Effect of irradiation on kinetic behavior of Salmonella Typhimurium and Staphylococcus aureus in lettuce and damage of bacterial cell envelope

Abstract

This study evaluated effect of gamma irradiation on survival of Salmonella Typhimurium and Staphylococcus aureus on lettuce and damage of cell envelope. S. Typhimurium and S. aureus were inoculated on red leaf lettuce, and they were irradiated at 0, 0.5, 1, 1.5, 2, 2.5, and 3 kGy, and the samples were then stored at 7 and 25 degrees C for 7 days. Survival of S. Typhimurium and S. aureus were enumerated on xylose lysine deoxycholate agar and Baird-Parker agar, respectively. D-10 value (dose required to reduce 1 log CFU/leaf) was calculated, and kinetic parameters (maximum specific growth rate; mu(max) and lag phase duration; LPD) were calculated by the modified Gompertz model. In addition, cell envelope damage of the pathogens was observed by scanning electron microscope (SEM) and transmission electron microscope (TEM). D-10 values were 0.35 and 0.33 kGy for S. Typhimurium and S. aureus, respectively. During storage at 7 degrees C, S. Typhimurium and S. aureus had significant (P < 0.05) growth only on non-irradiated samples up to about 2.5 and 4 log CFU/Ieaf at 0.42 and 1.28 log CFU/leaf/day of mu(max), respectively. At 25 degrees C, cell counts of S. Typhimurium and S. aureus on the samples irradiated at 0 and 0.5 kGy increased (P < 0.05) up to 3-6 log CFU/leaf. The mu(max) of both pathogens were higher in 0 kGy (1.08-2.27 log CFU/leaf/day) and 0.5 kGy (0.58-0.92 log CFU/leaf/day), and LPDs ranged from 1.53 to 3.14 day. SEM and TEM observations showed that cells irradiated at 1.5 and 3 kGy showed disrupted cell membrane. These results indicate that gamma irradiation could be a useful decontamination technology to improve food safety of lettuce by destroying cells of S. Typhimurium and S. aureus. (C) 2012 Elsevier Ltd. All rights reserved.