Efficacy of methenamine and methenamine salts as topical antibacterial agents were evaluated. The minimum inhibitory concentrations of methenamine hippurate, methenamine, and methenamine mandelate in vitro against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa suspensions of 105 cells per milliliter were 12.5 mg/ml, 12.5 mg/ml, and 50 mg/ml, respectively, at a pH determination of 6.5. Each MIC increased with an increased pH value. The correlation between MIC and pH is related to the amount of formaldehyde liberated in acid media at various pH determinations. Application of methenamine hippurate (4.5 gm of 10% cream) twice daily reduced the mortality from contaminated third-degree burns from 78% to 17% and from contaminated crush wounds from 72% to 13%.
Matsumoto T, Wolferth CC, Hayes MF. Methenamine Salts as Topical Antibacterial AgentI. Experimental Studies for Burn and Crush Wound. Arch Surg. 1970;101(1):71-77. doi:10.1001/archsurg.1970.01340250073016