The article is a bit confusing in that they refer to two separate studies. Both of the studies are open access

(Neotame) https://www.mdpi.com/1420-3049/23/2/367 (6 sweeteners) https://www.mdpi.com/1420-3049/23/10/2454

Also there seems to be a typo in the article:

> ...the bacteria found in the digestive system became toxic.

I don't think they mean the bacteria themselves became toxic.