Identification and characterization of antifungal active substances of Streptomyces hygroscopicus BS-112

Optimization of tetramycin production in Streptomyces ahygroscopicus S91

BACKGROUND

Tetramycin is a 26-member tetraene antibiotic used in agriculture. It has two components, tetramycin A and tetramycin B. Tetramycin B is obtained by the hydroxylation of tetramycin A on C4. This reaction is catalyzed by the cytochrome P450 monooxygenase TtmD. The two components of tetramycin have different antifungal activities against different pathogenic fungi. Therefore, the respective construction of high-yield strains of tetramycin A and tetramycin B is conducive to more targeted action on pathomycete and has a certain practical value.

RESULTS

Streptomyces ahygroscopicus S91 was used as the original strain to construct tetramycin A high-yield strains by blocking the precursor competitive biosynthetic gene cluster, disrupting tetramycin B biosynthesis, and overexpressing the tetramycin pathway regulator. Eventually, the yield of tetramycin A in the final strain was up to 1090.49 ± 136.65 mg·L^- 1. Subsequently, TtmD, which catalyzes the conversion from tetramycin A to tetramycin B, was overexpressed. Strains with 2, 3, and 4 copies of ttmD were constructed. The three strains had different drops in tetramycin A yield, with increases in tetramycin B. The strain with three copies of ttmD showed the most significant change in the ratio of the two components.

CONCLUSIONS

A tetramycin A single-component producing strain was obtained, and the production of tetramycin A increased 236.84% ± 38.96% compared with the original strain. In addition, the content of tetramycin B in a high-yield strain with three copies of ttmD increased from 26.64% ± 1.97 to 51.63% ± 2.06%.

Substantial improvement of tetraene macrolide production in Streptomyces diastatochromogenes by cumulative drug resistance mutations

Tetraene macrolides remain one of the most reliable fungicidal agents as resistance of fungal pathogens to these antibiotics is relatively rare. The modes of action and biosynthesis of polyene macrolides had been the focus of research over the past few years. However, few studies have been carried out on the overproduction of polyene macrolides. In the present study, cumulative drug-resistance mutation was used to obtain a quintuple mutant G5-59 with huge tetraene macrolide overproduction from the starting strain Streptomyces diastatochromogenes 1628. Through DNA sequence analysis, the mutation points in the genes of rsmG, rpsL and rpoB were identified. Additionally, the growth characteristic and expression level of tetrRI gene (belonging to the large ATP binding regulator of LuxR family) involved in the biosynthesis of tetraene macrolides were analyzed. As examined with 5L fermentor, the quintuple mutant G5-59 grew very well and the maximum productivity of tetramycin A, tetramycin P and tetrin B was as high as 1735, 2811 and 1500 mg/L, which was 8.7-, 16- and 25-fold higher than that of the wild-type strain 1628, respectively. The quintuple mutant G5-59 could be useful for further improvement of tetraene macrolides production at industrial level.

High Efficacy of the Volatile Organic Compounds of Streptomyces yanglinensis 3-10 in Suppression of Aspergillus Contamination on Peanut Kernels

Aspergillus flavus and Aspergillus parasiticus are saprophytic fungi which can infect and contaminate preharvest and postharvest food/feed with production of aflatoxins (B1, B2, and G). They are also an opportunistic pathogen causing aspergillosis diseases of animals and humans. In this study, the volatile organic compounds (VOCs) from Streptomyces yanglinensis 3-10 were found to be able to inhibit mycelial growth, sporulation, conidial germination, and expression of aflatoxin biosynthesis genes in A. flavus and A. parasiticus in vitro. On peanut kernels, the VOCs can also reduce the disease severity and inhibit the aflatoxins production by A. flavus and A. parasiticus under the storage conditions. Scanning electron microscope (SEM) observation showed that high dosage of the VOCs can inhibit conidial germination and colonization by the two species of Aspergillus on peanut kernels. The VOCs also showed suppression of mycelial growth on 18 other plant pathogenic fungi and one Oomycetes organism. By using SPME-GC-MS, 19 major VOCs were detected, like in other Streptomyces, 2-MIB was found as the main volatile component among the detected VOCs. Three standard chemicals, including methyl 2-methylbutyrate (M2M), 2-phenylethanol (2-PE), and β-caryophyllene (β-CA), showed antifungal activity against A. flavus and A. parasiticus. Among them, M2M showed highest inhibitory effect than other two standard compounds against conidial germination of A. flavus and A. parasiticus. To date, this is the first record about the antifungal activity of M2M against A. flavus and A. parasiticus. The VOCs from S. yanglinensis 3-10 did not affect growth of peanut seedlings. In conclusion, our results indicate that S. yanglinensis 3-10 may has a potential to become a promising biofumigant in for control of A. flavus and A. parasiticus.

