 (100 mg/kg body weight) or vehicle (0.5% carmellose) every day, 5 days/week for a further 4 weeks. Several markers of inflammation, fibrosis and steatosis were evaluated. Whole transcriptome sequencing and panel RNA expression analysis (NanoString) were performed. Results: IFM-514 inhibited IL-1ß production in mice challenged with 20 mg/kg lipopolysaccharide, and in mouse and human inflammatory cells in vitro. IFM-514 inhibited hepatic inflammation in the in vivo non-alcoholic steatohepatitis model assessed by H&E staining and in the hepatic gene expression of inflammasome-related proinflammatory cytokines. This effect was associated with significant reduction in caspase-1 activation. Similarly, IFM-514 was efficacious in vivo in MDC-fed ApoE -/- mice, markedly reducing portal pressure, Sirius red staining and 4-hydroxyproline content compared to vehicle-treated mice. Moreover, IFM-514 significantly reduced hepatic steatosis in MCD-fed ApoE -/- mice, as evidenced by NAFLD scores, oil red O staining, hepatic triglycerides and gene expression. In WD treated animals, similar trends in inflammation and fibrosis were observed, although not sufficient IFM-514 levels were reached. Conclusion: Overall, IFM-514 reduced liver inflammation and fibrosis, with mild effects on liver steatosis in experimental murine NASH. Blocking of NLRP3 may be an attractive therapeutic approach for NASH patients.

The ecological role of bacterial seed endophytes associated with wild cabbage in the United Kingdom.

Endophytic bacteria are known for their ability in promoting plant growth and defense against biotic and abiotic stress. However, very little is known about the microbial endophytes living in the spermosphere. Here, we isolated bacteria from the seeds of five different populations of wild cabbage (Brassica oleracea L) that grow within 15 km of each other along the Dorset coast in the UK. The seeds of each plant population contained a unique microbiome. Sequencing of the 16S rRNA genes revealed that these bacteria belong to three different phyla (Actinobacteria, Firmicutes, and Proteobacteria). Isolated endophytic bacteria were grown in monocultures or mixtures and the effects of bacterial volatile organic compounds (VOCs) on the growth and development on B. oleracea and on resistance against a insect herbivore was evaluated. Our results reveal that the VOCs emitted by the endophytic bacteria had a profound effect on plant development but only a minor effect on resistance against an herbivore of B. oleracea. Plants exposed to bacterial VOCs showed faster seed germination and seedling development. Furthermore, seed endophytic bacteria exhibited activity via volatiles against the plant pathogen F. culmorum. Hence, our results illustrate the ecological importance of the bacterial seed microbiome for host plant health and development.

Microbiome Patterns in Matched Bile, Duodenal, Pancreatic Tumor Tissue, Drainage, and Stool Samples: Association with Preoperative Stenting and Postoperative Pancreatic Fistula Development.

Postoperative complications after pancreatic surgery are still a significant problem in clinical practice. The aim of this study was to characterize and compare the microbiomes of different body compartments (bile duct, duodenal mucosa, pancreatic tumor lesion, postoperative drainage fluid, and stool samples; preoperative and postoperative) in patients undergoing pancreatic surgery for suspected pancreatic cancer, and their association with relevant clinical factors (stent placement, pancreatic fistula, and gland texture). For this, solid (duodenal mucosa, pancreatic tumor tissue, stool) and liquid (bile, drainage fluid) biopsy samples of 10 patients were analyzed using 16s rRNA gene next-generation sequencing. Our analysis revealed: (i) a distinct microbiome in the different compartments, (ii) markedly higher abundance of Enterococcus in patients undergoing preoperative stent placement in the common bile duct, (iii) significant differences in the beta diversity between patients who developed a postoperative pancreatic fistula (POPF B/C), (iv) patients with POPF B/C were more likely to have bacteria belonging to the genus Enterococcus, and (v) differences in microbiome composition with regard to the pancreatic gland texture. The structure of the microbiome is distinctive in different compartments, and can be associated with the development of a postoperative pancreatic fistula.

