Although in vitro tests of susceptibility to antibiotics are widely used and considered useful, there have been cases in which the in vitro tests have not predicted how well the drug would work when...

Although in vitro tests of susceptibility to antibiotics are widely used and considered useful, there have been cases in which the in vitro tests have not predicted how well the drug would work when administered to humans. This works both ways. Some antibiotics that are effective in vitro fail when administered to humans, and some antibiotics that test resistant in vitro actually work when used therapeutically. Give some explanations for both of these outcomes.