This study reports the impact of different ozone treatments on a Pseudomonas syringae strain known for its ice nucleation activity (INA). Ozone is a very powerful germicidal agent used for water treatment. The effect of ozone on viability and on cultivability of P. syringae was determined by flow cytometry analysis and by plate counting respectively. The impact of ozone on the outer membrane using the INA as marker was investigated by the drop freezing technique. The destruction curve followed a shoulder pattern with a slight reduction in population with CT values between 0 and 8 min. For an initial population of 9.3 log CFU mL(-1), the cultivability was lost starting at 14 min and a loss of viability was observed after 16 min of ozone treatment at 0.45 mg L-1. Microscopic observations at this point revealed whole but aggregated bacilli. INA decreased after 8 min of ozone treatment but did not disappear. This decrease could be due to the progressive disruption of ice nucleating sites in the outer membrane. It was however partially restored after long storage at 4 degrees C of dead cells treated for 16 min. (C) 2013 Elsevier Ltd. All rights reserved.