Activity of selected oxidizing microbicides against the spores of Clostridium difficile: relevance to environmental control.

BACKGROUND

Clostridium difficile is an increasingly common nosocomial pathogen, and its spores are resistant to common environmental surface disinfectants. Many high-level disinfectants (eg, aldehydes) are unsuitable for environmental decontamination because they need several hours of contact to be sporicidal. This study tested the potential of selected oxidative microbicides to inactivate C. difficile spores on hard surfaces in relatively short contact times at room temperature.

METHODS

The spores of a clinical isolate of C. difficile were tested using disks (1 cm diameter) of brushed stainless steel in a quantitative carrier test. The spores of C. sporogenes and Bacillus subtilis, common surrogates for evaluating sporicides, were included for comparison. The clostridia were grown separately in Columbia broth (CB), and B. subtilis was grown in a 1:10 dilution of CB. Each disk received 10 microL test spores with an added soil load, and the inoculum was dried. One disk each was placed in a glass vial and overlaid with 50 microL test formulation; controls received an equivalent volume of normal saline with 0.1% Tween 80. At the end of the contact time the microbicide was neutralized, the inoculum recovered from the disks by vortexing, the eluates were membrane filtered, and the filters placed on plates of recovery medium. The colony-forming units (CFU) on the plates were recorded after 5 days of incubation. The performance criterion was > or = 6 log(10) (> or = 99.9999%) reduction in the viability titer of the spores. The microbicides tested were domestic bleach with free-chlorine (FC) levels of 1000, 3000, and 5000 mg/L; an accelerated hydrogen peroxide (AHP)-based product with 70,000 mg/L H2O2 (Virox STF); chlorine dioxide (600 mg/L FC); and acidified domestic bleach (5000 mg/L FC).

RESULTS

Acidified bleach and the highest concentration of regular bleach tested could inactivate all the spores in < or = 10 minutes; Virox STF could do the same in < or = 13 minutes. Regular bleach with 3000 mg/L FC required up to 20 minutes to reduce the viability of the all the spores tested to undetectable levels; chlorine dioxide and the lowest concentration of regular bleach tested needed approximately 30 minutes for the same level of activity.

CONCLUSIONS

Acidified bleach, Virox STF, and regular bleach (3000-5000 mg/L FC) could inactivate C. difficile spores on hard environmental surfaces in approximately 10 to 15 minutes under ambient conditions. All of these products are strong oxidizers and should be handled with care for protection of staff, but acidified and regular bleach with high levels of FC also release chlorine gas, which can be hazardous if inhaled by staff or patients.