The relative germicidal activity of tri-iodide and diatomic iodine.

A freshly prepared aqueous solution of iodine at a pH less than eight will contain mainly two forms of free iodine, namely diatomic iodine, I2, and triiodide, Ii,. The ratio of these two forms is determined by the concentration of iodide which is usually added to bactericidal prepaations for the purpose of increasing the solubility of the iodine. The solubility of diatomic iodine at 25 C is 0.034 per cent. In the presence of an iodide salt like potasium iodide the solubility of free iodine may be increased several hundredfold, the increase being in the form of triiodide according to the equation: (1) Ta+ 1 Ir. A quantitative investigation of the relative sporicidal activity of diatomic iodine and triiodide carried out by Wyss and Strandskov (1945) showed that the triiodide ion had negligible activity. Their test organism was Bacillus meties spores. Subsequent publications by Knox et al. (1948) and Marks and Strandskov (1950) have emphasized that the mode of bactericidal action on spores may be quite different from that on vegetative cells and that generalization of the relative sporicidal activity of the two forms of iodine to include other microorganisms is not warranted. Salle (1954) states that both forms of iodine seem to be equally effective. It is the purpose of this paper to present experimental data on the relative lethal effects of I2 and Itoward the vegetative cells of Micrococcus pyogenes var. aureus and Escherichia coli, and to consider the possible relative effects of these forms of iodine on infectious agents in general. A quantitative measure of the activity of I2 and I,toward vegetative cells was sought by determining the time required to achieve a given level of mortality for various iodine solutions. Our experimental results were frequently erratic. Even the relationship between the time rate of kill and the concentration of diatomic iodine where the concentration of the latter was varied by the simple addition of water could not be reproduced except very qualitatively. The reasons for this behavior apparently stem from the very low concentration of iodine required to kill vegetative cells. In this connection our experience was similar to that adequately described in the recent work of Chang and Morris (1953). They stated that "attempts to evaluate the fundamental bactericidal behavior of iodine in pure water were not successful. A high degree of variability was encountered, probably because the concentrations of iodine required to give measurable rates of kill were so small that they could be completely neutralized by mere traces of reactive impurities. In some tests, concentrations of iodine as low as 0.02 ppm exhibited rapid bactericidal action while in other tests at the same concentration there was no measurable killing". In the present instance, matters were complicated further by the fact that several tenths of a per cent of potassium iodide are required to convert most of the diatomic iodine to triiodide. At these concentrations a slight bactericidal activity was observed for the iodide salt alone. Whether this is due to the intrinsic effect of the iodide ion or possibly the minute quantities of iodine produced by the action of dissolved oxygen in water is not clear. Experimental conditions were sought so as to render the effect of potasium iodide negligible. In order to minimize the side effects due to trace quantities of impurities an attempt was made to increase greatly the dosage of iodine, yet maintaining an experimentally measurable rate of mortality of bacteria. Two possible courses were open. One was to work at low temperatures; the other was to carry out the bactericidal measurements in the presence of high concentration of bacteria or in the presence of organic matter, e.g., human blood serum. The latter course was chosen because of the correlation of such procedure to the therapeutic use of iodine.