Visualising diverse extracellular nucleic acids in biofilms with DNA-binding dyes

ABSTRACT Extracellular nucleic acids (eNA) are central components of bacterial biofilms, contributing to structural integrity, antibiotic tolerance, and emerging functions such as extracellular electron transfer and peroxidase-like catalysis. While extracellular DNA has traditionally been assumed to adopt the canonical B-DNA conformation, biofilms are now known to contain non-canonical structures, including Z-DNA/RNA (Z-NA), G-quadruplex DNA/RNA (G4-NA), and substantial amounts of extracellular RNA. Conventional nucleic acid-binding dyes are widely used for rapid eNA detection, yet their specificity for these diverse structures has not been systematically evaluated. Here, we compare the fluorescence properties of eleven membrane-impermeant dyes (TOTO™, BOBO™, YOYO™, and POPO™ series, SYTOX™ Green, SYTOX™ Red, and propidium iodide) against synthetic B-DNA, Z-DNA, G4-DNA, A-RNA, Z-RNA, and G4-RNA oligonucleotides, with Z-NA stabilised through brominated guanosine analogues synthesised in-house. A clear pattern emerged: green-fluorescent dyes preferentially bound canonical B-DNA, whereas red-fluorescent counterparts displayed broader specificity that extended to non-canonical structures. TOTO™-3 and SYTOX™ Red bound G4-NA with higher fluorescence than B-DNA, and propidium iodide showed an unexpected preference for A-RNA over B-DNA. These observations were validated in Staphylococcus aureus biofilms by parallel immunolabelling with structure-specific antibodies. TOTO™-3, YOYO™-3, BOBO™-3, POPO™-3, and propidium iodide reproduced the eNA distribution at the bacterial cell surface. Finally, we introduce poly-A tailing with fluorescently labelled ATP as a stringent, RNA-specific imaging method for biofilms. Together, these results provide practical guidelines for visualising the structural diversity of eNA in biofilms. HIGHLIGHTS - Biofilms contain non-canonical structures of extracellular DNA and RNA - This study tests the ability of DNA-binding dyes to visualise such structures - Propidium iodide visualises RNA with brighter fluorescence than DNA - Red-fluorescent dyes were more versatile than green-fluorescent dyes - Combining several dyes enabled the detection of non-canonical structures