In Vitro Activity of Chlorine Dioxide versus a Hypochlorite Disinfectant Against Wild Type and NAP1 C. difficile

Background: Hypochlorite (HC)-based compounds are felt to represent the superior hospital disinfectants for C. difficile (CD), but the bleach smell precludes their routine use while a patient is in the room. Chlorine dioxide (ClO₂) has a milder odor, suggesting that ClO₂-based compounds can be used in occupied hospital rooms if they prove active against CD spores.

Objective: To test the in vitro activity of a ClO₂-based disinfectant against wild-type (WT) and NAP1 clinical strains of CD, and to compare this activity to a commercial HC-based preparation.

Methods: We employed a variation of the EPA/AOAC protocol to assess disinfectant sporicidal activity. Specifically, porcelain cylinders were inoculated with CD stocks derived from clinical isolates of a WT and NAP1 CD strain, selected for their ability to  consistently grow to high titer. Cylinders were exposed to solutions of the test compounds (ClO₂ 200 ppm and HC 5500 ppm) for varying durations of time. After neutralization the cylinders were sonicated for 5 min in water and then serially diluted and cultivated under anaerobic conditions on taurocholate-supplemented Brucella agar for 48 hrs. Median log₁₀-transformed counts per cylinder (n = 20 cylinders/experiment) are presented.

Results: For the WT CD strain, median counts/cylinder decreased by > 1 log₁₀ within 1 min exposure to ClO₂ (unexposed 5.61 log₁₀ reduced to 4.16); the median count after 5 min exposure was 0, with 75% of cylinders demonstrating no detectable spores; after 10 min exposures, no viable CD were detected on any cylinders. Similar results were obtained for the NAP1 strain. The activity of ClO₂ against WT CD then was compared with that of HC. After a 2.5 min exposure, HC demonstrated slightly greater activity (unexposed 5.18 log₁₀/cylinder reduced to median 0) compared with ClO₂ (reduced to median 0.17 log₁₀), but no spores were detected on any cylinders after 10 min exposure to either disinfectant. Again, similar results were detected with NAP1. Cylinders inoculated with additional WT and NAP1 clinical isolates (4 each, all with unexposed counts in excess of 5.0 log₁₀/clyinder) then were exposed for 10 min to ClO₂; in all instances the median spore count per cylinder after exposure was 0, with 16-20 (out of 20) cylinders inoculated with the WT strains having no detectable spores, and 14-20 (out of 20) cylinders inoculated with NAP1 strains having no detectable spores.                     

Conclusions: ClO₂ at 200 ppm exhibited rapid in vitro sporicidal activity against both WT and NAP1 CD spores. Although ClO₂ had slightly less activity compared with 5500 ppm HC at short exposure times, its milder odor may allow effective daily disinfection of occupied hospital rooms