Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 30, 2002
Publication Date: April 1, 2003
Citation: GAST, R.K., HOLT, P.S. INCUBATION OF EGG CONTENTS POOLS TO SUPPORT RAPID DETECTION OF SALMONELLA ENTERICA SEROVAR ENTERITIDIS. JOURNAL OF FOOD PROTECTION. 2003. Vol. 66. No.4. p. 656-659 2003. Interpretive Summary: Detecting internal contamination of eggs with Salmonella Enteritidis is an important aspect of efforts to identify infected laying flocks. Some new methods can detect S. Enteritidis contamination very rapidly, but these approaches typically require very large numbers of bacteria (as many as 10 million per ml) to be present. Because egg contamination typically involves very small initial numbers of S. Enteritidis cells, standard procedures for finding this pathogen in pools of liquid egg contents often include a preliminary incubation step to encourage bacterial multiplication to levels that can be consistently detected. The present study determined the rate at which very small initial numbers of S. Enteritidis (approximately 10 cells) multiplied in 10-egg pools to reach levels at which rapid detection methods are usually effective. Supplements (of iron or concentrated bacterial enrichment media) were added to some pools to promote the growth of S. Enteritidis. Incubation of supplemented pools at 37 C resulted in multiplication of S. Enteritidis to levels detectable by all rapid methods in as little as 12 hours and incubation of supplemented pools at 25 C achieved these levels in as little as 27 hours. Multiplication in pools without supplements was much slower. This study suggests that the length of incubation time necessary for consistent rapid detection of small numbers of S. Enteritidis in egg contents pools depends on the incubation temperature used, on whether the egg pools are supplemented to increase the rate of bacterial multiplication, and on the actual sensitivity of rapid tests that are used.
Technical Abstract: When egg contents pools are tested for Salmonella Enteritidis, a preliminary incubation step is often employed to allow small initial numbers of contaminants to multiply to more easily detectable levels. Consistent detection of S. Enteritidis in egg pools by direct plating requires the presence of at least 105 CFU/ml, whereas some very rapid methods can require as many as 107 CFU/ml. The present study determined the rates at which initial inocula of approximately 10 S. enteritidis cells multiplied in 10-egg pools, some of which were supplemented with concentrated non-selective enrichment broth or with a source of iron. At 37 C, S. Enteritidis levels in supplemented egg pools usually reached 105 CFU/ml within 12 hours and 107 CFU/ml by 12-15 hours of incubation. At 25 C, S. Enteritidis levels in supplemented egg pools typically attained 105 CFU/ml by 18-27 hours and 107 CFU/ml by 27-36 hours of incubation. At both temperatures, S. Enteritidis multiplication was significantly slower in unsupplemented pools. Accordingly, the length of incubation time necessary for consistent detection of small numbers of S. Enteritidis in egg contents pools depends on the incubation temperature used, on whether the egg pools are supplemented to increase the rate of bacterial multiplication, and on the sensitivity of subsequent tests applied to the incubated pools.