No internet access when creating bridge with ethernet

I am using Linux Mint. I am trying to create etheet bridge between interface eth0 and another virtual interface at0. In order to use inteet my interface eth0 have to coect with vpn(pptp) which is provided by my ISP. I lost my coection to inteet when I try to add eth0 to my bridge.

brctl addbr br0
brctl addif br0 eth0
brctl addif br0 at0
ifconfig at0 0.0.0.0 up
ifconfig br0 up
dhclient br0

What should I do to bridge these interfaces?