Cold atmospheric air plasma sterilization against spores and other microorganisms of clinical interest

Abstract

Physical cold atmospheric surface microdischarge (SMD) plasma operating in ambient air has promising properties for the sterilization of sensitive medical devices where conventional methods are not applicable. Furthermore, SMD plasma could revolutionize the field of disinfection at health care facilities. The antimicrobial effects on Gram-negative and Gram-positive bacteria of clinical relevance, as well as the fungus Candida albicans, were tested. Thirty seconds of plasma treatment led to a 4 to 6 log(10) CFU reduction on agar plates. C. albicans was the hardest to inactivate. The sterilizing effect on standard bioindicators (bacterial endospores) was evaluated on dry test specimens that were wrapped in Tyvek coupons. The experimental D(23)(°)(C) values for Bacillus subtilis, Bacillus pumilus, Bacillus atrophaeus, and Geobacillus stearothermophilus were determined as 0.3 min, 0.5 min, 0.6 min, and 0.9 min, respectively. These decimal reduction times (D values) are distinctly lower than D values obtained with other reference methods. Importantly, the high inactivation rate was independent of the material of the test specimen. Possible inactivation mechanisms for relevant microorganisms are briefly discussed, emphasizing the important role of neutral reactive plasma species and pointing to recent diagnostic methods that will contribute to a better understanding of the strong biocidal effect of SMD air plasma.

Fig 1

SMD plasma device. Shown are a photograph (A) and a frontal view and cross-section drawing (B). 1, spore test sample; 2, plasma electrode system; 3, lid.

Fig 2

SMD air plasma at 1 kHz and 10 kV_pp. (A) Discharge pattern and prominent plasma species. (B) Optical emission spectrum of the discharge dominated by excited N₂.

Fig 3

Reduction of plasma-treated vegetative microorganisms at 1 kHz and 10 kV_pp. (A) Kinetic study of S. aureus. (B) Thirty-second treatments. The bars indicate standard errors (n = 3).

Fig 4

Reduction of plasma-treated bacterial endospores at 1 kHz and 10 kV_pp. △, G. stearothermophilus; ○, B. pumilus; ♢, B. atrophaeus; □, B. subtilis; *, detection limit. The error bars indicate standard errors (n ≥ 3).

Fig 5

Reduction of plasma-treated G. stearothermophilus spores at 1 kHz and 10 kV_pp. △, stainless steel; ○, PTFE; ♢, PVC; □, glass; *, detection limit. The error bars indicate standard errors (n ≥ 3).