Characterization of Endophytic Streptomyces griseorubens MPT42 and Assessment of Antimicrobial Synergistic Interactions of its Extract and Essential Oil from Host Plant Litsea cubeba.

Nguyen Nguyen,Quach Quach,Bui Bui,Li Li,Khieu Khieu,Chu Chu,Sarter Sarter,Phi Phi,Chu-Ky Chu-Ky,Hoang Hoang,Vu Vu

doi:10.3390/antibiotics8040197

Abstract

The present study aimed to evaluate the synergistic effects of the crude ethyl acetate extract (CEAE) from endophytic actinomycete MPT42 and essential oil (EO) of the same host plant Litsea cubeba. The isolate MPT42, exhibiting broad-spectrum antimicrobial activities and harboring all three antibiotic-related biosynthetic genes pks-I, pks-II, and nrps, was identified as Streptomycete griseorubens based on an analysis of the morphology, physiology, and 16S rDNA sequence. Minimum inhibitory concentrations (MICs) and the fractional inhibitory concentration index were used to estimate the synergistic effects of various combined ratios between CEAE or antibiotics (erythromycin, vancomycin) and EO toward 13 microbial strains including pathogens. L. cubeba fruit EO, showing the main chemical constituents of 36.0% citral, 29.6% carveol, and 20.5% limonene, revealed an active-low against tested microbes (MICs ≥ 600 μg/mL). The CEAE of S. griseorubens culture exhibited moderate–strong antimicrobial activities against microbes (MICs = 80–600 μg/mL). Analysis of the mechanism of action of EO on Escherichia coli ATCC 25922 found that bacterial cells were dead after 7 h of the EO treatment at 1 MIC (5.5 mg/mL), where 62% cells were permeabilized after 2 h and 3% of them were filament (length ≥ 6 μm). Combinations of CEAE, erythromycin, or vancomycin with EO led to significant synergistic antimicrobial effects against microbes with 4–16 fold reduction in MIC values when compared to their single use. Interestingly, the vancomycin–EO combinations exhibited a strong synergistic effect against five Gram-negative bacterial species. This could assume that the synergy was possibly due to increasing the cell membrane permeability by the EO acting on the bacterial cells, which allows the uptake and diffusion of antimicrobial substances inside the cell easily. These findings in the present study therefore propose a possible alternative to combat the emergence of multidrug-resistant microbes in veterinary and clinics.

Highlights

The misuse and overuse of antibiotics for human and animal health management has become a main factor contributing to the rapid emergence and dissemination of antibiotic-resistant bacteria, which poses a serious threat to global public health [1]
In order to screen the antibacterial activity of isolates, 35 out of 143 endophytic actinomycetes potentially producing antibiotics were isolated from different organs of L. cubeba (Lour.) Pers plants
This study underlines that the endophytic actinomycete S. griseorubens MPT42 associated with medicinal plant L. cubeba can be a potential producer of broad-spectrum antimicrobial substances

Summary

Introduction

The misuse and overuse of antibiotics for human and animal health management has become a main factor contributing to the rapid emergence and dissemination of antibiotic-resistant bacteria, which poses a serious threat to global public health [1]. Medicinal plants are rich sources of essential oils (EOs) and secondary metabolite compounds that are often used as natural and safe medicines in the treatment of infectious diseases. The combination of antimicrobial agents (EO and antibiotics) may lead to synergistic effects against pathogens that can reduce the side effects of antibiotics, enhance the bioavailability, lower the therapeutic dose, and minimize the antimicrobial resistance [3]. The Litsea cubeba species is frequently found in southern China, Japan, Taiwan, and South-East Asia including Vietnam, and is widely and safely used in cosmetics, and traditional medicine for the treatment of headache, fatigue, muscle pain, depression, sores and furuncles [5,6,7]. Rich-EO-content medicinal plants like L. cubeba and Cinnamomum spp. could be one of the prevalent sources for antibiotic-producing endophytic actinobacteria conveying novel features [12,13]. It has been previously demonstrated that the ecology and evolution of endophytic microorganisms might be affected by the chemical compositions of host plant EO and the participation of endophytes in the biotransformation of EO [14]

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Discussion

Conclusion