Radiation tolerance of Bacillus cereus pre-treated with carvacrol alone or in combination with nisin after exposure to single and multiple sub-lethal radiation treatment

Currently little information exists on the response of Bacillus cereus after repetitive exposure to combined mild treatments involving antimicrobials and gamma irradiation. Therefore, the aim of this present study was to evaluate the radiation stress on growth and physiology of B. cereus LSPQ 2872 vegetative cells at stationary phase, following exposure to single and repetitive sub-lethal γ-radiation treatment at 1 kGy simultaneously with carvacrol alone or combined with nisin at sub-inhibitory concentrations. Results indicated that the combination of carvacrol and nisin significantly enhance the radiation sensitivity of B. cereus since lower D10 values were recorded after both single and repetitive irradiation treatments. Flow cytometric analysis of radiation-stressed B. cereus cells following repetitive treatment revealed the heterogenic behaviour of the bacterium leading to the induction of a radiation tolerance response. When compared to carvacrol alone, the combination of carvacrol and nisin developed also increased radioresistance if repetitively processed with γ-radiation at 1 kGy, since the decrease percentage of dead cells was accompanied by an increase in the number of injured cells. However, good agreement was not found between classical plate counting (log cfu reductions) and flow cytometry method. For both antimicrobials, the increase of radioresistance after repetitive mild treatment was not accompanied by augmentations of lag phase or growth rate. The structural changes of the outer membranes were assessed by TEM analysis and results revealed that radioresistance might be related to changes in the cell wall.