Heavy metal tolerance in rare actinobacteria from Moroccan mining sites: first report of copper, zinc, and cobalt tolerance and zinc-associated antibacterial activity in Lentzea sp.

The present work aims to investigate the heavy metal tolerance, antibiotic resistance and Zn2+-associated antibacterial effect of two rare actinobacterial strains belonging to the genera Lentzea and Amycolatopsis isolated from Moroccan mining sites. Interestingly, this study reports for the first time the tolerance profile of Lentzea sp. to three divalent metal cations: copper (Cu2+), cobalt (Co2+), and zinc (Zn2+). To our knowledge, no prior studies have reported the tolerance of Lentzea strains to these metals. In parallel, we investigated Amycolatopsis sp., a genus previously reported to resist Zn2+, nickel (Ni2+) and Cu2+, and here we provide novel data on its response to Co2+ exposure. Both strains demonstrated significant tolerance to the tested divalent metal cations, especially Zn2+, with maximum tolerated concentrations (MTCs) between 575 and 675 mg/L, as well as remarkable tolerance to Ni2+ and Cu2+ (MTCs = 275-625 mg/L). In addition, Amycolatopsis sp. displayed notable tolerance to Co2+, with an MTC of 600 mg/L. A high multiple antibiotic resistance index (0.58) was found in both strains, suggesting multidrug resistance profile despite their isolation from mining area with limited anthropogenic antibiotic pressure. Furthermore, Zn2+ stress was associated with a dose-dependent increase in the antibacterial activity of Lentzea sp. towards Bacillus subtilis, with the highest activity recorded at 35 mg/L, where the diameter of the inhibition zone reached 52.33 ± 0.57 mm, compared to only 4.66 ± 0.57 mm in the control. These findings highlight the biotechnological potential of rare actinobacteria, from mining area, as promising sources of metal resistance characters and stress-induced antibiotics.