The Role of Macrophage-Inducible C-Type Lectin in Different Stages of Chronic Liver Disease.

The macrophage-inducible C-type lectin (mincle) is part of the innate immune system and acts as a pattern recognition receptor for pathogen-associated molecular patterns (PAMPS) and damage-associated molecular patterns (DAMPs). Ligand binding induces mincle activation which consequently interacts with the signaling adapter Fc receptor, SYK, and NF-kappa-B. There is also evidence that mincle expressed on macrophages promotes intestinal barrier integrity. However, little is known about the role of mincle in hepatic fibrosis, especially in more advanced disease stages. Mincle expression was measured in human liver samples from cirrhotic patients and donors collected at liver transplantation and in patients undergoing bariatric surgery. Human results were confirmed in rodent models of cirrhosis and acute-on-chronic liver failure (ACLF). In these models, the role of mincle was investigated in liver samples as well as in peripheral blood monocytes (PBMC), tissues from the kidney, spleen, small intestine, and heart. Additionally, mincle activation was stimulated in experimental non-alcoholic steatohepatitis (NASH) by treatment with mincle agonist trehalose-6,6-dibehenate (TDB). In human NASH, mincle is upregulated with increased collagen production. In ApoE deficient mice fed high-fat western diet (NASH model), mincle activation significantly increases hepatic collagen production. In human cirrhosis, mincle expression is also significantly upregulated. Furthermore, mincle expression is associated with the stage of chronic liver disease. This could be confirmed in rat models of cirrhosis and ACLF. ACLF was induced by LPS injection in cirrhotic rats. While mincle expression and downstream signaling via FC receptor gamma, SYK, and NF-kappa-B are upregulated in the liver, they are downregulated in PBMCs of these rats. Although mincle expressed on macrophages might be beneficial for intestinal barrier integrity, it seems to contribute to inflammation and fibrosis once the intestinal barrier becomes leaky in advanced stages of chronic liver disease.

Biological activities associated with the volatile compound 2,5-bis(1-methylethyl)-pyrazine.

Pyrazines are 1,4-diazabenzene-based volatile organic compounds and known for their broad-spectrum antimicrobial activity. In the present study, we assessed the antimicrobial activity of 2,5-bis(1-methylethyl)-pyrazine, produced by Paenibacillus sp. AD87 during co-culture with Burkholderia sp. AD24. In addition, we were using transcriptional reporter assays in E. coli and mammalian cells to decipher the possible mode of action. Bacterial and mammalian luciferase reporter strains were deployed to elucidate antimicrobial and toxicological effects of 2,5-bis(1-methylethyl)-pyrazine. At high levels of exposure, 2,5-bis(1-methylethyl)-pyrazine exerted strong DNA damage response. At lower concentrations, cell-wall damage response was observed. The activity was corroborated by a general toxicity reporter assay in E. coli ΔampD, defective in peptidoglycan turnover. The maximum E. coli cell-wall stress activity was measured at a concentration close to the onset of the mammalian cytotoxicity, while other adverse outcome pathways, such as the activation of aryl hydrocarbon and estrogenic receptor, the p53 tumour suppressor and the oxidative stress-related Nrf2 transcription factor, were induced at elevated concentrations compared to the response of mammalian cells. Because of its broad-spectrum antimicrobial activity at lower concentrations and the relatively low mammalian toxicity, 2,5-bis(1-methylethyl)-pyrazine is a potential bio-based fumigant with possible applications in food industry, agriculture or logistics.

Growth promotion and inhibition induced by interactions of groundwater bacteria.

