A minimal salts medium adjusted to physiological pH and osmolality was buffered with either 0.3% phosphate or 1.2% borate and evaluated for antimicrobial activity. The borate-buffered medium, either with or without a carbon source, exhibited significant antimicrobial activity against 15 Pseudomonas strains, 12 strains of enteric bacteria, and 7 strains of staphylococci. The borate-buffered system appears suitable for use as a generic vehicle for ophthalmic pharmaceutical agents.In 1958, Kingma summarized the pharmacology and toxicology of boron compounds (10 on neonates (5), and by two pharmacokinetic studies of oral (14) and intravenous (9) administration.The present study evaluated the antimicrobial activity of 1.22% borate buffer in a minimal salts medium, with and without a carbon source (glucose). For comparison, phosphate-buffered and unbuffered minimal salts media were also evaluated. The antimicrobial activity of the solutions was evaluated as a function of the log change in viable cell count over 28 days. The significance of the results as they relate to the preservative effectiveness test described by the United States Pharmacopeia (USP XXI) and their application to ophthalmic pharmaceuticals is discussed. MATERIALS AND METHODSMicoorganisms. The microorganisms that were tested are listed in Tables 1 to 4. They were obtained from several different sources, including clinical, environmental, and product isolates.Defined media. Minimal media used with the Pseudomonas strains contained 0.04% (NH4)2SO4 and 0.05% MgSO4, either unbuffered, buffered with 0.10% KH2PO4 7H20 and 0.02% Na2HPO4, or buffered with 1.00% boric acid and 0.22% Na2B407 10H20. The minimal medium for the enteric bacteria and staphylococci contained 0.04% (NH4)2SO4, 0.02% MgSO4 7H2O, 0.02% CaC12 * 2H20, 0.0025% Na,MoO4 2H20, and 1.00% vitamin solution. The vitamin solution was prepared separately at 10Ox concentration and filter sterilized. The final concentrations were 0.0005% calcium pantothenate, 0.0001% thiamine hydrochloride, 0.0001% pyridoxine hydrochloride, 0.0001% biotin, and 0.0001% nicotinic acid. Each solution was made isotonic with 0.9% NaCl by calculating the salt equivalent of each ingredient and adding the appropriate amount of NaCl. The pH of each solution was adjusted to 7.00 ± 0.2 with hydrochloride or KOH. The glucose, MgSO4, CaCl2, and NaMoO4 were autoclaved separately at 10Ox concentration and then aseptically transferred to the minimal media.The microorganisms were transferred from nutrient broth to the defined minimal medium containing phosphate and glucose. After the microorganisms adapted and began growing in the minimal medium, a second 24-h culture was used for each experiment. The respective cultures were harvested by centrifugation, washed three times in distilled water, and standardized at A500 to give approximately 106 CFU/ml.The staphylococci were unable to grow in the minimal medium without the addition of amino acids. They were therefore harvested directly from the tryptic soy broth, washed three times in d...

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