The in-vitro activity of povidone–iodinecream against Staphylococcus aureus and its bioavailability in nasal secretions

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Due to the emergence of mupirocin-resistance in some epidemic strains of methicillin resistantStaphylococcus aureus (EMRSA) and the appearance of EMRSA with intermediate resistance to vancomycin, we evaluated the in-vitro activity of 5% povidone–iodine (‘Betadine’) cream as a possiblealternative to mupirocin for the elimination of nasal carriage of S. aureus. As judged by enrichment culture, povidone–iodine was bactericidal against three mupirocin-sensitive strains of S. aureus from nasal carriers, and against mupirocin-resistant and -sensitive strains of EMRSA types 3, 15 and 16, after incubation with povidone–iodine for 1.0 min at 32°C. Mupirocin nasal ointment did not prevent growth after 180 min incubation. In a quantitative suspension test, 1:100 dilution of povidone–iodine cream completely eliminated an inoculum of 108cfu/mL of all nine test organisms after incubation at 32°C for 1.0 min, and 1:1000 dilution reduced cfu, by a factor of 105. After direct inoculation of the povidone–iodine cream to give 105cfu/g, none of the test strains were recoverable after 30 s, giving a killing rate of approximately 104cfu/s; for mupirocin nasal ointment, the maximum reduction of mupirocin-sensitive strains was ten fold after 3 h. Povidone–iodine activity was not detectable in sensitivity-testing agar, although 0.025% of povidone–iodine was detectable in a 15% nutrient strength tryptone soya agar. Using this minimal medium, the addition of nasal secretions (from any of 11 samples) reduced the activity of povidone–iodine by 80–90%, but mupirocin activity was unaffected. One millilitre of nasal secretions inactivated the equivalent of approximately 22.5 mg of povidone–iodine. These results suggest that povidone–iodine cream may have a role in the prevention of colonization and infection caused by MRSA, including mupirocin-resistant strains.