Abstract
Actinobacteria are important producers of bioactive compounds. Extreme ecosystems cause evolution of novel secondary metabolic pathways of Actinobacteria and increase the possible discovery of new biological functions of bioactive compounds. Here, we isolated 65 Actinobacteria from rhizosphere soil samples of Opuntia stricta. An Actinobacteria strain (named SCA3-4) was screened against Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4, ATCC 76255). The strain produced pink–white aerial mycelia and brown substrate mycelium on Gause No. 1 agar. Biverticillate chains of cylindrical spores were observed by scanning electron microscopy (SEM). Based on alignment of 16S rRNA sequences, a constructed phylogenetic tree showed that strain SCA3-4 shared a 99.54% similarity with Streptomyces lilacinus NRRL B-1968T. The morphological, biochemical, physiological, and molecular characteristics further indicated that strain SCA3-4 belongs to the Streptomyces sp. It can grow well on medium with the following antibiotics chloramphenicol, streptomycin, penicillin-G, gentamicin, erythromycin, nystatin or neomycin sulfate. The polymerase chain reaction (PCR) amplification of types I and II polyketide synthase genes (PKS-I and PKS-II) suggested its bioactive potential. Under treatment with 100 μg/ml of ethyl acetate extracts isolated from Streptomyces sp. SCA3-4, growth of Foc TR4 was inhibited and cell membrane was destroyed. Crude extracts also showed a broad-spectrum antifungal activity against 13 phytopathogenic fungi including Foc TR4 and displayed the lowest minimum inhibitory concentration (MIC) (0.781 μg/ml) against Colletotrichum fragariae (ATCC 58718). A total of 21 different compounds identified by gas chromatography–mass spectrometry (GC-MS) were composed of phenolic compound, pyrrolizidine, hydrocarbons, esters, and acids. Besides the known active compounds, Streptomyces sp. SCA3-4 possesses antimicrobial or other biological activities. Further attention will be paid on other compounds with no functional annotation, aiming at the discovery of new bioactive substances.
Highlights
The secondary metabolites isolated from microbes exhibit either antimicrobial or antiviral activities, usually called “classical antibiotics.” the broadest definition of antibiotics should contain the bioactive compounds obtained from all living organisms (Bérdy, 2005)
Results showed that a 1,249-bp sequence of PKS-I (Accession Number MK205361) and a 614-bp sequence of PKS-II were obtained from Streptomyces sp
Strain SCA3-4 was isolated from the rhizosphere soil of O. stricta in a dry hot valley by the serial dilution technique
Summary
The secondary metabolites isolated from microbes exhibit either antimicrobial or antiviral activities, usually called “classical antibiotics.” the broadest definition of antibiotics should contain the bioactive compounds obtained from all living organisms (Bérdy, 2005). Actinobacteria are a large group of high G+C Gram-positive bacteria (Barka et al, 2016) and the primary sources of bioactive compounds in particular commercially available antibiotics (Barka et al, 2016; Rangseekaew and Pathom-aree, 2019). Several members of the Actinobacterial taxa were identified to produce a wide range of chemical compounds, including alkaloid, polyene macrolide, saccharide, pyrazoloisoquinolinone, butenolide, nucleoside, etc. These metabolites demonstrated significant biological activities such as antifungal, antitumor, antibacterial, anti-inflammatory, and enzyme inhibition (Lim et al, 2003; Bérdy, 2005; Feng et al, 2007)
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