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Detecting inactivated endospores in fluorescence microscopy using propidium monoazide

Published online by Cambridge University Press:  17 January 2012

Alexander Probst
Department for Microbiology and Archaea Centre, University of Regensburg, Universitaetsstrasse 31, 93053 Regensburg, Germany
Alexander Mahnert
Department for Microbiology and Archaea Centre, University of Regensburg, Universitaetsstrasse 31, 93053 Regensburg, Germany
Christina Weber
Compliance – Advice and Services in Microbiology GmbH, Robert-Perthel-Straße 49, 50739 Cologne, Germany Deutsches Wollforschungsinstitut DWI, Rheinisch-Westfälische Technische Hochschule Aachen (RWTH), Interactive Materials Research, Pauwelsstraße 8, 52056 Aachen, Germany
Klaus Haberer
Compliance – Advice and Services in Microbiology GmbH, Robert-Perthel-Straße 49, 50739 Cologne, Germany
Christine Moissl-Eichinger*
Department for Microbiology and Archaea Centre, University of Regensburg, Universitaetsstrasse 31, 93053 Regensburg, Germany


The differentiation between living and dead bacterial endospores is crucial in many research areas of microbiology. The identification of inactivated, non-pathogenic Bacillus anthracis spores is one reason why improvement of decontamination protocols is so desirable. Another field interested in spore viability is planetary protection, a sub-discipline of astrobiology that estimates the bioburden of spacecraft prior to launch in order to avoid interplanetary cross-contamination. We developed a dedicated, rapid and cost-effective method for identifying bacterial endospores that have been inactivated and consequently show a compromised spore wall. This novel protocol is culture-independent and is based on fluorescence microscopy and propidium monoazide (PMA) as a fluorescent marker, which is suggested to bind to DNA of spores with compromised spore coat, cortex and membranes based on our results. Inactivated preparations (treated with wet heat, irradiation, ultracentrifugation) showed a significant increase in spores that were PMA stained in their core; moreover, Bacillus atrophaeus, Bacillus safensis and Geobacillus stearothermophilus seemed to be best suited for this technique, as the spore cores of all these endospores could be positively stained after inactivation. Lastly, we describe an additional counter-staining protocol and provide an example of the application of the coupled staining methods for planetary protection purposes. The introduction of this novel protocol is expected to provide an initial insight into the various possible future applications of PMA as a non-viability marker for spores in, for example, B. anthracis-related studies, food microbiology and astrobiology.

Research Article
Copyright © Cambridge University Press 2012

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