Inducing and Enhancing Antimicrobial Activity of Mining-Soil-Derived Actinomycetes Through Component Modification of Bennett’s Culture Medium

This study investigated the effect of different culture agar media, derived from Bennett’s medium, on the antimicrobial activity of 15 Streptomyces sp. and 1 Lentzea sp. strains isolated from mining environments. The media were prepared from the standard Bennett’s medium by suppressing one, two, or three ingredients—yeast extract (YE), beef extract (BE), or casein (Cas)—while maintaining glucose (Gluc) or by substituting it with fructose (Fruc) or galactose (Gal) and keeping the same suppressions. The antimicrobial activity was investigated against Candida albicans ATCC 10231, Staphylococcus aureus ATCC 29213, Bacillus subtilis ATCC 6633, and Escherichia coli K12. The antimicrobial activity of actinomycete strains was positively influenced by media modifications, though the response was actinomycete strain and target pathogen-dependent. Unexpectedly, thirteen strains exhibited poor growth on a pure agar-agar medium, including six Streptomyces strains (AS34, AS3, BS59, BS68, BS69, and DAS104) that showed notable antimicrobial activity, with inhibition zone diameters ranging from 10.75 ± 1.06 to 18.00 ± 0.00 mm. Modifications of Bennett’s medium, including replacing glucose with fructose or galactose and maintaining yeast extract or both yeast extract and beef extract, induced and enhanced the antimicrobial activity of several actinomycete strains. Notably, the new media induced antimicrobial activity in strains that showed no activity in Bennett’s medium. They led, compared to Bennett’s medium, to the detection of eight additional active strains against S. aureus, eight against B. subtilis, six against E. coli, and four against C. albicans. This study is the first to explore the modification of Bennett’s medium, either by subtraction or substitution, in order to investigate the effect on antimicrobial activity of actinomycete strains. These results highlight the importance of the composition of culture media on inducing or boosting antimicrobial activity in Streptomyces and Lentzea.