The real issue is that the body, not the antibiotics are what really heal, the antibiotics just tilt the balance to some degree.
Those without a functioning immune system dont respond to antibiotics.
As you age your resistance steadily reduces until you get close to be AIDS-like because controller hormones are deficient and inadequately produced leaving you immunocompromiesed. To bump them back up to functionality you have to fool mother nature. The fact is that the stray bacteria is getting in here and there through cuts, the lungs, the the mouth, and the various holes further south depending how they are stretched or otherwise battered.