Submitted to: Journal of Food Safety
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/26/2007
Publication Date: 11/1/2008
Citation: Kawasaki, T., Musgrove, M.T., Murata, M., Tominaga, N., Kawamoto, S. 2008. Comparative study of shell swab and shell crush methods for the recovery of Salmonella from shell eggs. Journal of Food Safety. 28:482-498. Interpretive Summary: Eggs are not a consistent research priority. As a result, improvement in techniques to recover microorganisms from egg shells had not been determined recently. A comparison of the typical method swab method for sampling egg shells in Japan (only the United States, Canada, and Japan require washing of retail shell eggs) and a crush method determined in the use was performed. Crushing egg shells recovered significantly more microorganisms than swabbing. Sophisticated techniques such as chemiluminescent viability showed that microorganisms recovered by crushing returned to a viable, healthy state quicker than those recovered by swabbing. It was also determined that heating the recovery buffer, the solution used to rinse eggs while crushing shells and membranes was even more effective when the solution was heated 37oC. In Japan, this information is being used by the National Food Research Institute, an agency similar to the Agricultural Research Service in the U.S. This work supports the use of the crushing technique developed in an ARS laboratory. Increasing the sensitivity of microbial recovery methods improves the efficacy of research that benefits consumers, regulatory agencies, and academia.
Technical Abstract: Swabbing (SW) is the standard methodology for the recovery of resident microorganisms from shell eggs in Japan. A comparative study of shell swab (SW) and a shell crush (CR) technique was performed to recover the laboratory-inoculated Salmonella from shell eggs. It was found that the recovery of Salmonella by CR methods was significantly higher (4.5-7.5 log CFU/egg) than that of SW methods (3.1-6.3 log CFU/egg). However, analyses with quantitative real-time PCR (invA as a target gene), fluorescent microscopic, and quantitative analyses with a Live/Dead BacLightTM bacterial viability kit revealed that not all of the inoculated Salmonella spp. populations were recovered as intact cells by either method. The chemiluminescent bacterial viability assay showed that viability began to increase after 30 min in CR samples; on the other hand, SW samples did not show any increase in viability after 2 hr. These results suggest that SW might cause more damage and lethality to cells than crushing. In addition, to determine the most appropriate method for recovering resident aerobic bacteria, coliforms, and Salmonella spp. from shell eggs, 4,000 commercial eggs were collected and sampled by SR and CR techniques using phosphate-buffered saline (PBS) warmed to different temperatures. PBS at 37oC was found to be the best recovery solution and temperature, respectively for recovering aerobic microorganisms from shell eggs by both methods and CR method for recovering higher populations than SR methods (4.9 v. 5.8 log CFU/egg for SR and CR methods, respectively; n = 500 eggs/method). Therefore CR method along with recovery buffer (PBS) at 37oC could be an effective technique for the recovery of microorganisms from post-processed shells.