Prescribed fire selects for a pyrophilous soil sub-community in a northern California mixed conifer forest.

Prescribed fire is a critical strategy for mitigating the effects of catastrophic wildfires. While the above-ground response to fire has been well-documented, fewer studies have addressed the effect of prescribed fire on soil microorganisms. To understand how soil microbial communities respond to prescribed fire, we sampled four plots at a high temporal resolution (two burned, two controls), for 17 months, in a mixed conifer forest in northern California, USA. Using amplicon sequencing, we found that prescribed fire significantly altered both fungal and bacterial community structure. We found that most differentially abundant fungal taxa had a positive fold-change, while differentially abundant bacterial taxa generally had a negative fold-change. We tested the null hypothesis that these communities assembled due to neutral processes (i.e., drift and/or dispersal), finding that >90% of taxa fit this neutral prediction. However, a dynamic sub-community composed of burn-associated indicator taxa that were positively differentially abundant was enriched for non-neutral amplicon sequence variants, suggesting assembly via deterministic processes. In synthesizing these results, we identified 15 pyrophilous taxa with a significant and positive response to prescribed burns. Together, these results lay the foundation for building a process-driven understanding of microbial community assembly in the context of the classical disturbance regime of fire.

Role for dithiolopyrrolones in disrupting bacterial metal homeostasis.

Natural products harbor unique and complex structures that provide valuable antibiotic scaffolds. With an increase in antibiotic resistance, natural products once again hold promise for new antimicrobial therapies, especially those with unique scaffolds that have been overlooked due to a lack of understanding of how they function. Dithiolopyrrolones (DTPs) are an underexplored class of disulfide-containing natural products, which exhibit potent antimicrobial activities against multidrug-resistant pathogens. DTPs were thought to target RNA polymerase, but conflicting observations leave the mechanisms elusive. Using a chemical genomics screen in Escherichia coli, we uncover a mode of action for DTPs-the disruption of metal homeostasis. We show that holomycin, a prototypical DTP, is reductively activated, and reduced holomycin chelates zinc with high affinity. Examination of reduced holomycin against zinc-dependent metalloenzymes revealed that it inhibits E. coli class II fructose bisphosphate aldolase, but not RNA polymerase. Reduced holomycin also strongly inhibits metallo-ß-lactamases in vitro, major contributors to clinical carbapenem resistance, by removing active site zinc. These results indicate that holomycin is an intracellular metal-chelating antibiotic that inhibits a subset of metalloenzymes and that RNA polymerase is unlikely to be the primary target. Our work establishes a link between the chemical structures of DTPs and their antimicrobial action; the ene-dithiol group of DTPs enables high-affinity metal binding as a central mechanism to inhibit metabolic processes. Our study also validates the use of chemical genomics in characterizing modes of actions of antibiotics and emphasizes the potential of metal-chelating natural products in antimicrobial therapy.

High Spatial Resolution Imaging Mass Spectrometry Reveals Chemical Heterogeneity Across Bacterial Microcolonies.

Microbes interact with the world around them at the chemical level. However, directly examining the chemical exchange between microbes and microbes and their environment, at ecological scales, i.e., the scale of a single bacterial cell or small groups of cells, remains a key challenge. Here we address this obstacle by presenting a methodology that enables matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) of bacterial microcolonies. By combining optimized sample preparation with subatmospheric pressure MALDI, we demonstrate that chemical output from groups of as few as ∼50 cells can be visualized with MALDI-IMS. Application of this methodology to Bacillus subtilis and Streptomyces coelicolor revealed heterogeneity in chemical output across microcolonies and asymmetrical metabolite production when cells grew within physiological gradients produced by Medicago sativa roots. Taken together, these results indicate that MALDI-IMS can readily visualize metabolites made by very small assemblages of bacterial cells and that even these small groups of cells can differentially produce metabolites in response to local chemical gradients.

Natural products in soil microbe interactions and evolution.

In recent years, bacterial interspecies interactions mediated by small molecule natural products have been found to give rise to a surprising array of phenotypes in soil-dwelling bacteria, especially among Streptomyces and Bacillus species. This review examines these interspecies interactions, and the natural products involved, as they have been presented in literature stemming from four disciplines: soil science, interspecies microbiology, ecology, and evolutionary biology. We also consider how these interactions fit into accepted paradigms of signaling, cueing, and coercion.

