Genetically well-characterized strains of antibiotic-producing soil bacteria (Streptomyces griseus and Streptomyces coelicolor) were used to examine the ecological role of antibiotic production. Streptomycetes were competed against sensitive and resistant Bacillus subtilis, another soil bacterium, on surface (agar) culture. The ecological role of antibiotics was examined in three levels of competition. (1) Capacity of antibiotics to allow invasion of producing organisms (B. subtilis established and streptomycetes added later). (2) Capacity of antibiotics to mediate competition between established populations (B. subtilis and streptomycetes co-inoculated). (3) Capacity of antibiotics to prevent invasion by competitors (streptomycetes established and B. subtilis added later). Antibiotic production was found to play a significant role in preventing the invasion of competitors in these experiments. Antibiotic production did not improve the ability of producers to invade a population of sensitive cells nor did it play a strong role in mediating competition between established populations. Antibiotic production also selected for antibiotic-resistant bacteria among invading competitors.
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