Agglutination of Legionella pneumophila by antiserum is accelerated in an ultrasonic standing wave

The agglutination of Legionella pneumophila (LP) by diluted anti-LP whole rabbit serum has been compared in conventional microwell plates and in capillary containers where the suspension was exposed to a 1 MHz ultrazonic standing wave field. A positive reaction in the standing wave field was detected as a series of cell agglutinates, separated by half an acoustic wavelenght (0.75 mm), distributed along the lenght of the capillary. Agglutination occured in 60 s or less with ultrasound, while the incubation period for a positive microwell test was often of the order of hours. At a given antiserum concentration, ultrasound-induced agglutination occurred at LP concentrations two-fold lower than those giving a positive result in the microwell plate assays. At cell concentrations near the lower limit for detection of a positive result in the microwell plates a positive reaction was detected in the standing wave field at antiserum concentrations up to 500-fold lower than those forming visible precipitates in the conventional assay.