Synergy between irradiation and chlorination in killing of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes

The objectives of the studies were to determine if radiation and chlorine acted in a simple additive process or if a nonadditive increase in inactivation occurred when chlorination followed gamma-irradiation both in vitro and in situ. Separate studies evaluated the effects of gamma-irradiation (0.2 kGy at 20 degrees C), chlorination (0.5 ppm for 10 min), or irradiation followed by chlorination of Salmonella, Escherichia coli O157:H7 Ent C9490, or Listeria monocytogenes cells suspended on membrane filters. Cocktails of Salmonella enterica serovars: Anatum, Infantis, Newport, and, Stanley or of L monocytogenes isolates: ATCC 7644, 15313, 43256, and 49594 (Scott A) were used. In each case, greater inactivation was observed from irradiation followed by chlorination than was predicted from the sum of the, inactivation of the 2 treatments when applied separately. Inactivation of Salmonella cells was determined also, with the pathogens adsorbed onto alfalfa seeds. Analysis of results of 3 sets of experiments with Salmonella adsorbed, onto alfalfa seeds also led to the conclusion that the combination treatments were synergistic and produced a greater inactivation than were expected from the sums of the treatments although experiments with larger number of seeds gave less evidence of synergy. The effectiveness of both interventions against Salmonella was significantly reduced when the pathogen was on alfalfa seeds.