Surface-active antibiotic production as a multifunctional adaptation for postfire microorganisms.

Wildfires affect soils in multiple ways, leading to numerous challenges for colonizing microorganisms. Although it is thought that fire-adapted microorganisms lie at the forefront of postfire ecosystem recovery, the specific strategies that these organisms use to thrive in burned soils remain largely unknown. Through bioactivity screening of bacterial isolates from burned soils, we discovered that several Paraburkholderia spp. isolates produced a set of unusual rhamnolipid surfactants with a natural methyl ester modification. These rhamnolipid methyl esters (RLMEs) exhibited enhanced antimicrobial activity against other postfire microbial isolates, including pyrophilous Pyronema fungi and Amycolatopsis bacteria, compared to the typical rhamnolipids made by organisms such as Pseudomonas spp. RLMEs also showed enhanced surfactant properties and facilitated bacterial motility on agar surfaces. In vitro assays further demonstrated that RLMEs improved aqueous solubilization of polycyclic aromatic hydrocarbons, which are potential carbon sources found in char. Identification of the rhamnolipid biosynthesis genes in the postfire isolate, Paraburkholderia kirstenboschensis str. F3, led to the discovery of rhlM, whose gene product is responsible for the unique methylation of rhamnolipid substrates. RhlM is the first characterized bacterial representative of a large class of integral membrane methyltransferases that are widespread in bacteria. These results indicate multiple roles for RLMEs in the postfire lifestyle of Paraburkholderia isolates, including enhanced dispersal, solubilization of potential nutrients, and inhibition of competitors. Our findings shed new light on the chemical adaptations that bacteria employ to navigate, grow, and outcompete other soil community members in postfire environments.

microbeMASST: a taxonomically informed mass spectrometry search tool for microbial metabolomics data.

microbeMASST, a taxonomically informed mass spectrometry (MS) search tool, tackles limited microbial metabolite annotation in untargeted metabolomics experiments. Leveraging a curated database of >60,000 microbial monocultures, users can search known and unknown MS/MS spectra and link them to their respective microbial producers via MS/MS fragmentation patterns. Identification of microbe-derived metabolites and relative producers without a priori knowledge will vastly enhance the understanding of microorganisms' role in ecology and human health.

Interspecies modulation of bacterial development through iron competition and siderophore piracy.

While soil-dwelling actinomycetes are renowned for secreting natural products, little is known about the roles of these molecules in mediating actinomycete interactions. In a previous co-culture screen, we found that one actinomycete, Amycolatopsis sp. AA4, inhibited aerial hyphae formation in adjacent colonies of Streptomyces coelicolor. A siderophore, amychelin, mediated this developmental arrest. Here we present genetic evidence that confirms the role of the amc locus in the production of amychelin and in the inhibition of S. coelicolor development. We further characterize the Amycolatopsis sp. AA4 - S. coelicolor interaction by examining expression of developmental and iron acquisition genes over time in co-culture. Manipulation of iron availability and/or growth near Amycolatopsis sp. AA4 led to alterations in expression of the critical developmental gene bldN, and other key downstream genes in the S. coelicolor transcriptional cascade. In Amycolatopsis sp. AA4, siderophore genes were downregulated when grown near S. coelicolor, leading us to find that deferrioxamine E, produced by S. coelicolor, could be readily utilized by Amycolatopsis sp. AA4. Collectively these results suggest that competition for iron via siderophore piracy and species-specific siderophores can alter patterns of gene expression and morphological differentiation during actinomycete interactions.

Surface-active antibiotic production is a multifunctional adaptation for postfire microbes.

