On my way to Mars: Plasma sterilization to prevent microbial cross-contamination

By Dean Williams on June 30, 2016

The elimination of contaminating microorganisms is absolutely essential in numerous fields, ranging from hospital hygiene, the food processing industry, all the way to the space industry. Marina Raguse and Ralf Moeller introduce their new article published in JPhysD carried out with colleagues in Germany, Portugal and the United States.

Credit: NASA/JPL-Caltech/MSSS

Microbial contamination arising from spacecraft exploration harbors the distinct potential to impact the development and integrity of life-detection missions on planetary bodies such as Mars and Europa. Such missions are subjected to strict regulations. In the context of the planetary protection guidelines, established by the Committee of Space Research (COSPAR) in 1967, it is essential to reduce or eliminate the biological burden on flight hardware prior to launch in order to prevent cross contamination of celestial bodies with environmental or human-associated microorganisms.

Plasma is often referred to as the fourth state of matter and is essentially an ionized gas comprising various reactive components. Although numerous studies have been carried out proving that plasma sterilization is very effective in inactivating various contaminating bacteria, the diversity of active bactericidal species in a plasma discharge (mainly chemically reactive species, (V)UV photons, and ions) makes it very difficult to pin point a particular mechanism responsible for inactivation.

Current sterilization procedures have reached their limit as they usually require elevated temperatures and prolonged exposure times, which are likely to introduce damage to advanced spaceflight hardware materials. Cold plasma sterilization is becoming a more and more suitable alternative decontamination approach to the commonly used methods for cleaning spacecraft and preventing contamination of extraterrestrial pristine environments. It is fast, efficient and gentle to heat-sensitive material, such as innovative medical plastics, due to low-temperatures operations.

Figure 1 Atomic force microscopy imaging of native (a) and low pressure-plasma treated (b) B. subtilis spores.

The bacterial spore is an exceptionally robust biological system that harbors a multitude of structural features and mechanisms that enable it to survive particularly hostile conditions as well as a broad spectrum of sterilization methods. Its resilience makes it an ideal candidate for the function of a biological indicator in order to verify the effectiveness of a plasma sterilization procedure. Therefore, we use spores of the common test organism Bacillus subtilis to focus our research on identifying the relevant underlying mechanism that leads to the rapid bacterial inactivation by low pressure plasma discharges and the relevance of involved spore-specific resistance features.

In our latest article we were able to treat spores with quantified intensities of plasma particles using a well-characterized double inductively coupled plasma setup and identified the role of the proteinaceous coat layers that surrounds the spore as a crucial morphological structure that protects the spore from plasma-induced damage. Plasma discharges significantly erode the outer spore layers and introduces surface alterations that may enhance the penetration of harmful plasma species to deeper spore layers damaging essential structures such as the spore’s genome or the revival apparatus, ultimately leading to spore death (Figure 1).

Plasma sterilization offers unique features for the innovative sterilization and decontamination procedures in a variety of applications, yet, the complex interaction of plasma components with spore structures raise many questions that require comprehensive future investigations.

About the authors

Marina Raguse is a PhD student at the German Aerospace Center (DLR) in Cologne, Germany. She has a background in microbiology. During her PhD project she investigated the underlying mechanisms involved in bacterial spore inactivation by low pressure plasma sterilization focusing on spore-specific protection mechanisms.

Dr. Ralf Moeller is a microbiologist working at the German Aerospace Center (DLR) in Cologne, Germany. He is the leader of the Space Microbiology Research Group. His major research interests are the study of the microbial response to environmental extremes (e.g., life and survival on Mars) and the evaluation of innovative decontamination techniques such as low pressure plasma sterilization.

Peter Awakowicz is the head of the Institute for Electrical Engineering and Plasma Technology at the Ruhr-University Bochum, Germany. His research is focused on characterization of plasmas used for etching, coating, and sterilization and the interaction of plasma with multifunctional surfaces. Since 2010, he is speaker of the Collaborative Research Center SFT-TR87.

Katharina Stapelmann is head of the Biomedical Applications of Plasma Technology group at Ruhr-University Bochum, Germany. She has a background in electrical engineering with a focus on plasma technology. Her research focusses on the interactions of technical plasmas with biological systems.

Marcel Fiebrandt is a PhD student at the Institute for Electrical Engineering and Plasma Technology at the Ruhr-University Bochum, Germany. His research is focused on characterization of plasma sources for analysis of sterilization mechanisms in low pressure plasmas used for sterilization and decontamination.

Benjamin Denis recently defended his PhD Thesis. Prior to this, he was working at the Institute for Electrical Engineering and Plasma Technology at the Ruhr-University Bochum, Germany. His research interests are plasma sterilization and plasma diagnostics in inductively coupled plasmas.

Patrick Eichenberger is an Associate Professor of Biology in the Center for Genomics and Systems Biology at New York University. He studies global gene transcription networks and mechanisms of spore coat assembly in Bacillus subtilis and related bacteria.

Peter Eaton is an expert in biological applications of atomic force microscopy (AFM), based at UCIBIO/Requimte, University of Porto, Portugal. He is also a Special Visiting Researcher at CNPq, Brazil.

Adam Driks is a Professor of Microbiology at Loyola University Chicago, in the Stritch School of Medicine. He earned his PhD at Brandeis University and did post-doctoral research at Harvard University. His research focuses on the structure and function of the protective layers that surround bacterial spores. (image source: http://ssom.luc.edu)