Microbial Antagonists Research Articles

Klebsiella pneumoniae type VI secretion system-mediated microbial competition is PhoPQ controlled and reactive oxygen species dependent.

Klebsiella pneumoniae is recognized as an urgent threat to human health due to the increasing isolation of multidrug resistant strains. Hypervirulent strains are a major concern due to their ability to cause life-threating infections in healthy hosts. The type VI secretion system (T6SS) is widely implicated in microbial antagonism, and it mediates interactions with host eukaryotic cells in some cases. In silico search for genes orthologous to T6SS component genes and T6SS effector genes across 700 K. pneumoniae genomes shows extensive diversity in T6SS genes across the K. pneumoniae species. Temperature, oxygen tension, pH, osmolarity, iron levels, and NaCl regulate the expression of the T6SS encoded by a hypervirulent K. pneumoniae strain. Polymyxins and human defensin 3 also increase the activity of the T6SS. A screen for regulators governing T6SS uncover the correlation between the transcription of the T6SS and the ability to kill E. coli prey. Whereas H-NS represses the T6SS, PhoPQ, PmrAB, Hfq, Fur, RpoS and RpoN positively regulate the T6SS. K. pneumoniae T6SS mediates intra and inter species bacterial competition. This antagonism is only evident when the prey possesses an active T6SS. The PhoPQ two component system governs the activation of K. pneumoniae T6SS in bacterial competitions. Mechanistically, PhoQ periplasmic domain, and the acid patch within, is essential to activate K. pneumoniae T6SS. Klebsiella T6SS also mediates anti-fungal competition. We have delineated the contribution of each of the individual VgrGs in microbial competition and identified VgrG4 as a T6SS effector. The DUF2345 domain of VgrG4 is sufficient to intoxicate bacteria and yeast. ROS generation mediates the antibacterial effects of VgrG4, and the antitoxin Sel1E protects against the toxic activity of VgrG4. Our findings provide a better understanding of the regulation of the T6SS in bacterial competitions, and place ROS as an early event in microbial competition.

Yeasts and Bacterial Consortia from Kefir Grains Are Effective Biocontrol Agents of Postharvest Diseases of Fruits.

Fungal pathogens in fruits and vegetables cause significant losses during handling, transportation, and storage. Biological control with microbial antagonists replacing the use of chemical fungicides is a major approach in postharvest disease control, and several products based on single antagonists have been developed but have limitations related to reduced and inconsistent performance under commercial conditions. One possible approach to enhance the biocontrol efficacy is to broaden the spectrum of the antagonistic action by employing compatible microbial consortia. Here, we explore commercial kefir grains, a natural probiotic microbial consortium, by culture-dependent and metagenomic approaches and observed a rich diversity of co-existing yeasts and bacterial population. We report effective inhibition of the postharvest pathogen Penicillium expansum on apple by using the grains in its fresh commercial and milk-activated forms. We observed few candidate bacteria and yeasts from the kefir grains that grew together over successive enrichment cycles, and these mixed fermentation cultures showed enhanced biocontrol activities as compared to the fresh commercial or milk-activated grains. We also report several individual species of bacteria and yeasts with biocontrol activities against Penicillium rots on apple and grapefruit. These species with antagonistic properties could be further exploited to develop a synthetic consortium to achieve enhanced antagonistic effects against a wide range of postharvest pathogens.

Dynamics of bacterial population growth in biofilms resemble spatial and structural aspects of urbanization

Biofilms develop from bacteria bound on surfaces that grow into structured communities (microcolonies). Although surface topography is known to affect bacterial colonization, how multiple individual settlers develop into microcolonies simultaneously remains underexplored. Here, we use multiscale population-growth and 3D-morphometric analyses to assess the spatiotemporal development of hundreds of bacterial colonizers towards submillimeter-scale microcolony communities. Using an oral bacterium (Streptococcus mutans), we find that microbial cells settle on the surface randomly under sucrose-rich conditions, regardless of surface topography. However, only a subset of colonizers display clustering behavior and growth following a power law. These active colonizers expand three-dimensionally by amalgamating neighboring bacteria into densely populated microcolonies. Clustering and microcolony assembly are dependent on exopolysaccharides, while population growth dynamics and spatial structure are affected by cooperative or antagonistic microbes. Our work suggests that biofilm assembly resembles certain spatial-structural features of urbanization, where population growth and expansion can be influenced by type of settlers, neighboring cells, and further community merging and scaffolding occurring at various scales.

