The current FDA Bacteriological Analytical Manual (BAM) method for the detection of Salmonella in eggs requires two weeks to complete, according to Guodong Zhang of the Center for Food Safety and Applied Nutrition, Food and Drug Administration in College Park, Maryland.

In a paper in Poultry Science, Zhang and colleagues explain that the objective of their project was to improve the BAM method for the detection and isolation of Salmonella in whole shell eggs.

A novel protocol, using 1,000 g of liquid eggs for direct pre-enrichment with two litres of tryptic soy broth (TSB) followed by enrichment using Rappaport-Vassiliadis and Tetrathionate broths, was compared with the standard BAM method, which requires 96 hours room temperature incubation of whole shell egg samples followed by pre-enrichment in TSB supplemented with ferrous sulphate (FeSO₄).

Four Salmonella ser. Enteritidis (four phage types) and one Salmonella ser. Heidelberg isolates were used in the study.

Bulk inoculated pooled liquid eggs, weighing 52 or 56kg - approximately 1,100 eggs - were used in each trial. Twenty 1,000-g test portions were withdrawn from the pooled eggs for both the alternative and the reference methods.

Test portions were inoculated with Salmonella at one to five 5 colony-forming units (cfu) per 1,000g eggs. Two replicates were performed for each isolate.

In the eight trials conducted with Salmonella ser. Enteritidis, the alternative method was significantly (P<0.05) more productive than the reference method in three trials, and significantly (P<0.05) less productive than the reference method in one trial. There were no significant (P<0.05) differences between the two methods for the other four trials.

For Salmonella ser. Heidelberg, combined data from two trials showed the alternative method was significantly (P<0.05) more efficient than the BAM method.

Zhang and co-authors concluded that their alternative method has the potential to replace the current BAM culture method for detection and isolation of Salmonella from shell eggs based on the following factors:

• the alternative method is four days shorter than the reference method
• it uses regular TSB instead of the more complicated TSB supplemented with ferrous sulphate, and
• it was equivalent or superior to the reference method in nine out of 10 trials for the detection of Salmonella in shell eggs.

Reference

Zhang G., E. Thau, E.W. Brown and T.S. Hammack. 2013. Comparison of a novel strategy for the detection and isolation of Salmonella in shell eggs with the Food and Drug Administration Bacteriological Analytical Manual method. Poult. Sci. 92(12):3266-3274. doi: 10.3382/ps.2013-03380.