Principles of separating micro-organisms from suspensions using ultrasound

C.A. MILES, M.J. MORLEY, W.R. HUDSON AND B.M. MACKEY. 1995. When particles in suspension are placed in a stationary ultrasonic field, they form bands of high concentration at half-wavelength intervals. Several theoretical papers have recently contributed to an understanding of the process but, for particles as small as bacteria, there are substantial discrepancies between the different theories, and a lack of published observations.
The threshold amplitudes required to band latex spheres (0.5-5.0 μm diameter) were measured in the frequency range 1–3 MHz and used to establish conditions, reported here for the first time, suitable for banding and moving vegetative bacterial cells in pure culture. Ultrasonic means for separating and concentrating cream and bacteria at opposite ends of a tube containing a mixture of Escherichia coli and diluted milk are also described and the breakdown of theoretical equations at low particle size is discussed.