Soil is an abundant and diverse system of microorganisms and, therefore, is an excellent source for microbial prospecting. In this study, a bacterium with a characteristic actinomycetes colony was isolated from the soil. Several authors have published papers on prospecting for this bacterial group in the soil [2, 9, 20]. Micromorphological identification revealed the presence of spiral spores and arthrospores, typical for Streptomyces bacteria. The same characteristics were observed by Ensign (1978), Al Dhabi et al. (2020), and Amorim et al. (2020), who isolated bacteria of the genus Streptomyces from environmental samples.
Identification of promising species is essential for understanding and cataloging the potential of this microbial group. Bacteria of the genus Streptomyces have been widely reported as important producers of metabolites of biotechnological interest, and the search for new species is of paramount importance for the biotechnology industry. Molecular identification analysis determined it to be a bacterium from the actinomycetes group, S. kronopolitis. Other authors have identified environmental isolates through similar molecular techniques. Saraswathi et al. (2020) isolated species of actinomycetes from rhizosphere soil and, after screening bacteria with biotechnological potential, used molecular techniques to identify Streptomyces cangkrigensis. Pagmadulan et al. (2020) isolated four species, Streptomyces canus, Streptomyces cirratus, Streptomyces bacillaris, and Streptomyces peucetius, from the soil in Mongolia and identified them using the ribosomal 16s technique.
Bacteria of the genus Streptomyces spp. are known for their ability to produce compounds of biotechnological interest, such as enzymes and antibiotics [2, 9]. In this study, the bacterium S. kronopolitis showed activity against different bacteria of clinical interest in tests in solid medium and submerged fermentation, demonstrating its ability to produce and secrete active compounds into the external environment. Abba et al. (2018) pointed out that its compounds after submerged fermentation have antimicrobial action against gram-positive bacteria with 20-mm inhibition halos for Bacillus subtilis bacteria. The secreted compound showed activity against S. aureus (ATCC 6538), C. diphtheriae (27012), C. diphtheriae (27010), and M. abscesses, with inhibition halos larger than 20 mm. The isolation of bacteria with activity against isolates of clinical interest is of paramount importance because of the increasingly frequent appearance of multidrug-resistant strains [12].
The MIC of the tested metabolites was established for the sensitive bacteria, with S. aureus having the lowest MIC. Amorim et al. (2020) reported that the MIC for P. aeruginosa was 0.5 mg/mL using metabolites from the same microorganism (Streptomyces spp.), even though the pathogen was gram-negative. However, Huang et al. (2021) tested purified metabolites from microorganisms against gram-positive bacteria and obtained an MIC50 of 12.5 µg/mL. Potentially, because MS is not yet purified, it requires a higher concentration to reach lethality; therefore, it shows promise for complementary future tests in its purified form.
Although bacteria of the genus Streptomyces are recognized as important sources of secondary metabolites with diverse biological activities [25, 26, 7, 6, 8] there are no reports on the biological action of S. kronopolitis against the genus Leishmania. The leishmanicidal activity of Streptomyces has been reported in several studies, such as that by Sreedharan and Rao (2017), who analyzed the effect of a potential protease inhibitor against Leishmania donovani. This study also corroborates the work of Amorin et al. (2020), who evaluated the bactericidal and leishmanicidal potential of S. ansochromogenes.
In this study, the low toxicity of the studied metabolite was also demonstrated, which corroborates the results of studies by Aliança (2012), where macroalgal extracts were more selective to parasites than to mammalian cells (IS > 1), and those of studies by Trombini (2020), which stated that metabolites can be cytotoxic and capable of affecting both cell proliferation and the immune system. In the hemolysis assay, low hemolytic activity was observed, demonstrating the low toxicity of the metabolite. Szabo (2015). conducted hemolysis tests with secondary metabolites of herbal medicines and concluded that there was no risk of hemolysis at concentrations where the index was equal to or lower than that of the negative control.