Please format your code using triple back-ticks (```) as described here:
Also, the title of your post should be a question. From the same page linked above:
The title of your question should make clear what your post is about. Good titles are questions you would ask your friend about Typst.
I don’t know what the “correct” spacing is, but you can remove the existing spacing by adding a zero width space symbol sym.zws:
$n_("Vibrio" Delta upright("ACD"))=5$\
$n_("Vibrio"#sym.zws;Delta#sym.zws;upright("ACD"))=5$
# enters code mode and ; exits it. Normally you don’t need a ; but here it’s important not to introduce another (unwanted) space to exit code mode.
Using the horizontal spacing function h() with its weak parameter set to true would be the same as using sym.zws. Usin it doesn’t even require a ; since the closing parenthesis ()) is there:
$n_("Vibrio"#h(0em, weak: true)Delta#h(0em, weak: true)upright("ACD"))=5$\