Rapid identification of chemically-related compounds produced by bacteria by Kendrick mass defect filtering applied to high resolution imaging mass spectrometry

Introduction
Over the last years, lots of progress have been done in the development of mass spectrometry imaging, making the technique more and more accessible for various applications, such as biomarkers discovery or bioactive compounds identification. However, the progresses made in terms of spatial and instrumental resolution has for consequences the dramatic increase of dataset size, shifting the burden from data production to data analysis and compounds identification. We propose here to use a semi-targeted method based on Kendrick mass defect (KMD) analysis to immediately identify the chemistry-related compounds in mass spectrometry imaging applied to microbiology samples. Thanks to a software developed in-house, we are now able to better understand the bacteria-bacteria interactions.

Materials and methods
Bacteria strains were inoculated on a semi-solid agar-based medium and incubated at 30°C. Region of interest was cut directly from the petri dish and transferred to the target ITO plate, previously covered with double sided conductive carbon tape. This assembly was then put in a vacuum desiccator until complete drying (overnight), and covered with HCCA matrix. Mass spectrometry images were obtained using a FT-ICR mass spectrometer (9.4T SolariX, Bruker Daltonics, Bremen, Germany). Data analysis was performed on an in-house software.

Results & Discussion
KMD filtering for mass spectrometry imaging enabled the rapid identification of chemically-related compounds such as lipopeptides or lipids, independently of the signal intensity and without the need of an extensive database search. For each detected family of compounds, an image was generated, enabling to link the chemically-related compounds identified with their spatial localization. The analysis of the bacteria-bacteria interaction was greatly simplified by our in-house software, and we were able to have a better understanding of the underlying chemical mechanisms involved.