The present study evaluated the response of Bacillus cereus LSPQ 2872 vegetative cells, following exposure to single and repetitive sublethal γ-radiation treatment at 1 kGy alone or in combination with nisin at its maximum tolerated concentration, in BHI broth supplemented with 0.5% glucose. Results showed that B. cereus has the capability to develop increased resistance to subsequent cycles of gamma irradiation and a significant increase (p ≤ 0.05) of the relative radiation resistance (D(10)) value was noticed after the fourth irradiation at 1 kGy. The sequence and the repetitive sublethal treatment of γ-radiation with nisin affected significantly (p ≤ 0.05) B. cereus radio-tolerance since lower D(10) values were recorded. Our results showed that the response of B. cereus to repetitive mild bactericidal treatment was accompanied by relevant modifications in the cell properties leading to the increased resistance. Transmission electron microscopy revealed that resistance might be related to changes in the cell wall. Multiparameter flow cytometry revealed effectively diverse physiological changes that B. cereus underwent during development of radio-resistance. The development of increased resistance to gamma irradiation was accompanied by an increase of the percentage of injured and viable cells at the expense of dead cells. Nisin treated cells developed also increased radio-resistance if repetitively processed with γ-radiation at sublethal doses which was not demonstrated by plate counting.
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