Can you say "screening for a" in English? I know this is not a "Correct Use of English Forum" but I'm afraid that people from outside the field won't be able to properly answer my question. Would it be correct to say "screening for a long peptide capable of killing Gram-positive bacteria"? Or you just have to turn the word "screening" into a noun like: "preparing a screening for a long peptide capable of killing Gram-positive bacteria"? The last one doesn't seem right to me at all, but I'm not sure since I'm not a native speaker :( Thank you for your time!

"Screening" can be a verb here ("we screen for the long peptide") so your first sentence would be correct.

Even in this biological context this might still be a question better fit for the English/Learning English forums though.