Introduction: There is an immediate need to discover and develop new antibiotics because of the steady rise of conventional antibiotic resistant bacteria which is an imminent threat to public health. Genomic sequence data have revealed the presence of silent biosynthetic gene clusters in the genomes of actinomycetes that encode for secondary metabolites, which are not detected under standard fermentation conditions. Co-cultivation-based elicitation can therefore lead to the production of compounds that are not produced in monoculture. This research is focused on the effect of Pseudomonas aeruginosa inoculum amount on antimicrobial compounds production in Streptomyces sp.AC117. Method: P.aeruginosa was cultured in liquid medium for 24 hours. Then, the cell numbers was adjusted to 1×107 cells mL−1. This inoculum was added at 0.25%, 0.5%, 1% and 1.5% (v/v) to the flask at the same time of Streptomyces inoculation. Every 48 hours the extracts from pure and co-culture were evaluated with agar well diffusion method against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Pseudomonas aeruginosa. Result: The best increase in inhibition zone diameter was at 1%(v/v).The diameter in elicited extracts compared to pure one was increased for 5mm against S.aureus and 2mm against B. subtilis. Conclusion: Co-culture could be a valuable method for activating silent genes or the ones that are not normally expressed. Moreover, some of factors such as the amount of inoculation could play a major role in activation. Although, detailed mechanisms of the interaction are rarely understood.

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