Wildfires affect soils in multiple ways, leading to numerous challenges for colonizing microbes. While it is thought that fire-adapted microbes lie at the forefront of postfire ecosystem recovery, the specific strategies that these microbes use to thrive in burned soils remain largely unknown. Through bioactivity screening of bacterial isolates from burned soils, we discovered that several Paraburkholderia spp. isolates produced a set of unusual rhamnolipid surfactants with a natural methyl ester modification. These rhamnolipid methyl esters (RLMEs) exhibited enhanced antimicrobial activity against other postfire microbial isolates, including pyrophilous Pyronema fungi and Amycolatopsis bacteria, compared to the typical rhamnolipids made by organisms such as Pseudomonas spp . RLMEs also showed enhanced surfactant properties and facilitated bacterial motility on agar surfaces. In vitro assays further demonstrated that RLMEs improved aqueous solubilization of polycyclic aromatic hydrocarbons, which are potential carbon sources found in char. Identification of the rhamnolipid biosynthesis genes in the postfire isolate, Paraburkholderia caledonica str. F3, led to the discovery of rhlM , whose gene product is responsible for the unique methylation of rhamnolipid substrates. RhlM is the first characterized bacterial representative of a large class of integral membrane methyltransferases that are widespread in bacteria. These results indicate multiple roles for RLMEs in the postfire lifestyle of Paraburkholderia isolates, including enhanced dispersal, solubilization of potential nutrients, and inhibition of competitors. Our findings shed new light on the chemical adaptations that bacteria employ in order to navigate, grow, and outcompete other soil community members in postfire environments. Significance Statement: Wildfires are increasing in frequency and intensity at a global scale. Microbes are the first colonizers of soil after fire events, but the adaptations that help these organisms survive in postfire environments are poorly understood. In this work, we show that a bacterium isolated from burned soil produces an unusual rhamnolipid biosurfactant that exhibits antimicrobial activity, enhances motility, and solubilizes potential nutrients derived from pyrolyzed organic matter. Collectively, our findings demonstrate that bacteria leverage specialized metabolites with multiple functions to meet the demands of life in postfire environments. Furthermore, this work reveals the potential of probing perturbed environments for the discovery of unique compounds and enzymes.

A Taxonomically-informed Mass Spectrometry Search Tool for Microbial Metabolomics Data.

MicrobeMASST, a taxonomically-informed mass spectrometry (MS) search tool, tackles limited microbial metabolite annotation in untargeted metabolomics experiments. Leveraging a curated database of >60,000 microbial monocultures, users can search known and unknown MS/MS spectra and link them to their respective microbial producers via MS/MS fragmentation patterns. Identification of microbial-derived metabolites and relative producers, without a priori knowledge, will vastly enhance the understanding of microorganisms' role in ecology and human health.

Discretely calibrated regulatory loops controlled by ppGpp partition gene induction across the 'feast to famine' gradient in Escherichia coli.

Bacteria comprehensively reorganize their global gene expression when faced with starvation. The alarmone ppGpp facilitates this massive response by co-ordinating the downregulation of genes of the translation apparatus, and the induction of biosynthetic genes and the general stress response. Such a large reorientation requires the activities of multiple regulators, yet the regulatory network downstream of ppGpp remains poorly defined. Transcription profiling during isoleucine depletion, which leads to gradual starvation (over > 100 min), allowed us to identify genes that required ppGpp, Lrp and RpoS for their induction and to deduce the regulon response times. Although the Lrp and RpoS regulons required ppGpp for their activation, they were not induced simultaneously. The data suggest that metabolic genes, i.e. those of the Lrp regulon, require only a low level of ppGpp for their induction. In contrast, the RpoS regulon was induced only when high levels of ppGpp accumulated. We tested several predictions of a model that explains how bacteria allocate transcriptional resources between metabolism and stress response by discretely tuning two regulatory circuits to different levels of ppGpp. The emergent regulatory structure insures that stress survival circuits are only triggered if homeostatic metabolic networks fail to compensate for environmental deficiencies.

Ecological drivers of division of labour in Streptomyces.

Division of labour occurs when different individuals, cells or tissues become specialised to perform complementary tasks that benefit the whole organism or social group. Although long studied in multicellular organisms and colonies of social insects, several recent studies have established that division of labour is common in microorganisms. We review recent work on the division of labour in unicellular and multicellular bacteria, with a particular focus on reproductive and metabolic divisions of labour in actinomycetes. Actinomycetes show enormous variation in sporophore morphology and spore production patterns that likely affect the potential for cooperative interactions within colonies. They also display both irreversible genetic and spatiotemporally regulated phenotypic divisions of labour that structure antibiotic production. We highlight outstanding questions in this group of multicellular bacteria and outline factors that can modify the expression of division of labour across microbes.

Harnessing Rare Actinomycete Interactions and Intrinsic Antimicrobial Resistance Enables Discovery of an Unusual Metabolic Inhibitor.

