Soil is a highly exploited ecological niche in which the occupants create a variety of biologically active natural chemicals, including antibiotics that are clinically relevant. Most antimicrobial drugs have been derived from the natural products of actinomycetes and fungi [1] [2] . Among the genera, streptomycetes are particularly ubiquitous in soil. Members of the genus Streptomyce are gram-positive, aerobic soil inhabitants and are widely distributed in nature, constituting a significant component of the microbial community in most soils [3] .

The most beneficial and important characteristic of actinomycetes is their ability to produce antibiotics and other secondary metabolites that exhibit a variety of biological activities like antibacterial, antifungal, antiviral, antitumoral, antimalarial, cytotoxic, cytostatic, immunosuppressive, anti-inflammatory, antiparasitic, antioxidant, anti-angiogenesis, pigments, and enzymes [4] - [8] . In addition, the substances produced by actinomycetes have a diverse array of chemical structures, including macrolides, tetracyclines, aminoglycosides, glycopeptides, and ansamicines, which are used in combating infectious diseases, while anthracyclines support chemotherapy for cancer [9] .

In spite of the significance of these biological activities, a plethora of secondary metabolites derived from actinomycetes have become ineffective due to multidrug-resistant bacteria [10] [11] [12] . As a result, there is an urgent need for the discovery and development of unique bioactive substances from new species of actinomycetes that have not yet been studied or exploited for their medical, agricultural, and industrial applications. Furthermore, the bioactivity of actinomycetes might also be variable depending on the kind of soil and its composition [13] [14] [15] .

Libyan soils may offer immense potential for discovering new actinomycetes due to their vast and largely unexplored regions. Therefore, exploring new areas and developing innovative techniques to reveal novel actinomycetes with potent antimicrobial activities has become increasingly urgent. Consequently, the present investigation was designed to isolate, examine, and characterize actinomycetes from Libyan soil as a pivotal source of bioactive metabolites.

Actinomycetes represent a diverse group of bacteria that have been extensively studied due to their capacity to produce bioactive compounds with potent antimicrobial activities. However, with the rising numbers of multidrug-resistant microbes, the need for new and powerful antimicrobial agents has become increasingly urgent. These actinomycetes are primarily found in soil and can vary depending on the kind of soil, making it essential to examine a wide range of areas for the discovery of valuable antimicrobial compounds. Isolation of actinomycetes from unexplored and underexploited habitats could provide a promising avenue for discovering novel and effective chemicals and bioactive compounds [13] [14] . Libya has vast and largely unexplored regions that could potentially produce unique and novel bioactive secondary compounds.

Hence, research on actinomycetes in Libya may uncover antimicrobial substances that have not yet been investigated as potential sources of therapeutic metabolites for their applications in pharmaceutical and agricultural techniques. By exploring these uncharted and untapped habitats, we may reveal new species of actinomycetes that have not yet been studied or exploited for their medical, agricultural, and industrial applications.

Our findings indicated that 164 actinomycete isolates were obtained from 300 soil specimens collected from 77 different locations in Libya. Out of 164 actinomycete isolates, 38 isolates (23.17%) were morphologically distinct. Our screening results reported that out of 38 isolates, 11 isolates (28.95%) showed antimicrobial activity against tested microorganisms. The antibacterial activity of the isolates was found to be effective against Staph. epidermides (90.9%), Staph. aureus and Mycob. Phlei (81.2%), B. subtilis and Sal. enterica (72.7%), E. coli (63.6%) and Strept. Pyogenes (18.2%), while (63.6%) of the isolates were active against A. niger and C. albicans. The lowest activity or the resistance of Strept. pyogenes to the most of the isolates may attributed to produce an enzyme, which inactivated any substance produced.

In addition, our results demonstrated that more isolates were active against Gram positive bacteria than Gram negative bacteria, as reported by Thakur et al. [25] and Gurung et al. [26] . This could be due to morphological differences in the cell walls of these microbes. The outer membrane of Gram-negative bacteria provides a formidable barrier that is impermeable to lipophilic compounds. Moreover, the lipid and protein compositions of the outer membrane have a major role in providing a barrier to hydrophobic antibiotics and other compounds, while porin acts as a selective permeability barrier to the hydrophilic solutes [26] [27] . Gram positive bacteria, on the other hand, have only a peptidoglycan layer, which is ineffective as an antibacterial agent permeability barrier [26] .

Of all the isolates assayed in the present study, strain 063, which was isolated from Wadan soil (a desert region), showed highly broad-spectrum antimicrobial activity and inhibited the growth of all the test organisms used in this investigation. It is therefore possible that an isolate might produce various metabolically active compounds. El-Barasi et al. [28] revealed that the chemical structure of Wadan soil contained a high concentration of calcium carbonate (CaCO₃). Soil samples treated with calcium carbonate were found to be most effective for the isolation of bioactive actinomycetes, which provide antimicrobial products [29] [30] [31] . Uzcátegui et al. [32] demonstrated that calcium carbonate was one of the most effective additive substances in promoting Actinomyces antimicrobial activity against pathogens. As a result, Wadan soil could be a significant source of new natural products derived from microbes.

Parameters such as spore chain morphology and ornamentation of the spore surface are fundamental for the classification and identification of the Actinomycetes species [17] [20] [23] [33] - [38] . Microscopic examination demonstrated that the isolates belong to the genus Streptomyces. The frequency and dominance of Streptomyces isolates among actinomycetes in various soil types were also reported by several investigators [39] - [45] . Based on the color of aerial mycelium, the selected isolate was closely related to the gray series. These results were in agreement with other previously obtained results, which showed that the gray and white color series of Actinomycetes were the predominant patterns in the soil [25] [46] . Microscopically, it was observed that the spore chains of the aerial mycelium of the selected isolate showed a spiral chain. Similarly, Thakur et al. [25] stated that the majority of the isolates were considered to be spiral and rectus-flexibilis sporophores.

In the current research, the morphological evaluation of the spore surface indicated that the tested isolate belonged to the smooth spore surface. Similar results were also reported by Okudoh and Wallis [47] . According to Pridham and Gottlieb [48] , the use of carbon compounds is crucial for the taxonomic classification of actinomycete species. Carbon sources such as glucose, xylose, arabinose, rhamnose, galactose, mannitol, and meso-inositol were all successfully metabolized by isolate 063. However, the chosen isolate was unable to use raffinose as a carbon source.

This study aimed to isolate actinomycetes from Libyan soils and assess their antimicrobial activities against a wide variety of pathogenic microorganisms. Our findings indicated that all isolates belong to the genus Streptomyces. The selected isolate 063 from an extreme geographic and climatic area (the Wadan soil of the Aljufra area) exhibited promising antimicrobial activity against all the test microbes, which might be related to the presence of several bioactive compounds. In general, the exploration of Libyan soil for novel actinomycetes is a promising avenue for discovering unique antimicrobial agents. Additionally, studying these actinomycetes using molecular techniques could lead to a better understanding of the mechanisms behind their ability to produce these valuable substances.

The authors would like to thank Mr. Ali Almaremi for his assistance and effort in the collection of soil samples from the southern region of Libya. We would like to extend our gratitude to Dr. Bashir Nwer and Dr. Abdulmotlub El-Hashmi for drawing a map for our research study.

This work was supported by a grant funded by the University of Benghazi.

The authors declare no conflict of interest.

Salem, G.S., Baiu, S.H. and Ali, A.A. (2023) Isolation, Examination and Characterization of Actinomycetes as a Source of Antimicrobial Agents from Libyan Soil. Advances in Bioscience and Biotechnology, 14, 322-336. https://doi.org/10.4236/abb.2023.146020