Microorganisms can produce a plethora of secondary metabolites, some acting as signaling compounds and others as suppressing agents. As yet, the potential of groundwater microbes to produce antimicrobial compounds to increase their competitiveness against other bacteria has not been examined. In this study, we developed an AlamarBlue® based high-throughput screening method that allowed for a fast and highly standardized evaluation of both growth-inhibiting and -promoting metabolites. With this technique, 149 screened bacterial isolates were grown in monocultures and in 1402 co-cultures. Co-cultivation did not increase the frequency of growth inhibition against the two tested model organisms (Staphylococcus aureus 533R4 and Escherichia coli WA321) compared to monocultures. Mainly co-cultivation of Proteobacteria induced growth inhibition of both model organisms. Only slightly increased growth promotion of S. aureus 533R4 was observed. Growth-promoting effects on E. coli WA321 were observed by supernatants from co-cultures between Bacteroidetes and Firmicutes. With the standardized screening for both growth-inhibiting and -promoting effects, this method will enable further studies to elaborate and better understand complex inter-specific interactions and networks in aquatic communities as well as in other environments.

The effect of isabelin, a sesquiterpene lactone from Ambrosia artemisiifolia on soil microorganisms and human pathogens.

Ambrosia artemisiifolia L. (common ragweed) is an invasive weed, which is well known for the strong allergenic effect of its pollen as well as for its invasiveness and impact in crop fields (e.g. causing yield losses). This species produces a broad range of sesquiterpenoids. In recent years, new bioactive molecules have been discovered in this plant, e.g. isabelin, a sesquiterpene dilactone. The bioactivity of isabelin has been already demonstrated on allergy-related receptors and its inhibitory effect on seeds of various plant species. Isabelin was tested for potential antimicrobial effects by using a selection of soil-borne bacteria and fungi and three human pathogens as model organisms. For the majority of microorganisms tested, no antimicrobial activity of isabelin was observed. However, isabelin revealed strong antimicrobial activity against the Gram-positive soil bacterium Paenibacillus sp. and against the Gram-positive, multidrug-resistant Staphylococcus aureus. The observed inhibitory activity of isabelin can enlighten the importance to study similar compounds for their effect on human pathogens and on soil and rhizosphere microorganisms.

Involvement of Burkholderiaceae and sulfurous volatiles in disease-suppressive soils.

Disease-suppressive soils are ecosystems in which plants suffer less from root infections due to the activities of specific microbial consortia. The characteristics of soils suppressive to specific fungal root pathogens are comparable to those of adaptive immunity in animals, as reported by Raaijmakers and Mazzola (Science 352:1392-3, 2016), but the mechanisms and microbial species involved in the soil suppressiveness are largely unknown. Previous taxonomic and metatranscriptome analyses of a soil suppressive to the fungal root pathogen Rhizoctonia solani revealed that members of the Burkholderiaceae family were more abundant and more active in suppressive than in non-suppressive soils. Here, isolation, phylogeny, and soil bioassays revealed a significant disease-suppressive activity for representative isolates of Burkholderia pyrrocinia, Paraburkholderia caledonica, P. graminis, P. hospita, and P. terricola. In vitro antifungal activity was only observed for P. graminis. Comparative genomics and metabolite profiling further showed that the antifungal activity of P. graminis PHS1 was associated with the production of sulfurous volatile compounds encoded by genes not found in the other four genera. Site-directed mutagenesis of two of these genes, encoding a dimethyl sulfoxide reductase and a cysteine desulfurase, resulted in a loss of antifungal activity both in vitro and in situ. These results indicate that specific members of the Burkholderiaceae family contribute to soil suppressiveness via the production of sulfurous volatile compounds.

The Ecological Role of Volatile and Soluble Secondary Metabolites Produced by Soil Bacteria.

The rich diversity of secondary metabolites produced by soil bacteria has been appreciated for over a century, and advances in chemical analysis and genome sequencing continue to greatly advance our understanding of this biochemical complexity. However, we are just at the beginning of understanding the physicochemical properties of bacterial metabolites, the factors that govern their production and ecological roles. Interspecific interactions and competitor sensing are among the main biotic factors affecting the production of bacterial secondary metabolites. Many soil bacteria produce both volatile and soluble compounds. In contrast to soluble compounds, volatile organic compounds can diffuse easily through air- and gas-filled pores in the soil and likely play an important role in long-distance microbial interactions. In this review we provide an overview of the most important soluble and volatile classes of secondary metabolites produced by soil bacteria, their ecological roles, and their possible synergistic effects.