Isolation and identification of endophytic actinobacteria from Citrullus colocynthis (L.) Schrad and their antibacterial properties

Background Antibiotic resistance poses a major threat to human health globally. Consequently, new antibiotics are desperately required to discover and develop from unexplored habitats to treat life-threatening infections. Microbial natural products (NP) are still remained as primary sources for the discovery of new antibiotics. Endophytic actinobacteria (EA) which are well-known producers of bioactive compounds could provide novel antibiotic against pathogenic bacteria. This research aimed to isolate EA from the Citrullus colocynthis plant and explore the antibacterial properties of their metabolites against pathogenic bacteria. Results The healthy samples were collected, dissected and surface-sterilized before cultured on four different selection media at 28 degrees C. Six endophytic actinobacteria were isolated from Citrullus colocynthis plant. They were taxonomically classified into two family namely Streptomycetaceae and Nocardiopsaceae, based on colony morphological features, scanning electron microscope analysis and molecular identification of isolates. This is the first report on the identification of EA form Citrullus colocynthis and their antibacterial activity. The strains generated a chain of vibrio-comma, cubed or cylindrical shaped spores with indenting or smooth surfaces. Three of those were reported as endophytes for the first time. The strain KUMS-C1 showed 98.55% sequence similarity to its closely related strains which constitutes as a novel species/ strain for which the name Nocardiopsis colocynthis sp. was proposed for the isolated strain. Five isolated strains had antagonist activity against S. aureus, P. aeruginosa, and E. coli. Among those, stain KUMS-C6 showed the broadest spectrum of antibacterial activity against all test bacteria, whereas the strain KUMS-C4 had no antibacterial activity. Conclusions NPs have a long history of safe and efficient use for development of pharmaceutical products. Our study highlights that Citrullus colocynthis is an untapped source for the isolation of EA, generating novel and bioactive metabolites by which might lead to discovery of new antibiotic(s). This study reveals the future of new antibiotic developments looks bright against multi-drug resistance diseases by mining under- or unexplored habitats.

Automated summary

Antibiotic resistance is a global threat to human health, and the discovery of new antibiotics from unexplored habitats is crucial. This study isolated endophytic actinobacteria (EA) from the Citrullus colocynthis plant and investigated their antibacterial properties. Six endophytic actinobacteria were isolated, and they were classified into two families based on morphological features and molecular identification. One of the isolated strains was proposed as a novel species/strain. Five strains showed antagonistic activity against pathogenic bacteria, indicating the potential of Citrullus colocynthis as an untapped source for the discovery of new antibiotics.