Co-Cultivation of Two Bacillus Strains for Improved Cell Growth and Enzyme Production to Enhance the Degradation of Aflatoxin B1

Metabolites and degradation pathways of microbial detoxification of aflatoxins: a review

The degradation of aflatoxins using nonpathogenic microbes and their enzymes is emerging as a safe and economical alternative to chemical and physical methods for the detoxification of aflatoxins in food and feeds. Many bacteria and fungi have been identified as aflatoxin degraders. This review is focused on the chemical identification of microbial degradation products and their degradation pathways. The microbial degradations of aflatoxins are initiated by oxidation, hydroxylation, reduction, or elimination reactions mostly catalyzed by various enzymes belonging to the classes of laccase, reductases, and peroxidases. The resulting products with lesser chemical stability further undergo various reactions to form low molecular weight products. Studies on the chemical and biological nature of degraded products of aflatoxins are necessary to ensure the safety of the decontamination process. This review indicated the need for an integrated approach including decontamination studies using culture media and food matrices, proper identification and toxicity profiling of degraded products of aflatoxins, and interactions of microbes and the degradation products with food matrices for developing practical and effective microbial detoxification process.

Biocontrol Capabilities of Bacillus subtilis E11 against Aspergillus flavus In Vitro and for Dried Red Chili (Capsicum annuum L.)

As a condiment with extensive nutritional value, chili is easy to be contaminated by Aspergillus flavus (A. flavus) during field, transportation, and storage. This study aimed to solve the contamination of dried red chili caused by A. flavus by inhibiting the growth of A. flavus and detoxifying aflatoxin B₁ (AFB₁). In this study, Bacillus subtilis E11 (B. subtilis) screened from 63 candidate antagonistic bacteria exhibited the strongest antifungal ability, which could not only inhibit 64.27% of A. flavus but could also remove 81.34% of AFB₁ at 24 h. Notably, scanning electron microscopy (SEM) showed that B. subtilis E11 cells could resist a higher concentration of AFB₁, and the fermentation supernatant of B. subtilis E11 could deform the mycelia of A. flavus. After 10 days of coculture with B. subtilis E11 on dried red chili inoculated with A. flavus, the mycelia of A. flavus were almost completely inhibited, and the yield of AFB₁ was significantly reduced. Our study first concentrated on the use of B. subtilis as a biocontrol agent for dried red chili, which could not only enrich the resources of microbial strains for controlling A. flavus but also could provide theoretical guidance to prolong the shelf life of dried red chili.

Biodegradation of Aflatoxin B1 in Maize Grains and Suppression of Its Biosynthesis-Related Genes Using Endophytic Trichoderma harzianum AYM3

Aflatoxin B1 is one of the most deleterious types of mycotoxins. The application of an endophytic fungus for biodegradation or biosuppression of AFB1 production by Aspergillus flavus was investigated. About 10 endophytic fungal species, isolated from healthy maize plants, were screened for their in vitro AFs-degrading activity using coumarin medium. The highest degradation potential was recorded for Trichoderma sp. (76.8%). This endophyte was identified using the rDNA-ITS sequence as Trichoderma harzianum AYM3 and assigned an accession no. of ON203053. It caused a 65% inhibition in the growth of A. flavus AYM2 in vitro. HPLC analysis revealed that T. harzianum AYM3 had a biodegradation potential against AFB1. Co-culturing of T. harazianum AYM3 and A. flavus AYM2 on maize grains led to a significant suppression (67%) in AFB1 production. GC-MS analysis identified two AFB1-suppressing compounds, acetic acid and n-propyl acetate. Investigating effect on the transcriptional expression of five AFB1 biosynthesis-related genes in A. flavus AYM2 revealed the downregulating effects of T. harzianum AYM3 metabolites on expression of aflP and aflS genes. Using HepaRG cell line, the cytotoxicity assay indicated that T. harazianum AYM3 metabolites were safe. Based on these results, it can be concluded that T. harzianum AYM3 may be used to suppress AFB1 production in maize grains.

Efficacy of the antifungal metabolites of Streptomyces philanthi RL-1-178 on aflatoxin degradation with its application to prevent aflatoxigenic fungi in stored maize grains and identification of the bioactive compound

Aflatoxin B₁ is a potent carcinogen produced by Aspergillus flavus (A. flavus) and Aspergillus. parasiticus (A. parasiticus), mainly during grain storage. The efficacy of the freeze-dried culture filtrate of Streptomyces philanthi (S. philanthi) strain RL-1-178 (DCF) on degradation of aflatoxin B₁ (AFB₁) were evaluated and its bioactive compounds were identified. The DCF at a concentration of 9.0% (w/v) completely inhibited growth and AFB₁ production of A. parasiticus TISTR 3276 and A. flavus PSRDC-4 after 7 days tested in yeast-extract sucrose (YES) medium and on stored maize grains after 28 and 14 days incubation, respectively. This indicated the more tolerance of A. parasiticus over A. flavus. The DCF and bacterial cells of S. philanthi were capable to degrade AFB₁ by 85.0% and 100% for 72 h and 8 days, respectively. This confirmed the higher efficacy of the DCF over the cells. After separation of the DCF on thin-layer chromatography (TLC) plate by bioautography bioassay, each active band was identified by liquid chromatography-quadrupole time of flight mass spectrometer (LC-Q-TOF MS/MS). The results revealed two compounds which were identified as azithromycin and an unknown based on mass ions of both ESI⁺ and ESI^- modes. The antifungal metabolites in the culture filtrate of S. philanthi were proved to degrade aflatoxin B₁. It could be concluded that the DCF may be applied to prevent the growth of the two aflatoxin-producing fungi as well as the occurrence of aflatoxin in the stored maize grains.

