There is one other factor: Sex (in a way).
Bacteria can exchange genetic material with neighboring cells, often even across species lines. Can be proven like this:
Put, let's say, a strain of streptococcus on a growth medium. Let this strain be resistant to penicilin. Add another, radically different, bacterium (or even something that blurs the lines, like Anthrax).
After a few days, add large amounts of penicillin, wait 15 minutes, and examine under a microscope. You will find d lot of dead bacteria of the second species, but still, a large portion (sometimes nearly all of them) will have survived, which means they had a resistance against Penicillin, the only source of which were the streptococcus' genes.
Voila, bacterial sex <img src="/images/graemlins/wink.gif" alt="" />