The ability of direct fluorescence-based, two-colour assays to detect different physiological states of oral streptococci

Aims: To investigate the ability of six fluorescent-based, two-colour viability assays to detect different physiological growth stages of two oral streptococci species. Methods and Results: The growth of Streptococcus sanguinis and Strep. mutans from 0 to 73 h culture periods was monitored by cell labelling with six mixtures of fluorescent stains, in addition to the growth parameters optical densitiy (O.D.), log values of the total cell counts (log BC ml(-1)) and of the colony-forming units (log cfu ml(-1)). Conclusions: In comparison with the corresponding cfu values as control, the vitality proportions determined by the Syto 9/PI test best reflected the dynamic growth pattern of both test strains. The direct fluorescent-based, two-colour assay Syto 9/PI provides valuable information about microbial viability stages. Significance and Impact of the Study: The detection of viable but non-culturable bacteria requires more precise direct methods such as the fluorescent staining technique presented here, in addition to the classical plate count method.