Gramicidin
A channel-forming linear pentadecapeptide, the first antibiotic ever used clinically, now confined to topical and eye-drop combinations because it destroys red blood cells if it reaches the bloodstream.
Also known as gramicidin D, gramicidin A, linear gramicidin, Neosporin Ophthalmic Solution (component), Soframycin (component)
Approved drug — Licensed by a major regulator for human use, with phase-3 trial data behind it.
Gramicidin was the first antibiotic used clinically, in 1939, and its topical combination products carry long-standing regulatory approval based on historical use rather than modern randomised trials. Its enduring scientific importance is as the standard model ion channel in membrane biophysics, not as a modern therapeutic.
How it works
Gramicidin D from Bacillus brevis is a mixture of gramicidins A, B and C — linear peptides whose strictly alternating L- and D-amino acids let them fold into a beta-helix. Two helices meet end-to-end across the membrane to form a continuous pore about 4 angstroms wide, selectively permeable to sodium, potassium and hydrogen ions but not anions or divalent cations. Uncontrolled cation flux dissipates the transmembrane potential and pH gradient, ATP synthesis stops, and the cell dies. Because the mechanism is purely physical and depends only on lipid bilayers, it works equally well on human cell membranes — which is exactly why systemic use causes massive haemolysis and why gramicidin has been a workhorse model system in biophysics for fifty years. Its clinical spectrum is gram-positive.
Targets: Lipid bilayer, Monovalent cation gradient, Membrane potential
Dosing
| Protocol | Dose | Frequency | Route |
|---|---|---|---|
| Ophthalmic combination dropsOne or two drops into the affected eye, up to every hour in severe infection for the first day or two. | — | every 4 hours | topical |
| Topical combination preparationsThin application to affected skin. | — | two to three times daily | topical |
- · In Neosporin Ophthalmic Solution gramicidin sits at 0.025 mg/mL alongside neomycin 1.75 mg/mL and polymyxin B 10,000 units/mL. Courses run 7-10 days.
- · Found in some skin creams outside the US, usually with neomycin or a corticosteroid. Concentrations are in the fractions of a milligram per gram.
Cycling
Topical courses of about a week. Longer use with the neomycin-containing combinations increases contact sensitisation and, for eye products, risk of corneal effects.
Pharmacology
- Half-life
- Not meaningfully characterised — gramicidin is used only topically and is not intended to reach the systemic circulation.
- Onset
- Immediate on contact; conjunctivitis typically improves over 2 to 3 days of drop therapy.
- Routes
- topical
- Molecule
- Natural-product linear pentadecapeptide with alternating L- and D-amino acids
- Sequence length
- 15 amino acids
- Molecular weight
- 1882.3 Da
Handling
- Diluent
- Not applicable - supplied as a ready-made solution or cream
- Lyophilised
- Not applicable.
- Reconstituted
- Eye drops stored at room temperature and discarded within about 28 days of opening.
Mixing
Gramicidin is essentially insoluble in water and is formulated in the product's vehicle; there is no reconstitution step for the user.
Side effects
- commonOcular stinging and burning on instillation— Usually settles within a minute.
- commonAllergic contact conjunctivitis or dermatitis— In combination products this is usually the neomycin rather than the gramicidin.
- rareHaemolysis— Only relevant to systemic exposure, which is why systemic gramicidin was abandoned in the 1940s. It is not a concern with correct topical use.
Do not use if
- Never for systemic, injectable or intravenous use — gramicidin is strongly haemolytic.
- Known hypersensitivity to gramicidin or to the neomycin or polymyxin B in the same product.
- Viral, mycobacterial or fungal eye infections, where an antibacterial drop can mask progression.
- Do not use non-ophthalmic gramicidin preparations in the eye.
Combining it
- synergypolymyxin-b — Standard co-formulation: gramicidin covers gram-positives, polymyxin B covers gram-negatives.
- synergyneomycin — Broadens coverage in the classic triple ophthalmic solution, at the cost of neomycin's high allergy rate.
What to monitor
- · None routinely for short topical courses.
- · Re-examine the eye if symptoms have not improved within 48 to 72 hours, since that usually means the diagnosis rather than the drug is wrong.
Legal status
Prescription-only ophthalmic solution in the US; available in some topical preparations elsewhere. Systemic formulations do not exist and would not be approvable.
References
- Neosporin Ophthalmic Solution US prescribing information (label)
- Kelkar & Chattopadhyay, the gramicidin ion channel as a model membrane system (review)
- Dubos 1939, original description of gramicidin from soil bacilli (other)
Mechanism in depth
A linear pentadecapeptide of alternating D- and L-amino acids that dimerises head-to-head in the membrane to form a beta-helical channel permeable to monovalent cations. The channel dissipates the transmembrane potential and ion gradients. It is one of the earliest antibiotics ever isolated and a textbook model of a channel-forming peptide.
What usually goes wrong
Its lack of selectivity is the whole story: gramicidin is genuinely toxic to human cells and is confined to topical and ophthalmic preparations for that reason. Any proposal to use it systemically misunderstands why it has been topical for eighty years.
Pharmacokinetics
- Crosses blood-brain barrier
- no
Receptor targets
- Lipid bilayer, forming a monovalent cation channel — Non-specific to membrane composition
Collapses ion gradients; lyses mammalian cells as readily as bacterial ones
What to expect, and when
Local and immediate.
Genuinely uncertain
- Selective delivery strategies that would allow systemic use remain research concepts.