Enhancement of metabolic flux toward ε-poly-l-lysine biosynthesis by targeted inactivation of concomitant polyene macrolide biosynthesis in Streptomyces albulus

ε-Poly-l-lysine (ε-PL) produced as a secondary metabolite of Streptomyces albulus has long been used as a natural food preservative in a number of countries, including Japan, the United States, South Korea, and China. To date, numerous studies employing classical biotechnological approaches have been carried out to improve its productivity. Here we report a modern and rational genetic approach to enhancing metabolic flux toward ε-PL biosynthesis. Based on in silico genome analyses, we revealed that S. albulus NBRC14147 produces five antifungal polyene antibiotics-tetramycin A and B, tetrin A and B, and a trace amount of nystatin A1-concomitantly with antimicrobial ε-PL. Targeted inactivation of the biosynthetic gene cluster for tetramycins and tetrins in a nystatin A1 production-deficient mutant completely abolished the production of polyene macrolides, which in turn led to an approximately 20% improvement in ε-PL production that closely correlated with the polyene defects. The biosynthetic flux for ε-PL was thus successfully enhanced by inactivation of the concomitant secondary metabolite biosynthetic pathways. Since this elimination of concomitantly produced metabolites also allows for simpler purification after fermentation production of ε-PL, the rational strain engineering strategy we show here will improve its industrial production.

Biocontrol of Aspergillus flavus on Peanut Kernels Using Streptomyces yanglinensis 3-10

The bacterium, Streptomyces yanglinensis 3-10, shows promise in the control of many phytopathogenic fungi. In this study, S. yanglinensis and its antifungal substances, culture filtrate (CF3-10) and crude extracts (CE3-10), were evaluated for their activity in reducing growth and aflatoxin AFB1 production by Aspergillus flavus, both in vitro and in vivo on peanut kernels. The results showed that in dual culture conditions, S. yanglinensis reduced the mycelial growth of A. flavus about 41% as compared to control. The mycelial growth of A. flavus was completely inhibited on potato dextrose agar amended with CF3-10 at 3% (v/v) or CE3-10 at 2.5 μg/ml. In liquid culture experiments, growth inhibition ranged from 32.3 to 91.9% with reduction in AFB1 production ranging from 46.4 to 93.4% using different concentrations of CF3-10 or CE3-10. For in vivo assays, CF3-10 at 0.133 ml/g (v/w) or CE3-10 at 13.3 μg/g (w/w) reduced the postharvest decay of peanut kernels by inhibiting visible growth of A. flavus leading to an 89.4 or 88.1% reduction in AFB1 detected, respectively. Compared with the controls, CF3-10 and CE3-10 in A. flavus shake culture significantly reduced expression levels of two AFB1 biosynthesis genes, aflR and aflS. Furthermore, electron microscopy observation showed that CF3-10 (2%, v/v) caused hyphae growth to be abnormal and shriveled, cell organelles to degenerate and collapse, large vacuoles to appear. These results suggest that S. yanglinensis 3-10 has potential as an alternative to chemical fungicides in protecting peanut kernels and other agricultural commodities against postharvest decay from A. flavus.

Isolation and identification of biocontrol agent Streptomyces rimosus M527 against Fusarium oxysporum f. sp. cucumerinum

Actinomycetes have received considerable attention as biocontrol agents against fungal plant pathogens and as plant growth promoters. In this study, a total of 320 actinomycetes were isolated from various habitats in China. Among which, 77 strains have been identified as antagonistic activities against Fusarium oxysporum f. sp. cucumerinum which usually caused fusarium wilt of cucumber. Of these, isolate actinomycete M527 not only displayed broad-spectrum antifungal activity but also showed the strongest antagonistic activity against the spore germination of F. oxysporum f. sp. cucumerinum. In pot experiments, the results indicated that isolate M527 could promote the shoot growth and prevent the development of the disease on cucumber caused by F. oxysporum f. sp. cucumerinum. The control efficacy against seedling fusarium wilt of cucumber after M527 fermentation broth root-irrigation was up to 72.1% as compared to control. Based on 16S rDNA sequence analysis, the isolate M527 was identified as Streptomyces rimosus.