Just because the DNA transfer is, traditionally, one way doesn't make us weaker without superior abilities to a bacteria.
Getting inside a cell and changing DNA is what a virus is good at. Our immune system doesn't produce viral structures.
Attaching antibodies (those 3-D shapes) to pathogens and using pyrogens and killer cells - THAT the immune system does.
Introducing DNA changes to the pathogens that prey upon you is an ineffective method, because you cannot ‘get’ them all at once, and the more detrimental the disadvantage, the more those unaffected will dominate subsequent generations.
So even if the immune system...
a) created viral like particles that could enter the bacterial cell and introduce new DNA.
b) the new DNA was to the disadvantage of the bacteria.
Then.....
c) the bacteria with the disadvantage would be out-competed by those without it.
I'm not a DNA researcher, and I'm not even a researcher about the human defense mechanisms, but, I have stayed at a Holiday Inn Express. ;)
Introducing DNA changes to the pathogens that prey upon you is an ineffective method, because you cannot ‘get’ them all at once, and the more detrimental the disadvantage, the more those unaffected will dominate subsequent generations.
You don't have to "get them all at once"; just the ones that constitute the original invasion or infection. Infections start out with a few "invaders" and the number of them get larger with time. So, if the body is able to attack the few before they become the "many", then there is no need to "get them all".