The Effect of a combination of growing media and biological agents to control bacterial wilt disease of potato (Ralstoniasolanacearum) in greenhouse

Potato (Solanumtuberosum L.) in Indonesia is a commodity that priorities todeveloped, and has the potential to be marketed in the domestic as well as export. One of the obstacles is important that faced is pathogen soil-borne which is caused by the Ralstoniasolanacearum. This pathogen can causing loss of yield of potato 40-100%. Bacillus sp. and Pseudemonas fluorescens are antagonistic microbes that are proven to be able to control bacterial wilt disease by 69,95 % and 61,67%. This study aimed to determine the effect of the comparison composition of growing media and the ability of some agents of biological within suppress the growth of the disease wilt bacteria of potatoes which is caused by R. solanacearum.This research using completely randomized design (CRD) nonfactorial with 10 treatments, namely: M0(top soil), M1(top soil, R. solanacearum), M2(top soil, R. solanacearum, P.fluorescens), M3(top soil, R. solanacearum, Bacillus subtilis), M4(top soil, Chicken manure, R. solanacearum, P.fluorescens), M5 until M9 are combination of them. The results showed that the application of biological agents was able to suppress the growth of the pathogen R. solanacearum, as well as significantly affect plant height, number of shoots, and production.

Plants and Associated Soil Microbiota Cooperatively Suppress Plant-Parasitic Nematodes.

Disease suppressive soils with specific suppression of soil-borne pathogens and parasites have been long studied and are most often of microbiological origin. As for the plant-parasitic nematodes (PPN), which represent a huge threat to agricultural crops and which successfully defy many conventional control methods, soil progression from conducive to suppressive state is accompanied by the enrichment of specific antagonistic microbial consortia. However, a few microbial groups have come to the fore in diminishing PPN in disease suppressive soils using culture-dependent methods. Studies with cultured strains resulted in understanding the mechanisms by which nematodes are antagonized by microorganisms. Recent culture-independent studies on the microbiome associated with soil, plant roots, and PPN contributed to a better understanding of the functional potential of disease suppressive microbial cohort. Plant root exudation is an important pathway determining host-microbe communication and plays a key role in selection and enrichment of a specific set of microbial antagonists in the rhizosphere as first line of defense against crop pathogens or parasites. Root exudates comprising primary metabolites such as amino acids, sugars, organic acids, and secondary metabolites can also cause modifications in the nematode surface and subsequently affect microbial attachment. A positive interaction between hosts and their beneficial root microbiota is correlated with a low nematode performance on the host. In this review, we first summarized the historical records of nematode-suppressive soils and then focused on more recent studies in this aspect, emphasizing the advances in studying nematode-microbe interactions over time. We highlighted nematode biocontrol mechanisms, especially parasitism, induced systemic resistance, and volatile organic compounds using microbial consortia, or bacterial strains of the genera Pasteuria, Bacillus, Pseudomonas, Rhizobium, Streptomyces, Arthrobacter, and Variovorax, or fungal isolates of Pochonia, Dactylella, Nematophthora, Purpureocillium, Trichoderma, Hirsutella, Arthrobotrys, and Mortierella. We discussed the importance of root exudates in plant communication with PPN and soil microorganisms, emphasizing their role in microbial attachment to the nematode surface and subsequent events of nematode parasitism. Comprehensive understanding of the plant-beneficial microbial consortia and the mechanisms underlying disease suppression may help to develop synthetic microbial communities for biocontrol of PPN, thereby reducing nematicides and fertilizers inputs.

