DNA-mediated bacterial aggregation is dictated by acid-base interactions

Extracellular DNA (eDNA) plays a significant role in bacterial biofilm formation and aggregation. Here, for the first time, we present a physico-chemical analysis of the DNA-mediated aggregation for three bacterial strains (Streptococcus mutans LT11, Pseudomonas aeruginosa PAO1 and Staphylococcus epidermidis 1457). Adsorption of DNA on bacterial cell surfaces increased with increasing DNA concentration, reaching a level of around 4-6 × 10^-9 μg DNA per bacterium, concurrent with maximal bacterial aggregation. Further increase in DNA adsorption caused a decrease in aggregation. Water contact angles on bacterial lawns increased upon adsorption of DNA and were highest at the DNA concentration, where maximal bacterial aggregation of the three strains occurred. Bacterial ζ-potentials became more negative with increasing DNA adsorption and were the most negative at the DNA concentrations yielding maximal bacterial aggregation. Extended DLVO-calculations of bacterial aggregation energies for the three bacterial strains indicated that DNA-mediated aggregation was caused by an interplay of attractive Lifshitz-van der Waals and acid-base interactions. For the streptococcal strain, AFM retract force-distance curves indicated stronger adhesion forces in the presence of naturally occurring eDNA than in its absence. Subsequent Poisson analysis of retract force-distance curves confirmed that acid-base interactions dictate DNA-mediated bacterial aggregation. The distance over which adhesion forces could be detected in the presence of eDNA was approximately 200 nm larger than in its absence, suggesting eDNA is present as loops with a length of about 400 nm, corresponding with 1.2 kb of DNA in a fully extended state.