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 mu m, width = 0.2 mu m) 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. (C) 2012 Elsevier B.V. All rights reserved.