Conventional disinfection and sterilization methods are often ineffective with biofilms, which are ubiquitous, hard-to-destroy microbial communities embedded in a matrix mostly composed of exopolysaccharides. The use of gas-discharge plasmas represents an alternative method, since plasmas contain a mixture of charged particles, chemically reactive species and UV radiation, whose decontamination potential for free-living, planktonic micro-organisms is well established. In this study, biofilms were produced using Chromobacterium violaceum, a Gram-negative bacterium present in soil and water and used in this study as a model organism. Biofilms were subjected to an atmospheric pressure plasma jet for different exposure times. Our results show that 99.6 % of culturable cells are inactivated after a 5 min treatment. The survivor curve shows double-slope kinetics with a rapid initial decline in c.f.u. ml "1 followed by a much slower declinewith D values that are longer than those for the inactivation of planktonic organisms, suggesting a more complex inactivation mechanism for biofilms. DNA and ATP determinations together with atomic force microscopy and fluorescence microscopy show that non-culturable cells are still alive after short plasma exposure times. These results indicate the potential of plasma for biofilm inactivation and suggest that cells go through a sequential set of physiological and morphological changes before inactivation. INTRODUCTIONMost studies dealing with the growth and physiology of bacteria have been carried out using planktonic cells in batch cultures. Although these studies have provided extensive information regarding the basic molecular mechanisms that control the growth of individual bacteria, most bacteria live primarily in communities referred to as biofilms (Stoodley et al., 2002), where cooperative effects become important. Biofilms are microbial communities embedded in a matrix mostly composed of exopolysaccharides together with some proteins and nucleic acids. Biofilm formation can be considered as a developmental cycle that begins when planktonic bacteria attach to a surface, divide and recruit additional planktonic cells that attach to the cells already on the surface. In some cases, this process results in the development of a mature biofilm, in which cells cluster in pillar-and mushroom-like structures with water channels between them, forming a primitive circulatory system (Kolter & Losick, 1998). In other cases, more compact, flat and homogeneous biofilm layers (Heydorn et al., 2000;Picioreanu et al., 2000), spherical clusters (Matsumoto et al., 2007), discrete microcolony structures (Massol-Deyá et al., 1995), ball-shaped microcolonies (Tolker-Nielsen et al., 2000) and honeycomb-like structures (Marsh et al., 2003;Russo et al., 2006) have been reported. Although it is out of the scope of this paper to extensively review the different biofilm structures reported, it is generally accepted that biofilms are not a continuous monolayer surface deposit but instead a thin base film,...
Stable glow-type discharge plasmas at elevated pressures can be generated and maintained easily when the plasma is spatially confined to cavities with critical dimensions below 1 mm ('microplasmas'). We studied the properties of several atmospheric-pressure microplasmas and their use in the remediation of volatile organic compounds (VOCs) and biological decontamination. The VOCs studied include individual prototypcal aliphatic and aromatic compounds as well as mixtures such as BTEX (benzene, toluene, ethylbenzene and xylene). The biological systems under study included individual bacteria as well as bacterial biofilms, which are highly structured communities of bacteria that are very resistant to antibiotics, germicides, and other conventional forms of destruction.
Bacterial spores are the most resistant form of life and have been a major threat to public health and food safety. Nonthermal atmospheric gas discharge plasma is a novel sterilization method that leaves no chemical residue. In our study, a helium radio-frequency cold plasma jet was used to examine its sporicidal effect on selected strains of Bacillus and Clostridium. The species tested included Bacillus subtilis, Bacillus stearothermophilus, Clostridium sporogenes, Clostridium perfringens, Clostridium difficile, and Clostridium botulinum type A and type E. The plasmas were effective in inactivating selected Bacillus and Clostridia spores with D values (decimal reduction time) ranging from 2 to 8 min. Among all spores tested, C. botulinum type A and C. sporogenes were significantly more resistant to plasma inactivation than other species. Observations by phase contrast microscopy showed that B. subtilis spores were severely damaged by plasmas and the majority of the treated spores were unable to initiate the germination process. There was no detectable fragmentation of the DNA when the spores were treated for up to 20 min. The release of dipicolinic acid was observed almost immediately after the plasma treatment, indicating the spore envelope damage could occur quickly resulting in dipicolinic acid release and the reduction of spore resistance.
BackgroundDrawing on social identity theory and positive psychology, this study investigated women’s responses to the social environment of physics classrooms. It also investigated STEM identity and gender disparities on academic achievement and flourishing in an undergraduate introductory physics course for STEM majors. One hundred sixty undergraduate students enrolled in an introductory physics course were administered a baseline survey with self-report measures on course belonging, physics identification, flourishing, and demographics at the beginning of the course and a post-survey at the end of the academic term. Students also completed force concept inventories, and physics course grades were obtained from the registrar.ResultsWomen reported less course belonging and less physics identification than men. Physics identification and grades evidenced a longitudinal bidirectional relationship for all students (regardless of gender) such that when controlling for baseline physics knowledge: (a) students with higher physics identification were more likely to earn higher grades; and (b) students with higher grades evidenced more physics identification at the end of the term. Men scored higher on the force concept inventory than women; although no gender disparities emerged for course grades. For women, higher physics (versus lower) identification was associated with more positive changes in flourishing over the course of the term. High-identifying men showed the opposite pattern: negative change in flourishing was more strongly associated with high identifiers than low identifiers.ConclusionsOverall, this study underlines gender disparities in physics both in terms of belonging and physics knowledge. It suggests that strong STEM identity may be associated with academic performance and flourishing in undergraduate physics courses at the end of the term, particularly for women. A number of avenues for future research are discussed.
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