Incubation of Supplemented Egg Contents Pools To Support Rapid Detection of Salmonella enterica Serovar Enteritidis

Abstract

Detecting internal contamination of eggs with Salmonella enterica serovar Enteritidis (Salmonella Enteritidis) is an important aspect of efforts to identify infected laying flocks. When egg contents pools are tested for Salmonella Enteritidis, a preliminary incubation step is often employed to allow small initial populations of contaminants to multiply to more easily detectable numbers. Consistent detection of Salmonella 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 Salmonella Enteritidis cells multiplied in 10-egg pools, some of which were supplemented with concentrated nonselective enrichment broth or with a source of iron. At 37°C, Salmonella Enteritidis concentrationsin supplemented egg pools usually reached 105 CFU/ml within 12 h and 107 CFU/ml by 12 to 15 h of incubation. At 25°C, Salmonella Enteritidis concentrations in supplemented egg pools typically attained 105 CFU/ml by 18 to 27 h and 107 CFU/ml by 27 to 36 h of incubation. At both temperatures, Salmonella Enteritidis multiplication was significantly slower in unsupplemented pools. Accordingly, the length of incubation time necessary for consistent detection of small numbers of Salmonella 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.