Help with referencing a foreign word in a paper!

I'm writing a paper on 'physis' for my philosophy class and the contradictions between Nietzsche's conception and Aristotle's. Naturally, I am using the word "physis" quite a bit in the paper.

The problem is, I don't know how to reference the Greek word physis.

I've been italicizing it as I did above consistently thus far. However, is it more appropriate to put it in quotations? If so, single or double quotations?

The paper is MLA if that makes a difference.

I don't know if this belongs here. I posted it in the writing section, but I feel that it doesn't really belong there. Help! 

Thanks!

Dave