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Effects of Gamma Irradiation on the Viability and Phenotypic Characteristics of Salmonella Enteritidis Inoculated into Specific-Pathogen-Free Eggs

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Abstract:

The goal of this study was to determine the effects of various levels of gamma irradiation on the phenotypic characteristics of 20 strains of Salmonella Enteritidis inoculated separately into specific-pathogen-free shell eggs. Bacterial strains were inoculated into egg yolks and exposed to 60Co radiation at doses of 0.49 to 5.0 kGy. The eggs were maintained at 25°C and analyzed for the presence of Salmonella on days 1, 2, 4, and 7, and the recovered Salmonella isolates were characterized biochemically. All strains were resistant to doses of 0.49, 0.54, 0.59, 0.8, and 1 kGy; colony counts were ≥105 CFU/ml of egg yolk except for one strain, which was detected at 96 h and at 7 days after irradiation at 1 kGy, with a population reduction of 2 log CFU/ml. For the other evaluated doses, 12 strains (60.0%) were resistant at 1.5 kGy and 7 strains (35.0%) were resistant at 3.0 kGy. Among all analyzed strains, 5.0 kGy was more effective for reducing and/or eliminating the inoculated bacteria; only two (10%) strains were resistant to this level of irradiation. Salmonella colony counts were significantly reduced (P < 0.01) with increasing doses from the day 1 to 7 of observation, when microbial growth peaked. Loss of mobility, lactose fermentation, citrate utilization, and hydrogen sulfide production occurred in some strains after irradiation independent of dose and postirradiation storage time. Increases in antibiotic susceptibility also occurred: seven strains became sensitive to β-lactams, two strains became sensitive to antifolates, and one strain each became sensitive to fluoroquinolone, phenicol, nitrofurans, tetracyclines, and aminoglycosides. The results indicate that up to 5.0 kGy of radiation applied to shell eggs inoculated with Salmonella Enteritidis at 4 log CFU per egg is not sufficient for complete elimination of this pathogen from this food matrix.

Document Type: Research Article

DOI: http://dx.doi.org/10.4315/0362-028X.JFP-11-086

Affiliations: 1: Biomanguinhos, Oswaldo Cruz Institute, Oswaldo Cruz Foundation, Av. Brasil, 4365-CEP 21040-900, Rio de Janeiro, Brasil; Laboratory of Enterobacteria, Oswaldo Cruz Institute, Oswaldo Cruz Foundation, Av. Brasil, 4365-CEP 21040-900, Rio de Janeiro, Brasil; Federal Rural University of Rio de Janeiro, BR 465, CEP 23890-000, Km 7, Seropedica, Rio de Janeiro, Brasil 2: Nuclear Instrumentation Laboratory, Federal University of Rio de Janeiro, S. 133, Bl. I, Centro de Tecnologia, CP 68509, CEP 21945-970, Rio de Janeiro, Brasil 3: Federal Rural University of Rio de Janeiro, BR 465, CEP 23890-000, Km 7, Seropedica, Rio de Janeiro, Brasil 4: Laboratory of Enterobacteria, Oswaldo Cruz Institute, Oswaldo Cruz Foundation, Av. Brasil, 4365-CEP 21040-900, Rio de Janeiro, Brasil

Publication date: December 1, 2011

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