The Effect of Laboratory Water Type on Bacterial Growth in Prepared Media

For life on earth, water serves as a reactant in metabolic pathways and as the universal solvent. In its role as a solvent, the use of distilled or even ultrapure water is often specified when preparing growth media. In preparation instructions, the rationale for this choice is rarely given. We wondered if water could be too pure, lacking essential trace elements not provided by the medium powders. Conversely, could tap water still retain compounds that would inhibit microbial growth? We hypothesized that, with current water purification methods, tap water would be suitable for preparing media in the microbiology laboratory. To test this hypothesis, we prepared two commonly used media, Nutrient Broth and Tryptic Soy Broth, in water from three sources: tap, deionized, and ultrapure. We then measured growth of three bacteria commonly used in the microbiology teaching laboratory. Growth was observed in all media preparations. There was no inhibition of growth when tap water was used to prepare the media. For Escherichia coli grown in Nutrient Broth, use of tap water appeared to lead to increased growth. Growth in media prepared with ultrapure water did not significantly differ from that in media prepared with tap water, with the one exception, or deionized water. The use of high purity water will remain important when the exact composition of a medium is required. However, our results suggest that tap water is adequate when preparing media for general purposes such as establishing working cultures for short-term use.