The presence of dual SOS boxes in the regulatory region of the most of colicin operons confines synthesis of colicin to times of stress, presumably to reduce the cost of its production. However, in presence of certain inducing agents, such as antibiotics, this tight control of colicin operon is usually lost. Although synthesis of most of colicins is known to be regulated by SOS response of host cell, different patterns of induction from distinct colicins against various inducing agents have been shown in recent years. In this study, we investigated the induction pattern of enzymatic colicin E9 (ColE9) synthesis following treatment with various concentrations (sub MICs) of the Norfloxacin (NOR) using pSBM23 construct which carries transcriptional fusion of SOS inducible promoter of pColE9 (ColE9p) and a fluorescent reporter gene (gfpmut2) into kanamycin resistant pColE9-J plasmid. Flow cytomtry analysis of the Escherichia coli cells containing pSBM23, following treatment with various concentrations showed that the SOS response mediated induction of the synthesis of ColE9 happens in a dose-dependent manner. In summary, our results suggest that the presence, even in a minute amount, of SOS response inducing agents such as fluoroquinolone antibiotic in natural habitat of colicinogenic population can promote such a costly antagonistic behaviour of microbes.