Bacterial natural products have historically been a deep source of new medicines, but their slowed discovery in recent decades has put a premium on developing strategies that enhance the likelihood of capturing novel compounds. Here, we used a straightforward approach that capitalizes on the interactive ecology of "rare" actinomycetes. Specifically, we screened for interactions that triggered the production of antimicrobials that inhibited the growth of a bacterial strain with exceptionally diverse natural antimicrobial resistance. This strategy led to the discovery of a family of antimicrobials we term the dynaplanins. Heterologous expression enabled identification of the dynaplanin biosynthetic gene cluster, which was missed by typical algorithms for natural product gene cluster detection. Genome sequencing of partially resistant mutants revealed a 2-oxo acid dehydrogenase E2 subunit as the likely molecular target of the dynaplanins, and this finding was supported by computational modeling of the dynaplanin scaffold within the active site of this enzyme. Thus, this simple strategy, which leverages microbial interactions and natural antibiotic resistance, can enable discovery of molecules with unique antimicrobial activity. In addition, these results indicate that primary metabolism may be a direct target for inhibition via chemical interference in competitive microbial interactions. IMPORTANCE Many antibiotics were originally discovered from microbes. However, in recent decades, resistance to current treatments has risen, while novel antibiotic discovery has become increasingly challenging. Thus, there is a need to develop new strategies to find novel antimicrobials. Here, we incorporated three levels of innovation into a single, simple discovery pipeline: focusing on understudied bacteria with a high potential for producing antibiotics, growing these bacteria in binary microbial interactions, and screening for activity against a multidrug-resistant bacterium. This led us to discover a family of antimicrobials that we call the dynaplanins, which are synthesized by genes that were not detected by typical prediction algorithms. We found that dynaplanins likely block the function of one of three related enzymes called 2-oxo acid dehydrogenases, which are vital to cellular metabolism. Overall, our strategy based on bacterial competition led to discovery of a novel antibiotic that inhibits the ability to metabolize nutrients.

Structure and biosynthesis of amychelin, an unusual mixed-ligand siderophore from Amycolatopsis sp. AA4.

Actinobacteria generate a large number of structurally diverse small molecules with potential therapeutic value. Genomic analyses of this productive group of bacteria show that their genetic potential to manufacture small molecules exceeds their observed ability by roughly an order of magnitude, and this revelation has prompted a number of studies to identify members of the unknown majority. As a potential window into this cryptic secondary metabolome, pairwise assays for developmental interactions within a set of 20 sequenced actinomycetes were carried out. These assays revealed that Amycolatopsis sp. AA4, a so-called "rare" actinomycete, produces a novel siderophore, amychelin, which alters the developmental processes of several neighboring streptomycetes. Using this phenotype as an assay, we isolated amychelin and solved its structure by NMR and MS methods coupled with an X-ray crystallographic analysis of its Fe-complex. The iron binding affinity of amychelin was determined using EDTA competition assays, and a biosynthetic cluster was identified and annotated to provide a tentative biosynthetic scheme for amychelin.

The cvn8 Conservon System Is a Global Regulator of Specialized Metabolism in Streptomyces coelicolor during Interspecies Interactions.

Interspecies interactions are known to activate specialized metabolism in diverse actinomycetes. However, how interspecies cues are sensed and ultimately lead to induction of specialized metabolite biosynthetic gene clusters remains largely unexplored. Using transcriptome sequencing (RNA-seq), we analyzed genes that were transcriptionally induced in the model actinomycete Streptomyces coelicolor during interactions with four different actinomycetes, including genes that encode unusual regulatory systems known as conservons. Deletions in one such system, encoded by the cvn8 genes, led to altered patterns of pigmented antibiotic production by S. coelicolor during interactions. Further transcriptomic analysis of mutants lacking each of the five genes in the cvn8 locus demonstrated that this system is a global regulator of at least four different specialized metabolite biosynthetic pathways. How conservon systems work at the mechanistic level to regulate gene expression is not well understood, although it has been hypothesized that they may function in a way similar to eukaryotic G-protein-coupled receptors. The data presented here indicate that the gene products of the cvnA8 and cvnF8 (SCO6939) genes likely function together in one part of the Cvn8 signaling cascade, while the cvnC8 and cvnD8 gene products likely function together in another part. Importantly, because cvnD8 likely encodes a Ras-like GTPase, these results connect G-protein-mediated signaling to gene regulation in a bacterium. Additionally, deletion of any of the cvn8 genes led to abnormally high expression of an adjacent cryptic lanthipeptide biosynthetic gene cluster, indicating that conservon systems may be fruitful targets for manipulation to activate silent specialized metabolite biosynthetic pathways. IMPORTANCE Interactions between different species of actinomycete bacteria often trigger one of the strains to produce specialized metabolites, such as antibiotics. However, how this induction occurs at the genetic level is poorly understood. Using transcriptomic methods, we show that an unusual regulatory system, known as a conservon system, is responsible for regulating expression of multiple specialized metabolite biosynthetic gene clusters in the organism Streptomyces coelicolor during interactions. Conservon systems are unusual because they appear to employ small GTPases as an important component of their signaling cascades. Small GTPases are common in eukaryotic signaling pathways, but the results presented here are notable since they implicate a system that includes a small GTPase in global gene regulation in a bacterium. Mutants lacking this conservon system also showed abnormally high expression of a gene cluster involved in making an unknown specialized metabolite, suggesting that conservon mutants might be useful for driving natural product discovery.

