Autofluorescence as a viability marker for detection of bacterial spores

Recent biological terrorism events have indicated that bacterial spores such as Bacillus anthracis are real threat agents. Real time detection of biological agents is possible with the use of an ultraviolet Fluorescent Aerodynamic Particle Sizer (FLAPS) that measures particles' intrinsic fluorescence. It is important to know whether intrinsic fluorescence could be used to estimate agents' viability. Two categories of Bacillus spore populations can be differentiated by the intensity of intrinsic fluorescence emitted by ultraviolet (UV) stimulation : autofluorescent and non-autofluorescent. This study was performed to determine whether intensity of autofluorescence correlates with spore viability. Spores were analyzed using flow cytometer (equipped with a cell sorter) to mimic optical properties of FLAPS. Autofluorescent and non-autofluorescent spores were sorted according to the intensity of autofluorescence emitted following UV stimulation. Culturability, membrane integrity, membrane potential and dipicolinic acid (DPA) content were assessed. Autofluorescent spores were 1.7 times more culturable than the corresponding non-autofluorescent population. Moreover, a small proportion of autofluorescent spores exhibited extracellular membrane damages. Autofluorescent spores also showed higher membrane potential activity and contained higher levels of DPA. In conclusion, this study documents that the overall viability potential of bacterial spores can be assessed by UV flow cytometry used in the FLAPS technology.