From my personal experience i can say that a person who is always negative (passing negative comments) is looking for social reward of ‘popularity’.
I can suggest you to just don’t give them that !
Say while you respect his thoughts, you just don’t share it.
And also don’t let the group sail with the negative thoughts.
The best way to combat negativity is to ‘isolate’ it.
Isolating negativity will have positive effect on the entire organisation and even on negative individual.
If the negativity continues to spread, report it and remove your co-worker from the bunch.