In vitro studies of the killing of clinical isolates by povidone-iodine solutions.

Two-hundred-and-thirty clinical isolates were surveyed for susceptibility to povidone-iodine (PVP-I) ('Betadine'). Of 95 staphylococcal isolates tested, only 18 (19%) were completely killed by exposure to 10% PVP-I (the usual stock concentration) for 15 s. Seventy-seven staphylococcal isolates (81%) exhibited varying degrees of apparent 'resistance' under these conditions. However, of 39 other Gram-positive isolates and 96 Gram-negative isolates tested (including 20 Pseudomonas aeruginosa and four Ps. cepacia), only one strain of Escherichia coli exhibited significant lack of killing. However, this apparent 'resistance' was completely lost by prolonging the contact time to as little as 30 s in 60% of the isolates, and all isolates were completely killed by 120 s. A paradoxical increase in killing by lower concentrations of PVP-I was observed (maximal killing about 0.1% PVP-I). A new PVP-I formulation ('SP-Betadine') was completely cidal for all isolates tested after only a 15-s contact time.