Comparison of the bactericidal effect of cold atmospheric pressure plasma (CAPP), antimicrobial photodynamic therapy (aPDT), and polihexanide (PHX) in a novel wet surface model to mimic oral cavity application.

PURPOSE: Cold atmospheric pressure plasma (CAPP) is increasingly used for medical applications. The first devices are available from commercial manufactures, promising to improve wound healing and disinfection. The underlying antimicrobial mechanisms of CAPP are discussed, while the first results on its bactericidal efficiency against common bacterial species have already been published, with promising results. Most of the plasma sources used in these studies were built by the investigators themselves, and are not commercially available or licensed for clinical use. To evaluate the postulated bactericidal effects in clinical practice, we studied a commercially available, ready-to-use CAPP-device, which is also designed to be used in the field of dental, oral, and maxillofacial treatment.

MATERIALS AND METHODS: Standardized bacterial cultures of two different pathogens (Acinetobacter baumannii and Staphylococcus aureus) were produced with defined colony-forming unit concentrations. Dilutions of these cultures were treated with a commercially available CAPP product according to the manufacturer's instructions in order to evaluate the antimicrobial activity of the technique. This in vitro study compared the CAPP treatment with established clinical therapies like polihexanide (PHX) and antimicrobial photodynamic therapy (aPDT).

RESULTS: The bactericidal effect was evaluated in terms of reduction in colony-forming units after treatment of the bacterial samples with a defined dose of plasma, aPDT, or PHX. For CAPP, the bactericidal effect was found to be stronger in the Gram-negative isolate (A. baumannii) than in the Gram-positive S. aureus. A strong depth dependency was observed, especially with the Gram-negative isolate. Good bactericidal effects, with a reduction in bacterial load of more than 2 × log10 , could only be observed in conditions of 0.3 mm of water-film thickness or less. Such a significant reduction in bactericidal effect depending on depth was not observed using aPDT or PHX in the studied depth range of 0.3-1.8 mm.

CONCLUSION: CAPP treatment performed by the device (Plasma ONE) and configuration we used in this study seems to be ill suited for sufficiently killing Acinetobacter baumannii or Staphylococcus aureus in a moist infection site, as would be expected in the oral cavity. Established local antimicrobial therapies using PHX or aPDT showed better disinfectant properties. The clinical effect of improved wound healing, described by the manufacturer and some scientists, could not be investigated using this model. Given the results, however, it seems unlikely to be a direct consequence of bactericidal effects of CAPP in a wet environment. Further development of CAPP devices, or a different configuration (e.g. with a higher output, resulting in reactive nitrogen species-dominated, gas-phase chemistry), may enhance antibacterial effects in future, while tissue compatibility of such techniques remains to be elucidated further.