Fixed Charge Density (FCD) of P. aeruginosa Alginate Biofilm Predicts Donnan-Mediated Cationic Antibiotic Partitioning

Two seemingly unrelated fields are at the heart of this hypothesis. The first is the biomechanics of cartilage — specifically, how the gel-like material in your joints manages water, pressure, and electrically charged molecules. Decades ago, scientists discovered that cartilage contains a high density of fixed negative charges, which creates an electrical imbalance that pulls positively charged molecules inward and pushes negatively charged ones out. This 'Donnan effect' is well-understood physics, used to model how cartilage handles ions under load. The second field is the study of bacterial biofilms — those slimy, antibiotic-resistant colonies that Pseudomonas aeruginosa bacteria build inside the lungs of cystic fibrosis patients and in infected wounds. These biofilms are notoriously hard to kill, and nobody fully understands why antibiotics fail to penetrate them. The hypothesis borrows the cartilage math and applies it to bacterial slime. P. aeruginosa wraps itself in a gel made largely of alginate, a seaweed-like polymer bristling with negatively charged chemical groups — similar in principle to cartilage. The idea is that this negative charge creates the same kind of Donnan electrical potential seen in joints, which would actually *attract* positively charged antibiotics like tobramycin into the biofilm rather than repelling them. But here's the twist: in low-salt environments like the liquid lining your airways, the math predicts this effect could concentrate tobramycin roughly three times more inside the biofilm than outside. In salty blood or wound fluid, the effect nearly vanishes. So the hypothesis isn't just 'charge matters' — it's a precise, quantitative prediction about when and where the physics kicks in. This is genuinely clever science-by-analogy. If the same equations that describe a knee joint can predict how antibiotics partition into a bacterial fortress, it would mean we've been sitting on the right tools for decades without realizing it. The honest caveat is that tobramycin also chemically bonds directly to alginate, and bacteria have many other tricks up their sleeves — so the Donnan effect might be one piece of a complicated puzzle rather than the whole story.

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