(not a scientist but that doesn't seem plausible. water takes up equal space to diet soda and also provides no nutrition.)

Ah, I should've written a more generic "takes the place of" instead of space. I meant to ask something along the lines of whether the artificial sweeteners of diet sugar might be confused for nutrition by the gut bacteria. Basically, I was picturing a situation where the gut bacteria might try to "eat" the artificial sweeteners, but then don't end up getting enough energy from it and can't do the things they need to do.

Like how xylitol can kill mouth bacteria because they eat it instead of other sugars but can't metabolise it.

Yes! That's pretty much the mechanism I was thinking of.

Couldn't you say the same thing about water?

Good point. I worded it very poorly and your answer would definitely fit. I meant to ask whether the artificial sweeteners in diet soda were taking the place of nutrition for the gut bacteria, but don't end up giving the bacteria enough energy/resources to live on.