Use of yeasts from different environments for the control of Penicillium expansum on table grapes at storage temperature

A wide range of fresh fruits and vegetables is attacked by Penicillium species causing diseases during their postharvest handling. Many of these species are psychrotrophic and they are able to cause food spoilage at refrigeration temperature as happens with table grapes. After the harvest, grape bunches are stored inside boxes with SO2 generator pads to reduce the contamination with fungal conidia. However, SO2 residues are dangerous to people allergic to sulfites and they negatively affect the quality of fresh fruit. Biological control of phytopathogens with microbial antagonists naturally present on fruit surfaces could be helpful against postharvest diseases. The present study aimed to select native yeasts isolated from fermentation microenvironments and the surface of refrigerated grapes for their use in the biological control of P. expansum on table grapes stored in cold rooms. Non-pathogenic and pathogenic Penicillium species were isolated, and the four most aggressive pathogen isolates were identified as Penicillium expansum. Twenty yeast isolates identified as Aureobasidium pullulans, Cryptococcus magnus, Metschnikowia pulcherrima and Rhodotorula glutinis presented positive antagonistic activity against Penicillium expansum; they controlled the development of at least one of the fungi, significantly reducing the disease incidence. The results showed that three antagonistic yeasts (M. pulcherrima 22, 36 and 43) reduced the disease incidence and severity of all 4 P. expansum isolates. It was also found that the fruit surface is not the only source for isolation of biological control agents. Microenvironments with different stress conditions could be a promising source to isolate antagonistic microorganisms.

Effect of microbial non-volatile metabolites on growth of Fusarium oxysporum f. sp. ciceri and lini

Rhizospheric fungal antagonists activity against phytopathogen by production of volatile and non-volatile metabolites. In vitro assessments revealed their antibiosis and mycoparasitic ability to affect growth of the pathogen. Soil borne fungi, bacteria and actinomycetes were identified and used as antagonistic microbes against some soil borne pathogens. The efficacy of volatile and non-volatile metabolites produced by 14 selected dominant microbes against the common fungal test pathogens (F. oxysporum f. sp. ciceri & lini) were suppressed with colony interaction, volatile, and non-volatile metabolites emanated from different leaf surface microbes like Aspergillus niger, A. terreus, A. luchuensis, Trichoderma harzianum, Penicillium citrinum, Streptomyces rimosus, and S. aureofaciens and prior application of these antagonists effectively reduced the development of Fusarium wilt in chick pea and linseed.

A review of the postharvest characteristics and pre-packaging treatments of citrus fruit

Once harvested, fruit continue to respire, which is further exacerbated by elevated temperatures in the field and during transport to packhouses. This favors the proliferation of pathogens, which is detrimental to the postharvest fruit quality and, consequently, results in a decrease in the fruit shelf life. The aim of this review is to highlight the common citrus postharvest disorders and the various pre-packaging treatments that can be used to alleviate such disorders and promote fruit quality. Hot water, surface coatings, ultra-violet irradiation, chlorine (hypochlorous), salt treatments and microbial antagonists have been beneficial in maintaining the citrus quality and reducing the prevalence of postharvest decay. Environmentally friendly anolyte water has also proven to be a favourable postharvest treatment. Integrated treatments, such as hot water treatments and chlorine disinfection, have been successfully used in the global citrus industry. The use of integrated pre-packaging treatments improved the quality and shelf life of citrus, compared to individual treatments. An effective combination of pre-packaging treatments should include: (1) disinfectant; (2) curative and (3) preventive treatments to control pre- and postharvest pathogens.

