Comparison of the Microbicidal activity of monochloramine and iodine

Recently, we showed that monochloramine (NH2Cl) has a significantly stronger bactericidal and fungicidal activity than chloramine T despite its lower oxidizing power. This phenomenon was explained by increased penetration because of the higher lipophilicity and smaller bulk of NH2Cl. As iodine (I2) has an even fivefold higher bulk than NH2Cl, a comparison of both compounds regarding their microbicidal activity became the aim of this study. Aqueous solutions of I2 at a concentration of 10·7 μmol l−1 killed 106 colony forming units per millilitre (CFU ml−1) of Escherichia coli, Staphylococcus aureus or Pseudomonas aeruginosa to the detection limit of 102 CFU ml−1 within 1 min at 20°C and pH 7·1, while a concentration of 36–355 μmol l−1 of NH2Cl was needed to achieve the same effect. Aspergillus fumigatus was inactivated within 5 min by 36 μmol l−1 I2 and by 355 μmol l−1 NH2Cl, Candida albicans within 1 min by 10·7 μmol l−1 I2 and by 355 μmol l−1 NH2Cl. The lipophilicity of I2, determined with the octanol/water method, was three powers of 10 higher than that of NH2Cl. The at least 10-fold stronger microbicidal activity of iodine suggests that the hindrance of penetration of the bulky molecule is outweighed by enhanced lipophilicity. Significance and Impact of the Study The microbicidal activity of active halogen compounds increases not only with their reactivity, but also with higher lipophilicity and lower bulk, as shown recently. In this study, iodine showed a higher microbicidal activity than monochloramine and a 1000-fold higher lipophilicity. Therefore, the lipophilicity of a disinfectant may be more important than the bulk for bactericidal activity. These facts should be considered upon the design of new antiseptics and their clinical application.