How can i change the layout of the initial chatter feed when a case is opened from the community site?

For example, when a user is opening a new case the chatter feed shows the following:  CommunityTest (Customer)

Case created

• Subject: test
• Priority: Medium
• Status: New
• Case Number: 00014303

 How can i change the layout and fields of this initial chatter post on the new case (which is also what the customer see in the community site)?