Pathogenic fungi associated with storage rot of Colocasia esculenta and evaluation of bioformulations against the pathogen

Taro (Colocasia esculenta) is a major tropical tuber crop widely cultivated in India. Post-harvest loss of C. esculenta root and tubers had emerged as one of the major constraints in the production of taro and might have detrimental effects on food security. Our study on storage rot of C. esculenta tubers revealed that Fusarium oxysporum was mostly responsible for the post-harvest loss of the crop. Pathogenicity test confirmed its role as the causal agent of storage rot in taro. The pathogen was further confirmed based on molecular identification by amplifying ITS region of the rDNA using ITS1 and ITS4. Based on BLST analysis it showed maximum homology (95-99%) with F. oxysporum. Efficacy of a few microbial antagonists were tested against the pathogen in vitro which revealed that the treatment with combination of Trichoderma viride + Pseudomonas fluorescens + Bacillus thuringiensis showed highest inhibition of F. oxysporum (94.05%) followed by treatment of T. viride alone (65.17%).

Isolation of potential biosurfactant bacteria from railway diesel engine yard soil, its pcr detection and antifungal activity Rhizoctonia solani of Potato

Environmental factors force us to focus on new sustainable eco-friendly approaches such as microbial antagonists for the momagemont of soil-borne pathogens, therefore soil samples were collected near Shakurbasti diesel engine shed yard, New Delhi and novel biosurfactant bacteria obtained were screened, isolated and processed. Out of 10 SKNDLS (1–10) bacteria, only SKNDLS-08 isolate was grown on GSP agar, MacConkey agar as well as on Pseudomonas agar plates that was identified as putative Pseudomonas spp. and exhibited positive biosurfactant activity through different screening methods i.e. penetration assay, oil displacement (+1.7cm), hemolytic activity (>2cm) and EI24 (80%), positive blue plate agar plate (>2.5cm), modified drop collapse method. The SKNDLS-08 isolate inhibited 44.91%±0.10 of Rhizoctonia solani mycelial growth within 72h of post inoculation under vitro test and at 3% conc. of culture filtrate in food poisoning assay. PCR detection with Pseudomonas genus (PA-30, 618bp) as well species-specific (PA-31, 956bp) primers confirms the identity as Pseudomonas aeruginosa. SKNDLS-08 isolate is an interesting bacterium to investigate for the genes involved in secondary metabolite production in other diseases management applications in agriculture research.

Pengendalian Penyakit Karat Putih (Puccinia horiana) dengan Menggunakan Bakteri Antagonis Pada Tanaman Krisan

The study aims to analyze the techniques of controlling white rust, Pucinia horiana by using antagonistic bacteria. The results showed that all antagonistic microbes tested for controlling white rust (P. horiana) showed good ability to suppress because all were significantly different from the control and Pseudomonas flourescens was proven to be the most effective at suppressing the development of rust disease which was equivalent to the use of synthetic chlorotalonyl fungicides. Besides that, P. flourescens application increases plant height growth and quality of chrysanthemum flower caused by the ability of the two microbes as Plant Growth Promoting Rhizobacteria.

Surfactin-producing Bacillus velezensis 1B-23 and Bacillus sp. 1D-12 protect tomato against bacterial canker caused by Clavibacter michiganensis subsp. michiganensis

Bacillus strains are known to produce cyclic lipopeptides that are capable of providing protection against plant pathogens. Such abilities could be utilized to protect greenhouse tomatoes against diseases including bacterial canker caused by Clavibacter michiganensis subsp. michiganensis (Cmm). In the present study, Bacillus velezensis strain 1B-23 and Bacillus sp. strain 1D-12 were assessed for their potential biocontrol abilities against Cmm strain 98–1 (Cmm98–1). Both Bacillus strains interfered with growth of Cmm98–1 in vitro, as determined by agar plate assays to screen for microbial antagonism. Inoculation of Cmm98–1 infected tomato plants with B. velezenis 1B-23 or Bacillus sp. 1D-12 lead to significantly reduced disease incidence in a greenhouse setting. Liquid Chromatography coupled to Mass Spectrometry (LC-MS) of 1B-23 and 1D-12 extracts identified [Leu7]surfactin C13 (often called surfactin A), [Leu7]surfactin C14 (often called surfactin B) and [Leu7]surfactin C15 (often called surfactin C) in fractions of extracts that inhibited growth of Cmm98–1.

