NON-LETHAL INJURY AND LIMITATIONS OF RECOVERY OF COLIFORM ORGANISMS ON SELECTIVE MEDIA1

Abstract

Cell injury as a factor in the enumeration of coliform bacteria with selective media was evaluated. Non-lethal injury reduced the ability of cells to produce outgrowth. Escherichia coli and Aerobacter aerogenes were equally sensitive. Brilliant green lactose bile broth, desoxycholate lactose agar, and violet red bile agar were similar in inhibitory effect. Cell injury occurred with sub-lethal heat treatments, exposure to chlorine, exposure to sodium chloride, and freezing and thawing cycles. Circumstances to inflict injury are found in common environmental conditions of the food industry. Presence of injured cells may provide a count with a selective medium that is only 10% of the total count obtained with standard plate count agar. The limiting factor for recovery in selective media was associated with the surfactant. An understanding of these limitations in the use of selective media in quality control and public health applications of the tests should contribute to proper interpretation of results.