Abstract
Gamma irradiation is one of the most popular ray treatments in food industry. It's used to control micro-organisms proliferation in a wide range of products however the response of bacteria to low doses is still unknown. In this study we mainly focus on morphological alteration and virulence gene expression in Listeria monocytogenes after gamma irradiation treatments. The atomic force micrographs (AFM) showed that 0.5 kGy dose has no effect on the membrane morphology of L. monocytogenes. However, after 0.7 kGy treatment, the cells lost their typical shape and smooth membrane and 1 kGy dose was totally destructive. Moreover, membrane fatty acid composition was analyzed by the chromatographic method after different gamma doses. Significant modifications on fatty acids composition were detected in the irradiated strain showing a novo synthesis of membrane lipids: C12:0; C14:0; C15:0; C16:0 and C18:0. In addition, we reported an increase of the saturated fatty acid, essential for membrane adaptation under stress conditions. The expression levels of three virulence genes (hlyA, fri and prfA) were studied in the same conditions using real-time PCR technique. The analysis revealed that both prfA and fri genes were up-regulated after gamma treatment. The induction of prfA, which is a regulator gene, may affect the expression other genes controlling the adaptive form in the treated strain. This study open prospects for further researches to explain the regulatory mechanisms of the adaptive response in Listeria monocytogenes when exposed to sublethal irradiation-stress.
Highlights
In the Agro-Food Industries, the irradiation is the process of applying ionizing energy to foodstuffs to improve microbiological safety and to extend the shelf life [1]
Like the control L. monocytogenes, the 0.5 kGy irradiated Listeria strains have a normal rod shape with a smooth surface (Figure1 A-B); whereas the treated cells with 0.7 kGy gamma irradiation dose present irregular shape and rough surface characterized by dispersed lumps (Figure1 C-D)
The membrane fatty acid composition of L. monocytogenes cultured under the different growth conditions was determined using a chromatographic method
Summary
In the Agro-Food Industries, the irradiation is the process of applying ionizing energy to foodstuffs to improve microbiological safety and to extend the shelf life [1]. Irradiation technology is used for decontamination and/or sterilization of dehydrated vegetables, fruits, meats, poultry, fish and seafood in order to improve product safety and shelf life [2, 3]. The treatment depended on the process objective with respect radiological safety and generally, the doses used in gamma irradiation treatment are between 0.3 and 3 kGy [5,6,7]. It has been reported that the Gram-positive bacteria is more resistant to gamma irradiation than other potential pathogenic bacteria like E. coli [11]
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