Candida albicans biofilms: comparative analysis of room-temperature and cryofixation for scanning electron microscopy

Biofilms are frequently related to invasive fungal infections and are reported to be more resistant to antifungal drugs than planktonic cells. The structural complexity of the biofilm as well as the presence of a polymeric extracellular matrix (ECM) is thought to be associated with this resistant behavior. Scanning electron microscopy (SEM) after room temperature glutaraldehyde-based fixation, have been used to study fungal biofilm structure and drug susceptibility but they usually fail to preserve the ECM and, therefore, are not an optimised methodology to understand the complexity of the fungal biofilm. Thus, in this work, we propose a comparative analysis of room-temperature and cryofixation/freeze substitution of Candida albicans biofilms for SEM observation. Our experiments showed that room-temperature fixative protocols using glutaraldehyde and osmium tetroxide prior to alcohol dehydration led to a complete extraction of the polymeric ECM of biofilms. ECM from fixative and alcohol solutions were recovered after all processing steps and these structures were characterised by biochemistry assays, transmission electron microscopy and mass spectrometry. Cryofixation techniques followed by freeze-substitution lead to a great preservation of both ECM structure and C. albicans biofilm cells, allowing the visualisation of a more reliable biofilm structure. These findings reinforce that cryofixation should be the indicated method for SEM sample preparation to study fungal biofilms as it allows the visualisation of the EMC and the exploration of the biofilm structure to its fullest, as its structural/functional role in interaction with host cells, other pathogens and for drug resistance assays.