Identification of Differentially Expressed Genes in Trichoderma koningii IABT1252 During Its Interaction with Sclerotium rolfsii.

Sclerotium rolfsii, a soil-borne fungal pathogen, infects more than 500 crop species and causes stem rot/collar rot/seed rot/southern blight/wilt in a wide variety of crops which results in significant yield loses. Presently, antagonistic microbes are gaining more importance in managing plant pathogens because they control the pathogen in an environment-friendly manner. Trichoderma is an antagonistic fungi and most popularly used biocontrol agent against phytopathogenic fungi. It is predominantly used to treat soil and seed for the control of Sclerotium rolfsii infestation. In this study, the Trichoderma koningii IABT1252 that performed better in controlling groundnut seed/ seedling rot caused by S. rolfsii in pot experiments were selected to know the molecular basis for the control. Differentially expressed genes in Trichoderma at two different stages of interaction (prior to contact and after contact with S. rolfsii) were identified. In both the stages, some of the differentially expressed genes included ones coding for hydrolytic enzymes, secondary metabolite biosynthesis, transcription factors, signaling proteins, transporter proteins, and proteins involved in mycoparasitic process of Trichoderma.

The Effects of Trichoderma Fungi on the Tunneling, Aggregation, and Colony-Initiation Preferences of Black-Winged Subterranean Termites, Odontotermes formosanus (Blattodea: Termitidae)

The black-winged subterranean termite, Odontotermes formosanus Shiraki, is a severe pest of plantations and forests in China. This termite cultures symbiotic Termitomyces in the fungal combs, which are challenged by antagonistic microbes such as Trichoderma fungi. In a previous study we showed that O. formosanus workers made significantly fewer tunnels in sand containing commercially formulated conidia of Trichoderma viride Pers. ex Fries compared with untreated sand. Herein, we hypothesize that fungi in the genus Trichoderma exert repellent effects on O. formosanus. Different choice tests were conducted to evaluate the tunneling and aggregation behaviors of O. formosanus workers reacting to sand/soil containing the unformulated conidia of seven Trichoderma fungi (Trichoderma longibrachiatum Rifai, Trichoderma koningii Oud., Trichoderma harzianum Rifai, Trichoderma hamatum (Bon.) Bain, Trichoderma atroviride Karsten, Trichoderma spirale Indira and Kamala, and T. viride). We also investigated the colony-initiation preference of paired O. formosanus adults to soil treated with Trichoderma conidia (T. koningii or T. longibrachiatum) versus untreated soil. Tunneling-choice tests showed that sand containing conidia of nearly all Trichoderma fungi tested (except T. harzianum) significantly decreased tunneling activity in O. formosanus workers compared with untreated sand. Aggregation-choice test showed that T. koningii, T. atroviride and T. spirale repelled O. formosanus workers, whereas T. longibrachiatum and T. hamatum attracted termites. There was no significant difference in proportions of paired adults that stayed and laid eggs in the soil blocks treated with conidia of Trichoderma fungi and untreated ones. Our study showed that Trichoderma fungi generally repelled tunneling in O. formosanus, but may exert varied effects on aggregation preference by workers.

In Vitro Efficacy of Microbial Antagonists, Botanical Extracts and Synthetic Chemicals against Mango Quick Wilt Pathogen Ceratocystis Manginecans

ABSTRACT Ceratocystis manginecans is a cause of mango quick wilt disease. The pathogen has inflicted significant yield loss to the mango industry of Pakistan. The present study was undertaken to evaluate the inhibitory activity of different microbial antagonists, botanical extracts and fungicides on C. manginecans growth in vitro. Among the three microbial antagonists tested, T. harzianum produced the highest inhibition (87.3%) of the pathogen in dual culture. Allium sativum showed the highest inhibitory (71.5%) activity against C. manginecans, while, Topsin-M (Thiophanate Methyl) was the most effective (84.7%) among fungicides in a poison food assay. This study addresses the gap of information on potential chemical and biological control agents that might find potential application to control mango quick wilt disease. To our knowledge, this is the first report of excellent antifungal activity of A. sativum against C. manginecans. Taken together these findings T. harzianum, A. sativum, and Topsin-M should be tested further at field level so that they can be included as components of integrated management program of mango quick wilt.

