How to Study Antimicrobial Activities of Plant Extracts: A Critical Point of View

Multiresistance to antibiotics is a global threat for our quality of life. New weapons are urgently needed, and natural compounds regain interest. Research on antimicrobial compounds should fulfill several claims—they should be only evaluated at concentrations that could be achieved on the target—the bacterial species tested should be in agreement with the chosen application—studies should not be limited to a single strain. Conventional antimicrobial research is conducted in vitro for substances affecting growth by determining the minimal inhibitory concentrations (MICs). Research can further be refined by determining the kill time, the growth rate at sub-MICs concentration, the post-antibiotic effect, and synergy with other compounds like antibiotics. But features not related to bacterial growth can also reduce the infectious risk like anti-biofilm, anti-adherence, and antitoxin strategies. They can be determined both in vitro and in vivo but only in vivo infection models show directly the effect on the expression of the pathogenicity. The search of the mode of action can give useful insights in toxicity and drug interactions. Scientific tools in genomics and chemistry are steadily increasing to improve this topic. Bio-guided isolation of active compounds enhances detection of active compounds in the yet underexploited nature.