By bottom string I assume you mean the high E string (thinnest string). If this is the case then I wouldn't worry to much about it. Just mute the string so it doesn't buzz and allow only the middle 4 strings to sound. This is pretty common. Many people are not lucky enough to have a finger joint that can bend far enough back to let that high E string ring cleanly while playing the A shape bar chord with 2 fingers. I've been playing for 20 years and still can't get the high E to ring clean while playing a 2 finger A shape bar chord. The chord will still sound fine as long as the high E string is muted and doesn't buzz.