The effect of various amino acids was examined with the aim to optimize the biosurfactant production by D. caeni PO5. Cultivation in MSM supplemented with 10 g/l arginine resulted in a more than twofold increase in biosurfactant production (2.81 g/l) compared with the control cultivate in the absence of amino acid (data not shown). In fact, biosurfactant production by D. caeni PO5 increased as a result of the direct uptake of amino acids as precursors for the surfactant biosynthesis, thus improving biosurfactant yield. Saimmai et al. (2013a) reported that adding amino acid to a medium resulted in a 1.5-fold enhancement of biosurfactant production from Selenomonas ruminantium CT2. The addition of arginine to the nedium also influenced the C:N ratio of the medium, with the optimal C:N ratio for biosurfactant production from D. caeni PO5 Changing to 40:11.