Genome Sequencing of Pantoea agglomerans strain NC, isolated from peanut bacterial wilt nursery

Pantoea agglomerans was ubiquitous in nature and it has been identified as an effective microbial antagonist against fungal pathogens associated with several plants. Here, we report the draft genome sequence of P. agglomerans strain NC which was isolated from bacterial wilt infected peanut (Arachis hypogaea) in Nan Chang city of China.

Biological control of early blight on potato caused by Altrnaria solani by microbial antagonists

Early blight (Alternaria solani) is a potential disease of potato that reduces its production globally both in conventional and tunnel cultivations. Due to variability in pathogenic isolates, prolonged active disease cycle phase and broad host range early blight is very difficult to manage. 8 microorganisms as a bioagent exhibiting inhibitory affects against Alternaria solani, were screened for their activity towards A. solani by a dual culture in vitro assay and in vivo (whole plant) test.in vitro studies indicated that the microorganism’s strains strongly inhibited the mycelial growth of the pathogen. The effect of microorganism’s strains on the mycelial growth (mm) of the pathogen proved to be highest with Trichoderma sp. (0.55) followed by Pseudomonas brassica cearum (0.74) and Pseudomonas jessenii (0.81) on the high concentration (106 cells ml-1) compared to the control (2.30). On the other hand, Bacillus mycoides (2.14) in vivo studies 9 microorganism’s strains were applied in two different application (foliar – soil) and two different varieties (Labella – Romano). The results showed significant reductions in the disease severity (%) with the treatment by Trichoderma sp (2%) followed by Bacillus thuringiensis (3%) and Bacillus mycoides (5%) compared with the control (46%) of Romano variety, while there were a less significant reductions in the disease severity (%)with the treatments compared with the control (16%) of Labella variety . The efficacy of antagonists to suppress the early blight disease varied in respect to the time and type of application.

The Impact of Lactoferrin on the Growth of Intestinal Inhabitant Bacteria.

Lactoferrin (Lf) is an iron-binding milk glycoprotein that promotes the growth of selected probiotic strains. The effect of Lf on the growth and diversification of intestinal microbiota may have an impact on several issues, including (i) strengthening the permeability of the epithelial cell monolayer, (ii) favoring the microbial antagonism that discourages the colonization and proliferation of enteric pathogens, (iii) enhancing the growth and maturation of cell-monolayer components and gut nerve fibers, and (iv) providing signals to balance the anti- and pro-inflammatory responses resulting in gut homeostasis. Given the beneficial role of probiotics, this contribution aims to review the current properties of bovine and human Lf and their derivatives in in vitro probiotic growth and Lf interplay with microbiota described in the piglet model. By using Lf as a component in pharmacological products, we may enable novel strategies that promote probiotic growth while conferring antimicrobial activity against multidrug-resistant microorganisms that cause life-threatening diseases, especially in neonates.

