Dear all scientists,

I am working on degradation of plastic by microorganisms. During the experiment I analysed my sample for the possible bio-surfactant production. I used oil displacement method in this regard. Therefore, I taken out 5 ml of my cultural aqueous and centrifuged it at 8,000 4 degree for 15 minutes. The supernatant was filtered and cell-free liquid was subjected to the oil-displacement test. In brief, 30 ml of distilled water was poured in petri dish and 1 ml of sunflower oil poured gently on top of the water layer. Then 0.5 ml of filtered media containing the possible bio-surfactant was added on the surface of thin oil-water layer.

and for my control, I used distilled water and mineral salt media containing plastic. Now, my question is this:

after I poured my sample, oil was displaced about 3-5 mm. I repeat the experiment with my controls and in both cases even distilled water, oil was displaced. How is it possible?

Based on literature my method should be exactly fine, but I cannot understand this displacement.

Can you please share your knowledge and experiment in this regard?

For your guidance I attached the test result. The green arrows are displacement and red arrows are sunflower oil on water.

Many many thanks for your help.