Efficacy of expired IDEXX Colilert media in E. coli and coliform detection: a laboratory assessment

Water quality testing for coliforms and E. coli using the IDEXX Colilert system is easy, quick, and reliable. However, the associated costs can become substantial when multiple tests are required, especially in research facilities and classrooms with limited budgets. IDEXX Colilert has a 1-year shelf life, which poses challenges in environments where the media are used intensively but infrequently, such as annual class experiments or periodic field sampling. This study explored the relationship between the use of expired media and the accuracy of detecting total coliforms, thermotolerant (fecal) coliforms, and E. coli. Limited data exist on the performance of expired media for coliform and E. coli testing, leaving researchers and educators unsure whether slightly or significantly expired media remain effective. The accuracy was assessed by evaluating the most probable number (MPN) results from diluted wastewater samples using Colilert of various ages, from unexpired through up to 10 years pastfig. 2 expiration date. The findings indicated that the media could be usable for detecting total and thermotolerant (fecal) coliform bacteria up to 120 days post-expiration date and E. coli potentially up to 330 days post-expiration date. However, accuracy and precision begin to decline as the media further aged. Media that had been expired for several years tended to turn the water sample yellow, reducing the difference in color saturation between positive and negative results for coliform in test wells. While expired Colilert media are not recommended for use in settings requiring regulatory compliance, they may still be adequate for educational experiments where the highest level of accuracy is not critical.