How to have spotlight index .tex files

The comments should normally show up in Spotlight results after you close an information window in Finder. You can tell Spotlight to reindex directories or files with mdimport, but it should do that automatically most of the time.

The Spotlight comments seem to be stored in both .DS_Store files and extended attributes. But if a file is owned by root, Finder doesn't write the comment to an extended attribute, and it won't be seen by Spotlight either.

If you use xattr to add the comments, they are picked up by Spotlight, but they aren't shown in Finder's information windows.

sudo touch test
sudo xattr -w com.apple.metadata:kMDItemFinderComment somecomment test

If you open Finder as root, they are saved in both extended attributes and .DS_Store files.

sudo /System/Library/CoreServices/Finder.app/Contents/MacOS/Finder

You can remove directories from the Spotlight Index under the Privacy tab in System Preferences > Spotlight

(Described in this other question)