Identification of a red-pigmented bacterium producing a potent anti-tumor N-alkylated prodigiosin as Serratia marcescens

A bacterial strain producing a novel prodigiosin analogue 2,2′-[3-methoxy-1′amyl-5′-methyl-4-(1″-pyrryl)] dipyrrylmethene (MAMPDM) possessing potent cytotoxic activity towards cancer cells was isolated and identified. The bacterial cells were spherical and occurred singly, and some of the biochemical tests matched with Micrococcus. Therefore, the isolate was earlier tentatively reported to be Micrococcus sp. In the present studies, analytical profile index (API) suggested this organism to be Klebsiella. However, Klebsiella is not known to produce the red pigment prodigiosin, which is produced by Serratia species and some other bacteria. Based on other biochemical characteristics, particularly DNase, gelatinase, lipase, ornithine decarboxylase, presence of a cell-associated N-alkylated prodigiosin (MAMPDM) and organic solvent tolerance, the strain has now been identified as a variant of Serratia marcescens. 16S rRNA gene analysis conclusively established this organism as S. marcescens ost3. The red pigment (MAMPDM) of this organism showed selective cytotoxic activity in cancer cell lines of different origin (LS-A and U937) and reduced toxicity to non-malignant cells. The LC50 of MAMPDM was 1.59 μM and 0.176 μM for U937 and LS-A cells, respectively, while there was no effect on the viability of L929, a non-malignant cell line, at these concentrations. Thus, S. marcescens ost3 may serve as a source of a new anti-cancer compound.