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Salmonella can survive in a low-pH food system regardless of acid adaptation.

S. Heidelberg is the fourth-most-commonly reported Salmonella serotype to cause human illness. There have been several outbreaks and product recalls caused by S. Heidelberg in ready-to-eat products. Several hundred people became ill from ingesting hummus shirazi contaminated with S. Heidelberg in 2007.

Scientists at the University of Delaware investigated the effects of storage conditions and acid adaptation on the behavior of S. Heidelberg in hummus. Their efforts show the ability of S. Heidelberg to grow in a low-pH food system regardless of acid adaptation.

A three-strain cocktail of S. Heidelberg was grown in unadapted tryptic soy broth (TSB) and in acid-adapted TSB that was supplemented with 1% glucose for 24 hours at 37 C. These were inoculated into a commercial formulation of hummus at either a low (3 log CFU per g) or high (7 log CFU per g) population. Then they were stored at either 4 C or 22 C. Scientists determined the populations on days 0, 1, 3, 5, 8, 13, 18 and 28 on xylose lysine deoxycholate XLD media. They only evaluated a high inoculum of acid-adapted cells.

The pH of hummus at day 0 was 4.3 and ranged from 3.8 to 5.2 throughout the study. After 28 days, unadapted populations in the low and high inocula samples at 4 C remained unchanged at about 1 log CFU per g. Salmonellae in the low inoculum were undetectable after seven days, while those in the high inoculum increased by 2.7 log CFU per g in hummus stored at 22 C.

The high inocula of acid-adapted Salmonella at 22 C increased by 3.2 log CFU per g and decreased by 2.4 log CFU per g at 4 C. Samples with about 0.5 log CFU per g at 22 C were enriched following FDA guidelines. In order to analyze the potential adaptation to acidic conditions, RNA samples from unadapted and acid-adapted cells in hummus at both temperatures were extracted and analyzed for expression of genes involved in log and stationary phase acid tolerance responses.

Further information. Kali E. Kniel, Department of Animal and Food Science, University of Delaware, 044 Townsend Hall, Newark, DE 19716; phone: 302-831-6513; fax: 302-831-2822; email:

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Publication:Microbial Update International
Date:Jun 1, 2009
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