Trust by this time , you got a solution. This is a pertinent problem, one will face in any project. The PM should be aware of this. Be proactive, talk to the people who are prone to be negative, understand their concerns. Try to address them in a honest way. Be positive and sow an open culture, where every one is respected as an individual. This is at times difficult to maintain and still you may observe some negative trends. Have a discussion with them individually, give them more responsibility and makee them accountable for their actions. Even after this, there is NO change, then involve HR and initiate necessary corrective action, before it spreads to the other team members.