I have no idea, sorry. But I wanted to thank you for bringing this up. I never even heard of pseudomembranous colitis before, and only now (after googling it) realise it is something I probably will have to watch out for in my daughter. I'll stock up some more on the probiotics for her!
She too is on Nitrofurantoin for prophylaxis. She would've loved to get neon coloured wee...

But hers just changed to a darker brown once she started with it.