Iron Supplementation to Enhance the Recovery of Salmonella enteritidis from Pools of Egg Contents

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Abstract
Direct-plating culture methods have been proposed for use in programs to test eggs for contamination by Salmonella enteritidis (SE) because of their speed and low cost, but direct plating has previously been observed to detect SE less frequently than more elaborate broth-enrichment culture methods. The present study used experimentally inoculated pools of egg contents to assess the comparative sensitivities of direct-plating and broth-enrichment culturing for detecting SE, to evaluate the ability of iron supplementation to increase the multiplication of SE during incubation of egg pools, and to determine whether iron supplementation could enhance the ability of direct plating to detect SE in egg pools that initially contained very few SE cells. Efficient detection of SE in egg pools was found to require an approximately 10,000-fold higher level of SE for direct plating than for broth-enrichment culturing. Iron supplementation of contaminated egg pools significantly (P < 0.005) increased the resulting final level of SE after 1 day of incubation at 37°C. Iron supplementation also significantly (P < 0.005) increased the percentage of 10-egg pools, initially inoculated with fewer than 10 SE cells each, that were identified as contaminated by direct plating after the pools had been incubated for 1 day at 37°C. Increasing the iron availability in incubating egg pools, therefore, increased the probability that a small initial number of SE cells would grow quickly to levels likely to be detected by direct plating.

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