Mechanisms and transformed products of aflatoxin B1 degradation under multiple treatments: a review

Aflatoxins, including aflatoxin B1, B2, G1, G2, M1, and M2, are one of the major types of mycotoxins that endangers food safety, human health, and contribute to the immeasurable loss of food and agricultural production in the world yearly. In addition, aflatoxin B1 (AFB1) mainly produced by Aspergilus sp. is the most potent of these compounds and has been well documented to cause the development of hepatocellular carcinoma in humans and animals. This paper reviewed the detoxification and degradation of AFB1, including analysis and summary of the major technologies in physics, chemistry, and biology in recent years. The chemical structure and toxicity of the transformed products, and the degradation mechanisms of AFB1 are overviewed and discussed in this presented review. In addition to the traditional techniques, we also provide a prospective study on the use of emerging detoxification methods such as natural products and photocatalysis. The purpose of this work is to provide reference for AFB1 control and detoxification, and to promote the development of follow-up research.

Functional Characterization and Whole-Genome Analysis of an Aflatoxin-Degrading Rhodococcus pyridinivorans Strain

Simple Summary

The microbiological degradation of AFB₁ has been a promising approach to control AFB₁ contamination. Here, we characterize a Rhodococcus pyridinivorans strain that can efficiently degrade AFB₁. The AFB₁-degrading capacity of this bacterial strain was characterized, and the completed genome was sequenced and analyzed. Further proteomic analyses of this strain identified a total of 723 proteins in an extracellular component that showed the strongest capacity to degrade AFB₁ (degradation rate 83.7%). Multiple potential AFB₁-degrading enzymes, and enzymes that are reported to respond to AFB₁ treatment, have been identified accordingly. These findings provide a genomic, proteomic, and experimental approach for characterizing an efficient AFB₁-degrading bacterial strain with great potential for use in the remediation of AFB₁ contamination.

Abstract

Aflatoxin B₁ (AFB₁) is one of the most toxic, naturally occurring carcinogen compounds and is produced by specific strains of fungi. Crop contamination with AFB₁ can cause huge economic losses and serious health problems. Many studies have examined the microbiological degradation of AFB₁, especially the use of efficient AFB₁-degrading microorganisms, to control AFB₁ contamination. Here, we reported the identification of a new Rhodococcus pyridinivorans strain (4-4) that can efficiently degrade AFB₁ (degradation rate 84.9%). The extracellular component of this strain showed the strongest capacity to degrade AFB₁ (degradation rate 83.7%). The effects of proteinase K, SDS, temperature, pH, incubation time, and AFB₁ concentration on the AFB₁ degradation ability of the extracellular component were investigated. We sequenced the complete genome of this strain, encoding 5246 protein-coding genes and 169 RNA genes on a circular chromosome and two plasmids. Comparative genomic analysis revealed high homology with other Rhodococcus strains with high AFB₁-degradation ability. Further proteomic analyses of this strain identified a total of 723 proteins in the extracellular component, including multiple potential AFB₁-degrading enzymes, along with enzymes that are reported to response to AFB₁ treatment. Overall, the results demonstrate that R. pyridinivorans 4-4 would be an excellent candidate for the biodegradation and detoxification of AFB₁ contamination.

Enhancing the degradation of Aflatoxin B1 by co-cultivation of two fungi strains with the improved production of detoxifying enzymes

After the co-culture of Aspergillus niger and Pleurotus ostreatus, the obtained extracellular crude enzymes solution was employed to aflatoxin B₁ (AFB₁) degradation. The maximum AFB₁ degradation with co-culture reached 93.4%, which increased by 65.9% and 37.6%, respectively, compared with those of the mono-culture of Pleurotus ostreatus and Aspergillus niger. The molecular weight of the key detoxifying enzymes isolated by ultrafiltration was 58 and 63 kDa by SDS-PAGE analysis. The purified detoxifying enzymes had a high detoxification effect on AFB₁ with the degradation rate of 94.7%. It was found that the co-culture of Pleurotus ostreatus and Aspergillus niger promoted the production of 58 and 63 kDa detoxifying enzymes to enhance the AFB₁ degradation. The chemical structure of major degradation products of AFB₁ by the mixed cultures were preliminarily identified by LC-Triple TOF MS. Two pathways of AFB₁ degradation were inferred with the high potential of fungal co-cultivations for AFB₁ detoxification applications.