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Inhibition of injured Escherichia coli by several selective agents
Abstract:
A population of Escherichia coli ML30 cells was exposed to a quaternary ammonium compound, and injury to the cells was measured by a comparison of counts on Trypticase Soy Agar and Violet Red Bile Agar. Substantial injury could not be detected with a minimal medium. The ingredients of Violet Red Bile Agar were tested against damaged cells. The bile salts mixture alone in the medium prevented as many injured cells from growing as did any combination of the selective agents and inhibited as many injured bacteria as were inhibited by Violet Red Bile Agar itself. These dyes and salts were similarly assayed in minimal agar, and comparable results were obtained. Individual bile salts and other potential selective agents were added to the minimal medium, and the media were tested for inhibition of injured E. coli. Sodium deoxycholate was the bile salt most inhibitory to damaged E. coli cells.
Insights
Quaternary ammonium compounds injure Escherichia coli. Bile salts in Violet Red Bile Agar, particularly sodium deoxycholate, significantly inhibit injured bacterial growth, impacting microbial testing accuracy.
Area of Science:
- Microbiology
- Food Safety
- Bacteriology
Background:
- Selective media are crucial for enumerating viable bacteria.
- Injury to bacterial cells can affect their recovery on selective media.
- Escherichia coli (E. coli) is a key indicator organism in food and water safety.
Purpose of the Study:
- To investigate the impact of selective agents in Violet Red Bile Agar (VRBA) on injured Escherichia coli cells.
- To identify specific components of VRBA that inhibit the growth of damaged E. coli.
- To assess the suitability of minimal media for detecting cell injury.
Main Methods:
- Exposing Escherichia coli ML30 cells to a quaternary ammonium compound to induce injury.
- Comparing bacterial counts on Trypticase Soy Agar (TSA) and Violet Red Bile Agar (VRBA).
- Assessing the inhibitory effects of individual VRBA ingredients and other selective agents on injured E. coli in minimal agar.
Main Results:
- Substantial cell injury was not detectable using a standard minimal medium.
- The bile salts mixture in VRBA alone inhibited injured E. coli growth as effectively as the complete VRBA medium.
- Sodium deoxycholate was identified as the most inhibitory bile salt towards damaged E. coli cells.
Conclusions:
- The bile salts in VRBA are the primary inhibitors of injured E. coli.
- Standard selective media like VRBA may underestimate the viability of stressed or injured bacterial populations.
- Further research is needed to develop methods for accurately assessing injured bacterial populations in food safety testing.