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United States Department of Agriculture

Agricultural Research Service

Research Project: Develop a New Microbiological Medium for Selective Cultural Enrichment of STEC
2011 Annual Report


1a.Objectives (from AD-416)
The objective of this cooperative research project is to develop a microbiological growth medium that is selective for Shiga toxin-producing Escherichia coli bacteria (STEC).


1b.Approach (from AD-416)
The principle of the STEC-selective medium is based upon competition between STEC and another microorganism. At least one recent scientific publication suggests the protozoan Tetrahymena thermophila is capable of killing E. coli that do not express Shiga toxin, while STEC can kill the protozoan. If STEC are present, they may be the only organisms capable of growth on the media. Developing the new media based on an existing E. coli selective medium will help in negative selection against non-E. coli toxin producers, which might otherwise compete against STEC and the protozoan. Additional selection may be applied in a modified pre-amplification culture, before plating on the new media. For validation, STEC will be spiked into foods and the Lower Limit Of Detection determined.


3.Progress Report

The start of this project was delayed by the unexpected departure of the California State University graduate student assigned to work on it. However, with the new student on-board we screened several commercial media for appropriateness in culture of both the protozoa and bacteria of interest. We have also developed experimental conditions that simultaneously support both predation of non-STEC and protozoan lethality with STEC. While this experimental system is now working, it remains to be determined whether we can create a robust commercial STEC-specific growth medium based on protozoan co-culture. The Cooperator provides media and technical advice. The ADODR documents material shipments and consults the Cooperator via telephone and email communications.


Last Modified: 10/22/2014
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