Investigation of enhancement of two processes, sedimentation and conjugation, when bacteria are concentrated in ultrasonic standing waves

Abstract

Cells aggregate and can be recovered from suspension when exposed to an ultrasonic standing wave field. The acoustic force on individual cells in a standing wave decreases with particle volume. A plane ultrasonic field generated by a transducer driven at 3.3 MHz was used here to investigate the removal of Escherischia coli, cells with dimensions of the order of 1.0 μm, from batch suspension by sedimentation over a range of concentrations (10³ to 10¹⁰ cells ml⁻¹). Cell removal efficiencies greater than 90% were achieved at initial concentrations of 10¹⁰ cells ml⁻¹. Removal efficiencies decreased gradually to zero, as initial bacterial concentration was reduced to 10⁷ cells ml⁻¹. It was found that, when low concentrations of E. coli (10³ to 10⁵ cells ml⁻¹) were added to suspensions of larger particles (i.e. yeast cells) that were of sufficient concentration to form aggregates in the sound field, E. coli could be harvested to an efficiency of 40%. The results imply that the E. coli became trapped and sediment with aggregates of larger particles. Some strains of bacteria are capable of DNA transfer by conjugation. The transfer rate of E. coli RP4 plasmid is order of magnitudes greater when conjugation occurs on solid medium rather than in liquid suspension. We have investigated whether the conjugation rate would also be higher in ultrasonically induced E. coli clumps than in free suspension. The donor strain was mixed with a recipient strain of E. coli, then sonicated in a capillary at 4.6 MHz in a tubular transducer for 5 min. The bacteria aggregated successfully. Results showed a three-fold increase in the rate of conjugation compared to a liquid mating control.