Growth of Salmonella enteritidis in artificially contaminated eggs: the effects of inoculum size and suspending media

Growth profiles of two isolates of Salmonella enteritidis phage type (PT) 4 inoculated into either the albumen of whole shell eggs or into separated a lbumen were found to be markedly affected by the size of the inoculum and t he composition of the medium used to suspend the cells prior to inoculation . Using our model with an inoculum of two cells, multiplication of the Salm onella was not seen in 93% of eggs held at 20 degreesC for 8 days. In appro ximately 7% of eggs, however, growth occurred during the 8 days of storage. If the inoculum equaled or exceeded 25 cells per egg when eggs were subsequ ently stored at 20 degreesC, or 250 cells per egg when eggs were stored at 30 degreesC, high levels of growth of Salmonella in the egg occurred signif icantly more frequently than when the inoculum was two cells. High levels o f growth were also seen more frequently if the inoculum was suspended in bu ffered peptone water or maximal recovery diluent rather than in phosphate b uffered saline. Growth of Salmonella in separated albumen occurred very inf requently (1.1% of samples) at low inoculum levels and did not become signi ficant until the inoculum was 250 cells or greater. Growth in the albumen w as unaffected by the composition of the suspending medium. Provided that th e inoculum was approximately 2 cells per egg and the bacteria were suspende d in PBS, observed growth profiles of S. enteritidis inoculated into the al bumen of whole eggs resembled those in naturally contaminated eggs. (C) 200 1 Elsevier Science B.V. All rights reserved.