During the MPn Presumptive Phase, what is the incubation temperature used when inoculating Lauryl Tryptose Broth?

• A. 25 C
• B. 35 C
• C. 45 C
• D. 37 C

Answer: (B)

During the MPn Presumptive Phase, the Lauryl Tryptose Broth is incubated at 35°C. This temperature is chosen because it supports lactose fermentation by coliforms and allows gas production to appear in the Durham tube within the typical testing window. It provides reliable, consistent results across laboratories. Lower temperatures like 25°C slow fermentation and can miss positives, while higher temperatures such as 45°C can inhibit coliform growth, and 37°C is not the standard specified for this phase. Gas production in the Durham tube indicates a presumptive positive for coliforms, guiding the next confirmatory steps.