Why is sodium thiosulfate added to sample bottles for total coliform testing?

Sodium thiosulfate is specifically added to sample bottles for total coliform testing to neutralize any chlorine residual present in the water sample. Chlorine is commonly used as a disinfectant in water treatment processes, but it can inhibit the growth of coliform bacteria during testing if it is still present in significant amounts. By adding sodium thiosulfate, any residual chlorine is effectively neutralized, allowing for an accurate assessment of total coliform presence without the interference that chlorine could cause in the detection process. This step is crucial to ensure that the results of the total coliform testing reflect the actual microbial quality of the water sample, rather than being affected by the chlorine that could kill the target organisms being tested for.