Purpose: This study was conducted to evaluate the extent of Salmonella contamination in shell eggs in South Korea.
Methods: A total of 7,700 shell eggs (385 pooled samples of 20 eggs) were collected in the period 2010-2011, and Salmonella isolation was performed by the culture method according to the FDA’s Bacteriological Analytical Manual (BAM). The surfaces of eggs were disinfected with solution consisting of three parts 70% alcohol to one part iodine/potassium iodide for at least 10 seconds. And egg contents were obtained by cracking eggs aseptically. A total of 385 different bulk pools made of 20 whole shell egg contents each were prepared. Egg contents placed in sterile bags were mixed manually and incubated for 96 h at room temperature (20-24 °C). The sample (25 ml) mixed with 225 ml of sterile TSB were followed by incubation at 37 °C for 24 h. The enriched TSB culture (0.1 ml and 1 ml) were transferred into 10 ml of RV and MKTTn and incubated at 37 °C and 42 °C for 24 h, respectively. A loopful of RV and MKTTn enrichment culture was streaked onto XLD Agar plates. The presumptive colonies with a positive result were confirmed as Salmonella with VITEK2(bioMérieux) and Salmonella-specific real-time PCR.
Results: Salmonella was detected in 27 of 385 pooled samples of shell eggs. All Salmonella isolates were typed as Salmonella Gallinarum. The isolated Salmonella strains were also investigated for antibiotic resistance and showed similar antibiotic resistance patterns. Egg isolates were further characterized with the automated repetitive sequence-based PCR (rep-PCR) system (DiversiLab). The rep-PCR produced 2 different genetic groups among these isolates, and isolates within the same group did show clear relationships expecially with respect to the efficacy.
Significance: The strains of S. Gallinarum isolated from eggs seems to be different from the vaccine strain being used in Korea. It may indicate that outbreaks of SG in layer chickens still occur in South Korea despite extensive vaccination since 2001. It seems that the present vaccines still need to be improved, expecially with respect to efficacy.