The better suppression of pepper Phytophthora blight by arbuscular mycorrhizal (AM) fungus than Purpureocillium lilacinum alone or combined with AM fungus

Phytophthora blight caused by Phytophthora capsici (Pc) is one of the most economically destructive soilborne diseases of pepper (Capsicum annum L.) on a global scale. Biocontrol using antagonistic microbes, such as Purpureocillium lilacinum (Pl) and arbuscular mycorrhizal (AM) fungus Funneliformis caledonium (Fc), is one of the significant strategies for ecologically sound plant disease management. The purpose of this work was to investigate the sole and combined suppression of pepper Phytophthora blight by Fc and Pl. The 14-week pot experiment with three pepper plants per pot included five treatments: control (non-inoculation), inoculation with Pc, inoculation with Pc and Pl (Pc + Pl), inoculation with Pc and Fc (Pc + Fc), and inoculation with Pc, Pl, and Fc (Pc + Pl + Fc). Pots were randomly arranged with eight replicates per treatment. The incidence and severity of Phytophthora blight at plant full productive stage were recorded. The biomasses and N, P, and K concentrations of pepper shoots, roots, and fruits were all measured. In addition, root mycorrhizal colonization rate and soil pH, phosphatase activity, and available P concentration were also tested. The inoculation of Pc induced both high incidence (92%) and severity (33%) of pepper Phytophthora blight, and the alleviating effects of Pl, Fc, and Pl + Fc were 46%, 79%, and 59%, respectively. The Fc significantly increased (P < 0.05) root mycorrhizal colonization, nutrient (N, P, and K) acquisition, plant biomass, and fruit yield of pepper, while Pl only significantly increased (P < 0.05) plant nutrient (N and P) acquisition and tended to increase the fruit yield. The Pc, Pl, and Fc all had additive effects on decreasing soil pH, but only Fc significantly increased (P < 0.05) soil phosphatase activity and available P concentration, contributing partly to the elevated P acquisition as well as the increased P concentrations in both shoot and root. Compared with the sole inoculation of Fc, the extra inoculation of Pl had negative effects on mycorrhizal colonization, soil P mobilization, and plant growth and nutrient acquisition. It suggests that AM fungus (Fc) has the superiority of formatting symbioses with plant roots and enhancing soil P mobilization for the suppression of pepper Phytophthora blight compared with Pl, and the joint application of different fungal agents to improve plant health needs careful consideration.

Negative plant-soil feedbacks are stronger in agricultural habitats than in forest fragments in the tropical Andes.

There is now strong evidence suggesting that interactions between plants and their species-specific antagonistic microbes can maintain native plant community diversity. In contrast, the decay in diversity in plant communities invaded by nonnative plant species might be caused by weakening negative feedback strengths, perhaps because of the increased relative importance of plant mutualists such as arbuscular mycorrhizal fungi (AMF). Although the vast majority of studies examining plant-soil feedbacks have been conducted in a single habitat type, there are fewer studies that have tested how the strength and direction of these feedbacks change across habitats with differing dominating plants. In a fragmented montane agricultural system in Colombia, we experimentally teased apart the relative importance of AMF and non-AMF microbes (a microbial filtrate) to the strength and direction of feedbacks in both native and nonnative plant species. We hypothesized that native tree species of forest fragments would exhibit stronger negative feedbacks with a microbial filtrate that likely contained pathogens than with AMF alone, whereas nonnative plant species, especially a highly invasive dominant grass, would exhibit overall weaker negative feedbacks or even positive feedbacks regardless ofthe microbial type. We reciprocally inoculated each of 10 plant species separately with either the AMF community or the microbial filtrate originating from their own conspecifics, or with the AMF or microbial filtrate originating from each of the other nine heterospecific plant species. Overall, we found that the strength of negative feedback mediated by the filtrate was much stronger than feedbacks mediated by AMF. Surprisingly, we found that the two nonnative species, Urochloa brizantha and Coffea arabica, experienced stronger negative feedbacks with microbial filtrate than did the native forest tree species, suggesting that species-specific antagonistic microbes accumulate when a single host species dominates, as is the case in agricultural habitats. However, negative feedback between forest trees and agricultural species suggests that soil community dynamics may contribute to the re-establishment of native species into abandoned agricultural lands. Furthermore, our finding of no negative feedbacks among trees in forest fragments may be due to a loss in diversity of those microbes that drive diversity-maintaining processes in intact tropical forests.