Novel cyanine dyes and homodimeric styryl dyes as fluorescent probes for assessment of lactic acid bacteria cell viability

• Four novel dyes for staining of live and dead bacterial cells were synthesized.
• The structures can be excited with conventional light source at 488 nm.
• A protocol for “live/dead” analysis of lactic acid bacteria was developed.
• The dyes showed high sensitivity at low concentrations without long pre-incubation.

Innovations in labeling techniques and in the design and synthesis of dye structures are closely related to the development of service equipment such as light sources and detection methods. Novel styryl homodimers and monomethine cyanine dyes were synthesized and their staining abilities for discrimination between live and dead lactic acid bacterial cells were investigated. The dyes were combined in pairs based on their excitation and emission maxima and the capacity to penetrate through cell membranes of viable bacterial cells. The absorption maxima in the same region and the large Stocks shifts of the styryl derivatives allowed viability analysis to be done with epifluorescent microscope with a very basic configuration – one light source about 480 nm and one filter for the fluorescent emissions. A staining protocol was developed and applied for live/dead analysis of Bulgarian yoghurt starters. The live cells quantification by the fluorescence dyes coincided well with the results of the much more time-consuming tests by plate counting. Thus, the proposed dye combinations are appropriate for rapid viability estimation in small laboratories that may have conventional equipment.