Coexistence of and interaction relationships between an aflatoxin-producing fungus and a bacterium

Anti-Fungal Analysis of Bacillus subtilis DL76 on Conidiation, Appressorium Formation, Growth, Multiple Stress Response, and Pathogenicity in Magnaporthe oryzae

In recent years, biological control has gained more attention as a promising method to combat plant disease. Such severe diseases cited include rice blasts caused by Magnaporthe oryzae. However, more effective microbial strains with strong adaptability still need to be identified. Therefore, we sought to assess the conidia germination, and formation of appressorium of DL76 in Magnaporthe oryzae. Besides, we also aimed at understanding the growth, multiple stress response and pathogenicity in Magnaporthe oryzae. We isolated Bacillus subtilis DL76 from a rice farm, which observed a strong antimicrobial effect on M. oryzae. The sterilized culture filtrate of DL76 inhibited the growth of M. oryzae, which motivated us to deduce the influence of DL76 on the pathogenicity of M. oryzae. We screened the effect of Bacillus subtilis DL76 on M. oryzae guy11. It demonstrated that sterilized culture filtrate (1 × 10⁷ CFU/mL) of DL76 can delay and even suppress the germination of conidia and (1 × 10⁷ and 1 × 10⁶ CFU/mL) prevent the formation of appressorium in vitro and in vivo. DL76 became hypersensitive to osmotic, oxidative, and cell wall degrading agents. In addition, the relative transcript levels of stress-responsive genes oxidative and osmotic were down-regulated by DL76 except for sod1, cat1, and cat2. In vivo assessment of the antifungal activity of Bacillus subtilis using conidia suspension of DL76 reduced the incidence and severity of rice blast. Conclusively, our results show that DL76 is essential for controlling rice blast by inhibiting conidiation, growth, multiple stress tolerance, and pathogenicity in M. oryzae.

Chemical signals driving bacterial-fungal interactions

Microorganisms reside in diverse environmental communities where interactions become indispensable due to close physical associations. These interactions are driven by chemical communication among different microbial kingdoms, particularly between fungi and bacteria. Knowledge about these communication signals provides useful information about the nature of microbial interactions and allows predictions of community development in diverse environments. Here, we provide an update on the role of small signalling molecules in fungal-bacterial interactions with focus on agricultural and medicinal environments. This review highlights the range of - and response to - diverse biochemicals produced by both kingdoms with view to harnessing their properties towards drug discovery applications.

Loci Identification of a N-acyl Homoserine Lactone Type Quorum Sensing System and a New LysR-type Transcriptional Regulator Associated with Antimicrobial Activity and Swarming in Burkholderia Gladioli UAPS07070

A random transposition mutant library of B. gladioli UAPS07070 was analyzed for searching mutants with impaired microbial antagonism. Three derivates showed diminished antimicrobial activity against a sensitive strain. The mutated loci showed high similarity to the quorum sensing genes of the AHL-synthase and its regulator. Another mutant was affected in a gene coding for a LysrR-type transcriptional regulator. The production of toxoflavin, the most well known antimicrobial-molecule and a major virulence factor of plant-pathogenic B. glumae and B. gladioli was explored. The absence of a yellowish pigment related to toxoflavin and the undetectable transcription of toxA in the mutants indicated the participation of the QS system and of the LysR-type transcriptional regulator in the regulation of toxoflavin. Additionally, those genes were found to be related to the swarming phenotype. Lettuce inoculated with the AHL synthase and the lysR mutants showed less severe symptoms. We present evidence of the participation of both, the quorum sensing and for the first time, of a LysR-type transcriptional regulator in antibiosis and swarming phenotype in a strain of B. gladioli

Pre- and Postharvest Strategies to Minimize Mycotoxin Contamination in the Rice Food Chain

Rice is part of many people's diet around the world, being the main energy source in some regions. Although fewer reports exist on the occurrence of mycotoxins in rice compared to other cereals, fungal contamination and the associated production of toxic metabolites, even at lower occurrence levels compared to other crops, are of concern because of the high consumption of rice in many countries. Due to the diversity of fungi that may contaminate the rice food chain, the co-occurrence of mycotoxins is frequent. Specific strategies to overcome these problems may be applied at the preharvest part of the crop chain, while assuring good practices at harvest and postharvest stages, since different fungi may find suitable conditions to grow at the various stages of the production chain. Therefore, the aim of this review is to present the state-of-the-art knowledge on such strategies in an integrated way, from the field to the final products, to reduce mycotoxin contamination in rice.