Pyrolyzed Substrates Induce Aromatic Compound Metabolism in the Post-fire Fungus, Pyronema domesticum.

Wildfires represent a fundamental and profound disturbance in many ecosystems, and their frequency and severity are increasing in many regions of the world. Fire affects soil by removing carbon in the form of CO2 and transforming remaining surface carbon into pyrolyzed organic matter (PyOM). Fires also generate substantial necromass at depths where the heat kills soil organisms but does not catalyze the formation of PyOM. Pyronema species strongly dominate soil fungal communities within weeks to months after fire. However, the carbon pool (i.e., necromass or PyOM) that fuels their rise in abundance is unknown. We used a Pyronema domesticum isolate from the catastrophic 2013 Rim Fire (CA, United States) to ask whether P. domesticum is capable of metabolizing PyOM. Pyronema domesticum grew readily on agar media where the sole carbon source was PyOM (specifically, pine wood PyOM produced at 750°C). Using RNAseq, we investigated the response of P. domesticum to PyOM and observed a comprehensive induction of genes involved in the metabolism and mineralization of aromatic compounds, typical of those found in PyOM. Lastly, we used 13C-labeled 750°C PyOM to demonstrate that P. domesticum is capable of mineralizing PyOM to CO2. Collectively, our results indicate a robust potential for P. domesticum to liberate carbon from PyOM in post-fire ecosystems and return it to the bioavailable carbon pool.

Genetic Network Architecture and Environmental Cues Drive Spatial Organization of Phenotypic Division of Labor in Streptomyces coelicolor.

A number of bacteria are known to differentiate into cells with distinct phenotypic traits during processes such as biofilm formation or the development of reproductive structures. These cell types, by virtue of their specialized functions, embody a division of labor. However, how bacteria build spatial patterns of differentiated cells is not well understood. Here, we examine the factors that drive spatial patterns in divisions of labor in colonies of Streptomyces coelicolor, a multicellular bacterium capable of synthesizing an array of antibiotics and forming complex reproductive structures (e.g., aerial hyphae and spores). Using fluorescent reporters, we demonstrate that the pathways for antibiotic biosynthesis and aerial hypha formation are activated in distinct waves of gene expression that radiate outwards in S. coelicolor colonies. We also show that the spatiotemporal separation of these cell types depends on a key activator in the developmental pathway, AdpA. Importantly, when we manipulated local gradients by growing competing microbes nearby, or through physical disruption, expression in these pathways could be decoupled and/or disordered, respectively. Finally, the normal spatial organization of these cell types was partially restored with the addition of a siderophore, a public good made by these organisms, to the growth medium. Together, these results indicate that spatial divisions of labor in S. coelicolor colonies are determined by a combination of physiological gradients and regulatory network architecture, key factors that also drive patterns of cellular differentiation in multicellular eukaryotic organisms.IMPORTANCEStreptomyces coelicolor is a multicellular bacterium that differentiates into specialized cell types and produces a diverse array of natural products. While much is known about the genetic networks that regulate development and antibiotic biosynthesis in S. coelicolor, what drives the spatial organization of these activities within a colony remains to be explored. By using time-lapse microscopy to monitor gene expression in developmental and antibiotic biosynthesis pathways, we found that expression in these pathways occurs in spatiotemporally separated waves. Normally, expression of the antibiotic biosynthesis pathway preceded expression in the developmental pathway; however, this order was compromised in a mutant lacking a key developmental regulator. Furthermore, when we disrupted the local gradients during S. coelicolor growth, we observed disordered patterns of gene expression within colonies. Together, these results indicate that spatial divisions of labor in S. coelicolor colonies are determined by a combination of regulatory network architecture and physiological gradients.

