Flocculation of bacteria by depletion interactions due to rod-shaped cellulose nanocrystals

We report depletion induced flocculation and phase separation of bacteria due to the presence of rod-shaped nanoparticles by using confocal laser microscopy and turbidity measurements. To investigate the depletion effect of rod shaped nanoparticles in bacteria systems, cellulose nanocrystals with length, L = 90 ± 10 nm, diameter, D = 8 ± 1 nm and zeta potential of 51.5 ± 0.8 mV was used. The flocculation of the Gram-negative bacterium Pseudomanas aeruginosa (average length = 1.2 μm, width = 0.2 μm) was tested. The zeta potential of P. aeruginosa suspended in 10 mM NaCl was −29.84 ± 0.6 mV. Bacterial flocculation was observed at the CNC concentration of less than 0.1% due to the depletion effect. We conclude from our results that rod shaped nanoparticles are very effective for the depletion flocculation of colloidal size bacteria and that phase separation of bacteria can occur at very low concentrations of rodlike CNC particles.

Illustration of depletion effect in a mixture of two big bacteria plus small rodlike cellulose nanocrystals. As the bacteria approach each other (a and b) rodlike nanoparticles cannot enter the gap between the bacteria and bacteria flocculates.Rod-shaped cellulose nanocrystal particles cause the depletion flocculation and phase separation of bacteria.Figure optionsDownload as PowerPoint slideHighlights
► Rod shaped nanoparticles cause depletion flocculation of bacteria.
► Phase separation of bacteria can occur at very low concentrations of rodlike CNC particles.
► The lower end of the concentration range at which phase separation is found is lower than the theoretical estimate.