What Is the Connection between Polymyxin B and Trimethoprim?

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  • Written By: Maggie J. Hall
  • Edited By: Susan Barwick
  • Last Modified Date: 01 April 2020
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Polymyxin B and trimethoprim are antibiotics that manufacturers often combine to make topical ophthalmic solutions that treat external eye and eyelid infections. Together, polymyxin B and trimethoprim destroy a large selection of gram-negative and gram-positive bacteria. Pharmaceutical companies also use polymyxin B in conjunction with other antibiotics in the formulation of over-the-counter antibiotic ointments. Physicians often prescribe trimethoprim alone or with sulfamethoxazole, another antibiotic, for the treatment of urinary tract infections.

The antibiotic polymyxin B comes from the Bacillus polymyxa bacterium and belongs to the group of anti-infectives known as cyclic lipopeptides. The antibiotic acts by binding to the negatively charged area on a bacterial cell membrane, rendering the cell permeable. Once inside, polymyxin B interferes with cellular respiration, killing the bacteria. The substance also binds to and inactivates the bacterial endotoxins that are released after cell death and that are responsible for inflammation. Polymyxin B effectively kills the majority of gram-negative bacteria with the exception of the species belonging to the group knows as Proteus.


Laboratories synthetically produce trimethoprim, which is a dihydrofolic reductase inhibitor. Trimethoprim enters bacterial cells and prevents the production of dihydrofolic acid, which is necessary for the synthesis of tetrahydrofolic acid. This action also prevents the thymidine triphosphate synthesis that is necessary for deoxyribonucleic acid (DNA) production. The medication also deprives the cells of folic acid and proteins. Unable to reproduce or carry on normal cellular activities, the bacteria die. The many bacteria eradicated by trimethoprim include the gram-negative and gram-positive organisms Escherichia coli, Proteus mirabilis and the Staphylococcus.

Patients usually apply one drop of a solution of polymyxin B and trimethoprim into an affected eye as prescribed. The solution is usually every three hours, up to six times a day, for a period of seven to ten days. Ophthalmic solutions containing polymyxin B and trimethoprim may produce burning or stinging upon application. Hypersensitivity reactions may produce redness and swelling of the eyelid or rash formation around the general area. Using a polymyxin B and trimethoprim solution longer than the prescribed length of time can create super infections resulting from the production of resistant bacteria.

Adverse reactions include burning, itching, and redness, which may begin at any time during use. If the infection persists or worsens, patients must consult with the prescribing physician. To maintain sterility of the container and solution, direct contact between the bottle applicator and any body part or foreign object must be avoided. The solution is usually stored at room temperature.


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