The effects of stress of coliform bacteria resulting from exposure to heat, radiation, or sodium chloride on behavior of the progeny were studied. After exposure to stress and subsequent growth on plating media, colonies were picked by random design and grown in nutrient broth for further comparison of their ability to form colonies on plate count and violet red bile agar. After 6 hr in nutrient broth, average counts on violet red bile agar were less than half those obtained with plate count agar. Sensitivity to the selective medium was lost by repeated transfer and growth in nutrient broth or by repeated picking from the selective medium and subculture of colonies.

Cultures with moderate sensitivity to violet red bile agar were obtained from raw sewage through picking of colonies from plate count agar. Attempts to obtain a stable sensitive strain through selective enrichment were unsuccessful. A laboratory strain of Escherichia coli, with extreme sensitivity to violet red bile agar, however, was used to determine that tolerance was acquired by stepwise adaptation to a selective medium. Occurrence of sensitive coliform cells in nature indicates their potential importance in tests for indicator organisms of public health significance.

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Author notes

1Published as Paper Number 3499, Journal Series, Nebraska Agricultural Experiment Station, Lincoln.