PRINCIPLES OF SEPARATING MICROORGANISMS FROM SUSPENSIONS USING ULTRASOUND

When particles in suspension are placed in a stationary ultrasonic hel d, they form bands of high concentration at half-wavelength intervals. Several theoretical papers have recently contributed to an understand ing of the process but, for particles as small as bacteria, there are substantial discrepancies between the different theories, and a lack o f published observations. The threshold amplitudes required to band la tex spheres (0.5-5.0 mu m diameter) were measured in the frequency ran ge 1-3 MHz and used to establish conditions, reported here for the fir st 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 Esche richia coli and diluted milk are also described and the breakdown of t heoretical equations at low particle size is discussed.