Multiple lineages of Streptomyces produce antimicrobials within passalid beetle galleries across eastern North America.

Some insects form symbioses in which actinomycetes provide defense against pathogens by making antimicrobials. The range of chemical strategies employed across these associations, and how these strategies relate to insect lifestyle, remains underexplored. We assessed subsocial passalid beetles of the species Odontotaenius disjunctus, and their frass (fecal material), which is an important food resource within their galleries, as a model insect/actinomycete system. Through chemical and phylogenetic analyses, we found that O. disjunctus frass collected across eastern North America harbored multiple lineages of Streptomyces and diverse antimicrobials. Metabolites detected in frass displayed synergistic and antagonistic inhibition of a fungal entomopathogen, Metarhizium anisopliae, and multiple streptomycete isolates inhibited this pathogen when co-cultivated directly in frass. These findings support a model in which the lifestyle of O. disjunctus accommodates multiple Streptomyces lineages in their frass, resulting in a rich repertoire of antimicrobials that likely insulates their galleries against pathogenic invasion.

Cooperation, Competition, and Specialized Metabolism in a Simplified Root Nodule Microbiome.

Microbiomes associated with various plant structures often contain members with the potential to make specialized metabolites, e.g., molecules with antibacterial, antifungal, or siderophore activities. However, when and where microbes associated with plants produce specialized metabolites, and the potential role of these molecules in mediating intramicrobiome interactions, is not well understood. Root nodules of legume plants are organs devoted to hosting symbiotic bacteria that fix atmospheric nitrogen and have recently been shown to harbor a relatively simple accessory microbiome containing members with the ability to produce specialized metabolites in vitro On the basis of these observations, we sought to develop a model nodule microbiome system for evaluating specialized microbial metabolism in planta Starting with an inoculum derived from field-grown Medicago sativa nodules, serial passaging through gnotobiotic nodules yielded a simplified accessory community composed of four members: Brevibacillus brevis, Paenibacillus sp., Pantoea agglomerans, and Pseudomonas sp. Some members of this community exhibited clear cooperation in planta, while others were antagonistic and capable of disrupting cooperation between other partners. Using matrix-assisted laser desorption ionization-imaging mass spectrometry, we found that metabolites associated with individual taxa had unique distributions, indicating that some members of the nodule community were spatially segregated. Finally, we identified two families of molecules produced by B. brevisin planta as the antibacterial tyrocidines and a novel set of gramicidin-type molecules, which we term the britacidins. Collectively, these results indicate that in addition to nitrogen fixation, legume root nodules are likely also sites of active antimicrobial production.

Inducible Antibacterial Activity in the Bacillales by Triphenyl Tetrazolium Chloride.

The world is in the midst of an antimicrobial resistance crisis, driving a need to discover novel antibiotic substances. Using chemical cues as inducers to unveil a microorganism's full metabolic potential is considered a successful strategy. To this end, we investigated an inducible antagonistic behavior in multiple isolates of the order Bacillales, where large inhibition zones were produced against Ralstonia solanacearum only when grown in the presence of the indicator triphenyl tetrazolium chloride (TTC). This bioactivity was produced in a TTC-dose dependent manner. Escherichia coli and Staphylococcus sp. isolates were also inhibited by Bacillus sp. strains in TTC presence, to a lesser extent. Knockout mutants and transcriptomic analysis of B. subtilis NCIB 3610 cells revealed that genes from the L-histidine biosynthetic pathway, the purine, pyrimidine de novo synthesis and salvage and interconversion routes, were significantly upregulated. Chemical space studied through metabolomic analysis, showed increased presence of nitrogenous compounds in extracts from induced bacteria. The metabolites orotic acid and L-phenylalaninamide were tested against R. solanacearum, E. coli, Staphylococcus sp. and B. subtilis, and exhibited activity against pathogens only in the presence of TTC, suggesting a biotransformation of nitrogenous compounds in Bacillus sp. cells as the plausible cause of the inducible antagonistic behavior.