Membrane filter Petri dishes should be incubated in which position?

Membrane filter Petri dishes should be incubated in an inverted position to prevent condensation from dropping onto the filter surface. When dishes are placed upright, moisture from the air can accumulate on the lid and eventually drip onto the agar or membrane filter, potentially leading to false positives or interfering with colony counts. The inverted position allows any condensation that may occur to run down the sides of the dish instead of pooling on the filter, thus ensuring that the conditions for microbial growth are optimal and that the results of the incubation are reliable. This practice is especially crucial for ensuring the accuracy of water quality testing, where precise counts of microorganisms are necessary for regulatory compliance and environmental assessments.