Irradiation Sensitivity of Planktonic and Biofilm-associated Listeria monocytogenes and L. innocua as Influenced by Temperature of Biofilm Formation

The human pathogen Listeria monocytogenes forms biofilms that are relatively resistant to chemical sanitizing treatments. Ionizing radiation effectively inactivates planktonic Listeria, but no information is available on the relative efficacy of the process against biofilm-associated Listeria. The irradiation sensitivity of planktonic or biofilm cells was determined for L. monocytogenes ATCC 43256 and ATCC 49594 and a commonly used surrogate Listeria innocua ATCC 51742. Biofilms were formed on sterile glass slides incubated for 48 h at 22°C, 28°C, or 37°C. The cultures were gamma irradiated and the irradiation D10 value was calculated for each combination of isolate/culture/ temperature. The effect of temperature of cultivation on the irradiation sensitivity of both planktonic cells and biofilm cells varied for each of the isolates. Depending on isolate and temperature, biofilm cells were equally sensitive or more sensitive (P<0.05) to irradiation. D10 values overall tended to increase with temperature of cultivation for L. monocytogenes 49594 and L. innocua 51742, but tended to decrease with increasing temperature for L. monocytogenes 43256. The D10 values of the various culture/temperature combinations differed significantly among the isolates examined. Irradiation effectively eliminates both planktonic and biofilm-associated cells. The extent to which the biofilm habitat modifies the antimicrobial efficacy of irradiation is dependent on the specific isolate examined and the temperature at which it forms. This study is the first inquiry to show that biofilm Listeria cells are as sensitive or more sensitive to irradiation compared with planktonic cells and that this response is dependent on biofilm formation conditions.