A doc once told me that antibiotics can't do the job by themselves. They need the body's help. So the 6 mp does seem suspect.
The problem with antibiotics is that they kill the good bacteria as well as the bad. So anybody on antibiotics should eat yogurt, which helps replenish the good bacteria.
Also, once antibiotics have killed all the bacteria, then the bad bacteria, not the good bacteria, are more likely to grow back, first.
Yogurt plus a probiotic like lactobacillus acidophilus or VSL 3 might help your body's immune system.