A great candidate to study microbial interactions is the emerging technique Imaging Mass Spectrometry (Imaging MS) that allows the in situ and real time monitoring of chemical compounds directly on biological samples such as petri dishes and tissues of animals, plants and fungi, enabling the monitoring of chemical compounds while the interacting organisms are in contact.

Although Imaging MS exploitation is still in its early days, this technique can contribute to solve some challenges to study co-cultures. First, keeping the organisms in full contact on solid surfaces during the analysis ensures that compounds involved in cross-talk are accumulated in the interplay region, instead of getting dissolved in the extracted sample, thus it can be a very sensitive and specific technique. Second, the secondary metabolites do not need to be extracted, and sample preparation is very straight forward and easy.

The Biomolecular Chemistry group has already successfully established Imaging MS methods at Hans Knöll Institute to investigate the role of secondary metabolites in inter-kingdom interactions. For instance, this approach contributed to the discovery of jagaricin, a highly antifungal compound produced by the bacteria Janthinobacterium agaricidamnosum, which is also effective against major human pathogens such as Candida albicans, Aspergillus fumigatus, and A. terreus. Imaging MS allowed the detection of jaragicin for the first time in button mushrooms tissues infected by the bacteria, where it causes the superficial lesions involved in soft rot disease.[1] Imaging MS also unraveled the ecological role of secondary metabolites in interactions involving plants. For example, Imaging MS provided evidences that banana plants produce the secondary metabolites phenylphenalenones to protect their roots against pathogen nematodes (Radopholus similes),[2] and that the secondary metabolites nostopeptolides, from the nitrogen-fixing cyanobacteria Nostoc punctiforme, have a governing role during the differentiation of motile filaments known as “hormogonia”, that are the basis of their symbiosis with plants.[3]

Figure 1. Imaging MS applications that investigated secondary metabolites in their ecological native environment. A) Jagaricin (m/z 1181, in green) in mushroom infected by the bacteria Janthinobacterium agaricidamnosum (modified from [1]); B) Anigorufone (m/z 271, higher intensity in green) in the lesions of banana plants (Musa spp.) cultivars resistant to the nematode Radopholus similes (modified from [2]); selected metabolite (m/z 899, higher intensity in light blue) only detected in the symbiosis between the cyanobacteria Nostoc punctiforme and the plant Gunnera manicata (1: N. punctiforme alone. 2: N. punctiforme in Gunnera gland nine month post symbiosis; modified from [3]).

Moreover, Imaging MS can work as a massive hypothesis generator by rapidly spotting potentially new secondary metabolites produced in their native ecological environment, helping to connect the structure of the compounds to their original biological function.

Imaging MS is very popular in the field of medical diagnosis, but only recently it has been used to look for natural products. Efforts to develop Imaging MS techniques in the last years have yielded important improvements, such as new ionization techniques like DESI (Desorption Electrospray Ionization) that operates at atmospheric pressure, enabling, for instance, the analysis of living microbial colonies.[4] 

Thus, Imaging MS is a great contribution to the research of natural products that can complement very well other powerful “omic” approaches, such as metabolomics and genome mining.


[1] Graupner, K., Scherlach, K., Bretschneider, T., Lackner, G., Roth, M., Gross, H., & Hertweck, C. (2012). Imaging mass spectrometry and genome mining reveal highly antifungal virulence factor of mushroom soft rot pathogen. Angewandte Chemie International Edition, 51(52), 13173-13177. 
[2] Hölscher, D., Dhakshinamoorthy, S., Alexandrov, T., Becker, M., Bretschneider, T., Buerkert, A., ... & Heklau, H. (2014). Phenalenone-type phytoalexins mediate resistance of banana plants (Musa spp.) to the burrowing nematode Radopholus similis. Proceedings of the National Academy of Sciences, 111(1), 105-110.
[3] Liaimer, A., Helfrich, E. J., Hinrichs, K., Guljamow, A., Ishida, K., Hertweck, C., & Dittmann, E. (2015). Nostopeptolide plays a governing role during cellular differentiation of the symbiotic cyanobacterium Nostoc punctiforme. Proceedings of the National Academy of Sciences, 112(6), 1862-1867.
[4] Watrous, J. D., & Dorrestein, P. C. (2011). Imaging mass spectrometry in microbiology. Nature Reviews Microbiology, 9(9), 683-694.