A new source of actinomycetes with heavy metal tolerance and antibacterial properties against multidrug-resistant Staphylococcus hominis

Abstract

Objective: This study aimed to isolate copper (Cu)- and nickel (Ni)- tolerant actinomycetes with antibacterial properties against multidrug-resistant Staphylococcus hominis and Escherichia coli.

Methods: Bacteria were isolated from soil samples of the floor and ceiling of Umm Jirsan Cave using starch nitrate agar medium. The isolates were identified using 16S rRNA, and their tolerance to heavy metals at various concentrations (50, 100, 150, 200, 250, and 300 mg/L) was screened, and the antibacterial activities were detected using an agar plug method. Also, the antibacterial potential of metal-resistant bacteria was further examined on metal-embedded media at the lowest concentration tested (50 mg/L).

Results: Ten different bacterial strains were isolated, identified, and characterized for their tolerance to Cu and Ni and their antibacterial activity against the bacterial pathogens, S. hominis and E. coli. Phylogenetic analysis of the strains confirmed that nine of them belong to Gram-positive bacteria, Actinobacteria phylum, Streptomyces genus, based on their shared 99-100% identity with known Streptomyces species. Only one isolate was a Gram-negative bacterium identified below the Proteobacteria phylum as Sphingomonas ursincola C6 (100% similarity). The results revealed a varying response of the bacterial species to the two heavy metals. At the highest tested concentration of metals (300 mg/L), 40% of the isolates showed Ni-resistance, and 10% of isolates were Cu-resistant, suggesting that these cave bacteria were more resistant to Ni than Cu. In addition, six isolates demonstrated good antibacterial potential against S. hominis, with inhibition zone diameters ranging from 11.0±0.59 to 21.5±0.5 mm, with the highest values, 21.5±0.5 and 21.0±0.63 mm, for Streptomyces vinaceusdrappus C2 and Streptomyces huasconensis C8, respectively.  Interestingly, the presence of Cu (50 mg/L) in the growth medium significantly enhanced the antibacterial activity of S. vinaceusdrappus C2 against S. hominis, with an inhibition zone of 27.5±0.5 mm. Similarly, S. huasconensis C8 recorded a higher inhibition zone of 27±0.23 mm against S. hominis in the medium supplemented with Ni (50 mg/L). No inhibition zones against E. coli were recorded by the tested bacteria.

Conclusion: The results conclude that bacteria, especially actinomycetes, from Umm Jirsan Cave are valuable candidates for pharmaceutical research and bioremediation, owing to their metal-tolerance and antibacterial properties.