When collecting total or fecal coliform bacteria samples, which bottle setup and handling is correct?

• A. Fill bottle to neck (leave airspace) and cap.
• B. Use a bottle without sodium thiosulfate treatment prefixed.
• C. Cap the bottle and invert after filling to mix, without leaving airspace.
• D. Use a sterilized 125 mL blue-capped bottle prefixed with Sodium Thiosulfate; fill bottle to neck (leave airspace) and cap; invert several times to mix.

Answer: (D)

Preserving the bacteria and ensuring a representative sample are essential steps in coliform testing. The correct approach uses a sterile 125 mL bottle that already contains sodium thiosulfate to neutralize any residual chlorine in the sample, which could otherwise kill bacteria and skew results. Fill the bottle to the neck, leaving a small airspace, then cap the bottle and invert several times to mix. This combination of preservative, proper fill level, and thorough mixing ensures the sample remains viable and representative for accurate analysis.

Choices lacking the preservative or using improper filling or mixing would risk chlorine residuals altering the bacterial count or producing an unhomogeneous sample, leading to unreliable results.