Several students at CSCR participate in bacteria testing at their various project sites. Of course, this includes students in the Bacteria Monitoring group. These students collect duplicate sterile samples at each of their sites. One of these samples is delivered to the Board of Health for professional lab testing, and the other is tested for bacteria by students in the CSCR lab. Students are often excited to see that the professional lab results match their own, indicating a job well done by our students.
CSCR students run tests for E. coli/total coliform at freshwater sites and enterococci at saltwater sites. To test for coliform, students add one packet of Colilert to a 100mL sample and gently roll or invert the bottle to dissolve the particles. At this point, students pour the sample into a bacteria tray labeled with the date, time, and site. The bacteria tray has several wells for the liquid and can be sealed using a heat sealer. Once the tray is sealed, it is placed in an incubator for approximately 24 hours until it can be read. Testing for enterococci is a nearly identical process with a few exceptions. Rather than using Colilert, the students use a similar powdered product called Enteroalert for bacteria to feast. Enteroalert requires a 10mL dilution, which is achieved by adding 90mL of autoclaved, distilled water to 10mL of sample water. Additionally, the enterococci trays must be incubated at a slightly higher heat than the coliform trays.
After 24 hours have been passed, students read the results by placing the tray under a UV light. Some of the wells light up fluorescent blue, indicating bacteria presence. Students count the number of small and large wells that fluoresce and use a chart provided by IDEXX (the company that produces Colilert and Enterolert) to determine the MPN. MPN stands for Most Probable Number, referring to the most probable number of bacteria colonies within the sample. Over the years students have found that rainfall leads to increased MPN, or higher bacteria levels, in their samples.