Dual flow immunochromatographic assay for rapid and simultaneous quantitative detection of ochratoxin A and zearalenone in corn, wheat, and feed samples

A one-step dual flow immunochromatographic assay (DICGA), based on a competitive format, was developed for simultaneous quantification of ochratoxin A (OTA) and zearalenone (ZEN) in corn, wheat, and feed samples. The limit of detection for OTA was 0.32 ng/ml with a detection range of 0.53‒12.16 ng/ml, while for ZEN it was 0.58 ng/ml with a detection range of 1.06‒39.72 ng/ml. The recovery rates in corn, wheat, and feed samples ranged from 77.3% to 106.3% with the coefficient of variation lower than 15%. Naturally contaminated corn, wheat, and feed samples were analyzed using both DICGA and liquid chromatography-tandem mass spectrometry (LC-MS/MS) and the correlation between the two methods was evaluated using a regression analysis. The DICGA method shows great potential for simple, rapid, sensitive, and cost-effective quantitative detection of OTA and ZEN in food safety control.

Identification of the potent toxin bongkrekic acid in a traditional African beverage linked to a fatal outbreak

In January 2015, 75 people died and 177 were hospitalized in the Mozambique village of Chitima after attending a funeral. The deaths were linked to the consumption of a traditional African beverage called pombe. Samples of the suspect pombe were subjected to myriad analyses and compared to a control sample. Ultimately, non-targeted liquid chromatography-mass spectrometry screening revealed the presence of the potent toxin bongkrekic acid, and its structural isomer, isobongkrekic acid. Quantitative analysis found potentially fatal levels of these toxins in the suspect pombe samples. Bongkrekic acid is known to be produced by the bacterium Burkholderia gladioli pv. cocovenenans. This bacterium could not be isolated from the suspect pombe, but bacteria identified as B. gladioli were isolated from corn flour, a starting ingredient in the production of pombe, obtained from the brewer's home. When the bacteria were co-plated with the fungus Rhizopus oryzae, which was also isolated from the corn flour, synergistic production of bongkrekic acid was observed. The results suggest a mechanism for bongkrekic acid intoxication, a phenomenon previously thought to be restricted to specific regions of Indonesia and China.

Suppression of Magnaporthe oryzae and interaction between Bacillus subtilis and rice plants in the control of rice blast

Magnaporthe oryzae, the causative pathogen of rice blast, has caused extensive losses to rice cultivation worldwide. Strains of the bacterium Bacillus subtilis have been used as biocontrol agents against rice blast. However, little has been reported about the interaction between B. subtilis and the rice plant and its mechanism of action. Here, the colonization process and induced disease resistance by B. subtilis SYX04 and SYX20 in rice plants was examined. Strains of B. subtilis labeled with green fluorescent protein reached population of more than 5 × 10⁶ CFU/g after 20 days on mature rice leaves and were detected after 3 days on newly grown leaves. Results showed that SYX04 and SYX20 not only inhibited spore germination, germ tube length, and appressorial formation but also caused a series of alterations in the structures of hyphae and conidia. The cell walls and membrane structures of the fungus showed ultrastructural abnormalities, which became severely degraded as observed through scanning electron microscopy and transmission electron microscopy. The mixture of both B. subtilis and M. oryzae resulted in enhanced activity of peroxidase, and polyphenol oxidase while there was significantly more superoxide dismutase activity in plants that had been sprayed with B.subtilis alone. The present study suggests that colonized SYX04 and SYX20 strains protected rice plants and exhibited antifungal activity and induced systemic resistance, thus indicating their potential biological control agents.