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 °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.

The evolution of spectrum in antibiotics and bacteriocins

A key property of many antibiotics is that they will kill or inhibit a diverse range of microbial species. This broad-spectrum of activity has its evolutionary roots in ecological competition, whereby bacteria and other microbes use antibiotics to suppress other strains and species. However, many bacteria also use narrow-spectrum toxins, such as bacteriocins, that principally target conspecifics. Why has such a diversity in spectrum evolved? Here, we develop an evolutionary model to understand antimicrobial spectrum. Our first model recapitulates the intuition that broad-spectrum is best, because it enables a microbe to kill a wider diversity of competitors. However, this model neglects an important property of antimicrobials: They are commonly bound, sequestered, or degraded by the cells they target. Incorporating this toxin loss reveals a major advantage to narrow-spectrum toxins: They target the strongest ecological competitor and avoid being used up on less important species. Why then would broad-spectrum toxins ever evolve? Our model predicts that broad-spectrum toxins will be favored by natural selection if a strain is highly abundant and can overpower both its key competitor and other species. We test this prediction by compiling and analyzing a database of the regulation and spectrum of toxins used in inter-bacterial competition. This analysis reveals a strong association between broad-spectrum toxins and density-dependent regulation, indicating that they are indeed used when strains are abundant. Our work provides a rationale for why bacteria commonly evolve narrow-spectrum toxins such as bacteriocins and suggests that the evolution of antibiotics proper is a signature of ecological dominance.

Bioactive Compounds from Marine Heterobranchs

The natural products of heterobranch molluscs display a huge variability both in structure and in their bioactivity. Despite the considerable lack of information, it can be observed from the recent literature that this group of animals possesses an astonishing arsenal of molecules from different origins that provide the molluscs with potent chemicals that are ecologically and pharmacologically relevant. In this review, we analyze the bioactivity of more than 450 compounds from ca. 400 species of heterobranch molluscs that are useful for the snails to protect themselves in different ways and/or that may be useful to us because of their pharmacological activities. Their ecological activities include predator avoidance, toxicity, antimicrobials, antifouling, trail-following and alarm pheromones, sunscreens and UV protection, tissue regeneration, and others. The most studied ecological activity is predation avoidance, followed by toxicity. Their pharmacological activities consist of cytotoxicity and antitumoral activity; antibiotic, antiparasitic, antiviral, and anti-inflammatory activity; and activity against neurodegenerative diseases and others. The most studied pharmacological activities are cytotoxicity and anticancer activities, followed by antibiotic activity. Overall, it can be observed that heterobranch molluscs are extremely interesting in regard to the study of marine natural products in terms of both chemical ecology and biotechnology studies, providing many leads for further detailed research in these fields in the near future.

Antimicrobial properties of the novel bacterial isolate Paenibacilllus sp. SMB1 from a halo-alkaline lake in India

Antibiotic-resistance is ever growing burden on our society for the past many years. Many synthetic chemistry approaches and rational drug-design have been unable to pace up and tackle this problem. Natural resources, more specifically, the microbial diversity, on the other hand, make a traditional and still the best platform to search for new chemical scaffolds and compounds. Here, we report the antimicrobial characteristics of novel bacterial isolate from a salt lake in India. We screened the bacterial isolates for their inhibitory activity against indicator bacteria and found that four novel species were able to prevent the growth of test strains studied in vitro. Further, we characterized one novel species (SMB1^T = SL4-2) using polyphasic taxonomic approaches and also purified the active ingredient from this bacterium. We successfully characterized the antimicrobial compound using mass spectroscopy and amino acid analysis. We also allocated two novel biosynthetic gene clusters for putative bacteriocins and one novel non-ribosomal peptide gene cluster in its whole genome. We concluded that the strain SMB1^T belonged to the genus Paenibacilllus with the pairwise sequence similarity of 98.67% with Paenibacillus tarimensis DSM 19409^T and we proposed the name Paenibacillus sambharensis sp. nov. The type strain is SMB1^T (=MTCC 12884 = KCTC 33895^T).

