Fluorescence microscopy of NaCl-stressed, elongated Salmonella and Listeria cells reveals the presence of septa in filaments

The cell morphology of Salmonella enteritidis and Listeria monocytogenes after the application of stress was examined. Cells were stressed by plating the bacteria on Tryptone Soya Agar (TSA) plates, with 5-10% NaCl. The plates were subsequently incubated for 6 days at 25 degrees C. Finally, the cells were harvested and subjected to different fluorescent probes in order to visualize the possible presence of septa in elongated cells. Use of the stain 4',6-Diamidino-2-phenylindole (DAPI), which is a blue fluorescent nucleic acid stain that preferentially stains double-stranded DNA, showed clearly the presence of dark spots, probably cellular partitions where no nucleic acids were present, in both Salmonella and Listeria cells. Another stain, FM 4-64, a lipophilic styryl dye for red staining of the inner membrane, showed the presence of highly fluorescent spots in Listeria cells, probably indicating the presence of membranes. For Salmonella, however, FM 4-64 was not successful in revealing septa in filaments. Double staining applied to elongated Listeria cells showed areas with high fluorescence in DAPI-staining (DNA-rich spots) which contained low fluorescence in FM 4-64-staining (membrane spots) and vice versa, which is a confirmation that the elongated cells are indeed composed of several normal size cells.