The radiosensitization of two pathogenic bacteria, Escherichia coli and Salmonella Typhi, was evaluated in the presence of thyme and its principal essential oil constituents (carvacrol and thymol) in ground beef. Ground beef was inoculated with E. coli or Salmonella Typhi (10(5) CFU/g), and each compound was added separately at various concentrations (0 to 3.5%, wt/wt). The antimicrobial potential of carvacrol, thymol, and thyme was evaluated in unirradiated meat by determining the MIC in percentage (wt/wt) after 24 h of storage at 4 +/- 1 degree C. Results showed a MIC of 0.88 +/- 0.12%, 1.14 +/- 0.05%, and 2.33 +/- 0.32% for E. coli in the presence of carvacrol, thymol, and thyme, respectively. MICs of 1.15 +/- 0.02%, 1.60 +/- 0.01%, and 2.75 +/- 0.17% were observed for Salmonella Typhi in the presence of the same compounds, respectively. The best antimicrobial compound (i.e., carvacrol) was selected and added to the sterilized ground beef along with ascorbic acid (0.5%, wt/wt) and tetrasodium pyrophosphate (0.1%, wt/wt). Meat samples (10 g) were packed in air and then irradiated in a 60Co irradiator at doses of 0 to 0.7 kGy for the determination of E. coli radiation D10 and 0 to 2.25 kGy for the determination of Salmonella Typhi radiation D10. Addition of carvacrol increased the relative sensitivity of both bacteria 2.2 times. The radiation D10 was reduced from 0.126 +/- 0.0039 to 0.057 +/- 0.0015 kGy for E. coli and from 0.519 +/- 0.0308 to 0.235 +/- 0.0158 kGy for Salmonella Typhi. The addition of tetrasodium pyrophosphate did not affect significantly (P > 0.05) the radiosensitization of either bacterium. However, the presence of ascorbic acid in the media reduced significantly (P < or = 0.05) the radiosensitivity of both bacteria. An additive effect of carvacrol addition and packaging under modified atmosphere conditions (60% O2-30% CO2-10% N2) was also observed on bacterial radiosensitization at 4 degrees C. Compared with the control packed under air, modified atmosphere packaging conditions in the presence of carvacrol and tetrasodium pyrophosphate improved the relative sensitivity of E. coli by 2.7 times and Salmonella Typhi by 9.9 times.