Use of Raman Spectroscopy and Phase-Contrast Microscopy To Characterize Cold Atmospheric Plasma Inactivation of Individual Bacterial Spores.

Abstract

Raman spectroscopy and phase-contrast microscopy were used to examine calcium dipicolinate (CaDPA) levels and rates of nutrient and nonnutrient germination of multiple individual Bacillus subtilis spores treated with cold atmospheric plasma (CAP). Major results for this work include the following: (i) >5 logs of spores deposited on glass surfaces were inactivated by CAP treatment for 3 min, while deposited spores placed inside an impermeable plastic bag were inactivated only ∼2 logs in 30 min; (ii) >80% of the spores treated for 1 to 3 min with CAP were nonculturable and retained CaDPA in their core, while >95% of spores treated with CAP for 5 to 10 min lost all CaDPA; (iii) Raman measurements of individual CAP-treated spores without CaDPA showed differences from spores that germinated with l-valine in terms of nucleic acids, lipids, and proteins; and (iv) 1 to 2 min of CAP treatment killed 99% of spores, but these spores still germinated with nutrients or exogenous CaDPA, albeit more slowly and to a lesser extent than untreated spores, while spores CAP treated for >3 min that retained CaDPA did not germinate via nutrients or CaDPA. However, even after 1 to 3 min of CAP treatment, spores germinated normally with dodecylamine. These results suggest that exposure to the present CAP configuration severely damages a spore's inner membrane and key germination proteins, such that the treated spores either lose CaDPA or can neither initiate nor complete germination with nutrients or CaDPA. Analysis of the various CAP components indicated that UV photons contributed minimally to spore inactivation, while charged particles and reactive oxygen species contributed significantly. Much research has shown that cold atmospheric plasma (CAP) is a promising tool for the inactivation of spores in the medical and food industries. However, knowledge about the effects of plasma treatment on spore properties is limited, especially at the single-cell level. In this study, Raman spectroscopy and phase-contrast microscopy were used to analyze CaDPA levels and kinetics of nutrient- and non-nutrient-germinant-induced germination of multiple individual spores of Bacillus subtilis that were treated by a planar CAP device. The roles of different plasma species involved in spore inactivation were also investigated. The knowledge obtained in this study will aid in understanding the mechanism(s) of spore inactivation by CAP and potentially facilitate the development of more effective and efficient plasma sterilization techniques in various applications.