Antibiotics and plant antimicrobials

Many plants have measurable antibacterial activity in the laboratory, and this is the fact that sells a great deal of product. The gap between a dish and a person is where the harm lives.

Why in-vitro activity rarely translates.

• Concentration. The concentration that inhibits a bacterium in a dish is often far above what can be achieved in tissue or blood without toxicity. Applying a 1% solution to skin is achievable; achieving an inhibitory concentration in blood is not.
• Protein binding and tissue distribution. Blood and tissue proteins bind many of these compounds, so the free concentration in the body is a fraction of the dish concentration.
• Biofilm. In a real infection bacteria are in biofilms and intracellular compartments, where they are far more resistant than in a broth culture.
• Selective pressure. A concentration that partially inhibits and does not kill is the optimal condition for selecting resistance — which is exactly what a sub-therapeutic plant antimicrobial delivers.

Where plant antimicrobials genuinely earn their place.

• Topical and mucosal surfaces, where a high local concentration is achievable: honey on a wound, tea tree oil at 5% on acne or a dermatophyte infection, a saline irrigant, a mouthwash. Topical applications are where the concentration gap is smallest.
• Honey on wounds has clinical trial evidence including against antibiotic-resistant organisms in a wound.
• Garlic's allicin is a genuine antimicrobial with a well-characterised mechanism — and it is unstable and formed only when the tissue is crushed, which is why crushed garlic should rest 10 minutes before cooking.

Where they cause harm.

• A systemic infection treated with a plant antimicrobial. Pneumonia, a urinary infection with fever, cellulitis, sepsis — these need effective antibiotics, and a delay is how someone dies.
• Partial courses. The single most important driver of antibiotic resistance is incomplete treatment and unnecessary use, and a plant antimicrobial used instead of or alongside a prescribed course is a partial course.
• Self-treating a "sinus infection" or a "chest infection" that may be viral, with an antimicrobial chosen by neither testing nor diagnosis.

Colloidal silver deserves a specific mention. Sold as a cure-all, it has no demonstrated clinical efficacy, it inactivates antibiotics when combined, and prolonged use causes argyria — an irreversible blue-grey discolouration of skin and eyes. It is the clearest example in this section of an antimicrobial sold on in-vitro data and a theory.