This study serves as an example of strategies used to increase the phage resistance of an important Irish Cheddar cheese starter, Lactococcus lactis DPC4268. It describes the emergence and persistence of a lytic bacteriophage, 4268, that has a relatively large burst size and exhibits no homology to the most common phage types encountered in Irish cheese plants. Inherent difficulties were encountered that prevented the effective introduction of conjugative phage‐resistance plasmids pNP40 and pMRC01 to strain DPC4268. In fact, pNP40‐associated Abi systems were naturally present in six of 19 starters. Control of phage 4268 was eventually achieved by generating a mutant of DPC4268, which was subsequently used for cheese manufacture.

The removal of Saccharomyces cerevisiae and Escherichia coli from 2.5 ml suspensions in ultrasonic standing wave formed at 1 or 3 MHz has been characterized. The standing wave was set up by a plane transducer and reflector mounted in the vertical plane. Cells in the ultrasonic field first concentrated in vertical planes at half wavelength separations. The ultrasound was then pulsed to allow clumps of concentrated cells to sediment in a controlled way during the short 'off' intervals. Yeast removal from suspension at a concentration of 3 x 10(9) ml-1 (14% volume v/v) was 99.5% in a total time of 4.5 min. Almost total (99.5%) clarification of prokaryote (E. coli) suspension was achieved here for the first time in a standing wave field. The clarification of a 1.3 x 10(11) ml-1 (16% v/v) E. coli suspension occurred over 11.5 min. The period decreased to 7 min in the presence of a polycationic flocculant, polyethyleneimine. The implications of the results for design of systems to further reduce clarification times are discussed. Removal efficiency for both S. cerevisiae and E. coli decreased with decrease in cell concentration. This concentration dependence is shown not to be simply a consequence of acoustic interaction between single cells. Flow cytometry of stained cells detected no loss of cell viability arising from the ultrasonic procedure.

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