Analysis of the Genome and Metabolome of Marine Myxobacteria Reveals High Potential for Biosynthesis of Novel Specialized Metabolites

Comparative genomic/metabolomic analysis is a powerful tool to disclose the potential of microbes for the biosynthesis of novel specialized metabolites. In the group of marine myxobacteria only a limited number of isolated species and sequenced genomes is so far available. However, the few compounds isolated thereof so far show interesting bioactivities and even novel chemical scaffolds; thereby indicating a huge potential for natural product discovery. In this study, all marine myxobacteria with accessible genome data (n = 5), including Haliangium ochraceum DSM 14365, Plesiocystis pacifica DSM 14875, Enhygromyxa salina DSM 15201 and the two newly sequenced species Enhygromyxa salina SWB005 and SWB007, were analyzed. All of these accessible genomes are large (~10 Mb), with a relatively small core genome and many unique coding sequences in each strain. Genome analysis revealed a high variety of biosynthetic gene clusters (BGCs) between the strains and several resistance models and essential core genes indicated the potential to biosynthesize antimicrobial molecules. Polyketides (PKs) and terpenes represented the majority of predicted specialized metabolite BGCs and contributed to the highest share between the strains. BGCs coding for non-ribosomal peptides (NRPs), PK/NRP hybrids and ribosomally synthesized and post-translationally modified peptides (RiPPs) were mostly strain specific. These results were in line with the metabolomic analysis, which revealed a high diversity of the chemical features between the strains. Only 6-11% of the metabolome was shared between all the investigated strains, which correlates to the small core genome of these bacteria (13-16% of each genome). In addition, the compound enhygrolide A, known from E. salina SWB005, was detected for the first time and structurally elucidated from Enhygromyxa salina SWB006. The here acquired data corroborate that these microorganisms represent a most promising source for the detection of novel specialized metabolites.

Homospermidine Lipids: A Compound Class Specifically Formed during Fruiting Body Formation of Myxococcus xanthus DK1622

The fascinating ability of myxobacteria to form multicellular spore filled fruiting bodies under starvation conditions was widely studied as a model for cooperative microbial behavior. The potential of a life cycle induced change of secondary metabolism, as a means to discover novel natural products, remains largely underexplored. We therefore studied the model organism Myxococcus xanthus DK1622 under submersed and solid cultivation conditions to find putatively life-cycle related compounds by applying statistical analysis on analytical data. Utilizing the advantageous characteristics of LC-MS, LC-MS/MS, and MALDI-MSI allowed the identification of compounds unambiguously associated with myxobacterial fruiting bodies. Our screening effort resulted in the purification and structure elucidation of a novel compound, the homospermidine lipid, from cultures that had undergone the fruiting process. A combination of molecular networking and targeted LC-MS/MS in conjunction with our in-house metabolomics database subsequently revealed alternative producers of the respective compound as well as a number of compounds belonging to the same structural class. Three further members of this compound class were isolated from an alternative producer and structurally elucidated by NMR. Insights into the biosynthesis of this novel compound class was gained by feeding of isotopically labeled substrates and in silico analysis.

The Potential of Indonesian Heterobranchs Found around Bunaken Island for the Production of Bioactive Compounds

The species diversity of marine heterobranch sea slugs found on field trips around Bunaken Island (North Sulawesi, Indonesia) and adjacent islands of the Bunaken National Marine Park forms the basis of this review. In a survey performed in 2015, 80 species from 23 families were collected, including 17 new species. Only three of these have been investigated previously in studies from Indonesia. Combining species diversity with a former study from 2003 reveals in total 140 species from this locality. The diversity of bioactive compounds known and yet to be discovered from these organisms is summarized and related to the producer if known or suspected (might it be down the food chain, de novo synthesised from the slug or an associated bacterium). Additionally, the collection of microorganisms for the discovery of natural products of pharmacological interest from this hotspot of biodiversity that is presented here contains more than 50 species that have never been investigated before in regard to bioactive secondary metabolites. This highlights the great potential of the sea slugs and the associated microorganisms for the discovery of natural products of pharmacological interest from this hotspot of biodiversity.