Investigation of Microbial Cooperation via Imaging Mass Spectrometry Analysis of Bacterial Colonies Grown on Agar and in Tissue During Infection

Understanding the metabolic consequences of microbial interactions that occur during infection presents a unique challenge to the field of biomedical imaging. Matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry represents a label-free, in situ imaging modality capable of generating spatial maps for a wide variety of metabolites. While thinly sectioned tissue samples are now routinely analyzed via this technology, imaging mass spectrometry analyses of non-traditional substrates, such as bacterial colonies commonly grown on agar in microbiology research, remain challenging due to the high water content and uneven topography of these samples. This paper demonstrates a sample preparation workflow to allow for imaging mass spectrometry analyses of these sample types. This process is exemplified using bacterial co-culture macrocolonies of two gastrointestinal pathogens: Clostridioides difficile and Enterococcus faecalis. Studying microbial interactions in this well-defined agar environment is also shown to complement tissue studies aimed at understanding microbial metabolic cooperation between these two pathogenic organisms in mouse models of infection. Imaging mass spectrometry analyses of the amino acid metabolites arginine and ornithine are presented as representative data. This method is broadly applicable to other analytes, microbial pathogens or diseases, and tissue types where a spatial measure of cellular or tissue biochemistry is desired.