Autofluorescence and green fluorescent protein-derived fluorescence in Listeria innocua

Fluorescent L. innocua M1 was generated by transformation with plasmid SB2019 carrying gfp3. The transformed organism exhibited 2 h longer lag phase than the parental strain. The transformation and gfp3 expression did not affect the growth rates (0.49 ± 0.02 and 0.47 ± 0.05 h−1) and the maximum optical densities (1.02 ± 0.01 and 0.90 ± 0.06) of the parental or the transformed strains. The transformation of L. innocua M1 with pSB2019 resulted in cell-concentration related fluorescence which was detectable from washed cells but not in growth media. Statistical discrimination between GFP3-driven and background fluorescence signals of transformed and parental strains occurred at cell optical densities of 0.1 and above which was partially due to a relatively high endogenous autofluorescence. Although the gfp-transformed L. innocua M1 developed in this study has the potential to be a marker organism for monitoring Listeria spp. responses in mixed cultures, more work is needed to optimize the GFP-based fluorescent signal.