4. Scaling up from in vitro to in vivo assays

Scaling up an antimicrobial assay from controlled, in vitro conditions to that of natural, in vivo conditions can be difficult if no proper considerations are taken. For instance, active concentrations for in vitro conditions frequently cannot be reached in vivo because the infecting microorganisms are never exposed to constant concentrations of an antimicrobial agent. Microorganisms in vivo are subject to competition from other microorganisms present in the tissue, so decreased microbial activity might be due to this competition rather than directly related to the antimicrobial activity of the plant extract. Moreover, temperature, pH, and humidity are more difficult to control in an in vivo system. Another issue to consider is that microorganisms in a microtiter plate are in the form of a suspension, whereas bacteria associated with different illnesses naturally form biofilms (organ and tissue infections, dental plaque, etc.), representing an extra challenge for antimicrobial agents [1]. Until now, only studies regarding fruit and vegetable infections have shown a parallel between in vitro and in vivo responses to Taraxacum extracts (Chapter 1; see Section 2.2.2), but studies in animal tissues and organs have not yet been performed directly.
