Why is a water sample usually packed in ice for bacteriological analysis?

The reason a water sample is usually packed in ice for bacteriological analysis is to retard changes in the number of bacteria. This practice is crucial because bacterial populations can change rapidly if the samples are not kept at a low temperature. When water samples are stored at higher temperatures, the metabolic activity of bacteria can increase, leading to multiplication or die-off of certain species. This results in inaccurate counts and potentially misleading results when analyzing the sample for contamination or other microbiological concerns.

Packing the samples in ice slows down the growth and reproduction of bacteria, effectively stabilizing the sample for more accurate analysis when it is eventually tested in a laboratory setting. This preservation method allows for reliable assessment of the water's microbiological quality as it minimizes the alterations that could occur while the sample is transported and until analysis is performed.