This study investigated ethidium bromide as a plasmid-curing agent in Streptomyces bacteria. The researchers treated spores of S. glaucescens and S. lividans carrying various plasmids with ethidium bromide at different concentrations and assessed plasmid loss and mutagenic effects. Ethidium bromide successfully eliminated certain plasmids: it efficiently cured the multicopy plasmid pIJ303 from S. glaucescens and the low-copy plasmid pIJ61 from S. lividans, but failed to cure the high-copy plasmid pIJ702. However, beyond plasmid curing, ethidium bromide induced numerous chromosomal mutations at high frequencies, including loss of streptomycin resistance, melanin production, sporulation ability, and amino acid biosynthesis. DNA analysis revealed these mutations were large chromosomal deletions affecting specific genes. The authors concluded that while ethidium bromide effectively cures plasmids from streptomycetes, it functions primarily as a potent, region-specific mutagen that generates deletions rather than random mutations, particularly in genetically unstable chromosomal regions.

Key findings

• Ethidium bromide efficiently cured low-copy plasmids (pIJ303, pIJ61) but was ineffective against high-copy plasmids (pIJ702) in Streptomyces strains
• Treatment induced high-frequency chromosomal mutations including loss of antibiotic resistance, melanin production, and sporulation ability, all resulting from large DNA deletions
• Mutations were not random but targeted specific chromosomal regions showing naturally elevated genetic instability
• Ethidium bromide functions as a potent region-specific mutagen rather than a simple plasmid-curing agent in streptomycetes