In a coliform presence-absence test, how long should a sample be incubated at 35°C?

In a coliform presence-absence test, the sample should be incubated at 35°C for 24 to 48 hours to effectively detect the presence of coliform bacteria. This incubation period allows enough time for the bacteria, if present, to grow to detectable levels. During this timeframe, coliform bacteria will metabolize the nutrients provided in the medium, which leads to observable changes indicating their presence.

The incubation temperature of 35°C is specifically chosen because it is conducive to the growth of coliforms, which thrive at this temperature range. Incubating for less than 24 hours may not allow sufficient time for growth, thereby increasing the risk of false negatives. Conversely, incubating beyond 48 hours may result in overgrowth or secondary organisms outcompeting the coliforms, complicating the interpretation of results. Thus, the 24 to 48-hour range is optimal for accurate detection consistent with standard testing protocols.