FT-IR micro-spectroscopy for imaging the extracellular matrix composition in biofilms
Microorganisms form granules by embedding themselves in an extracellular matrix through the secretion of extracellular polymeric substances (EPS). The extracellular matrix is a complex structure comprising of e.g. proteins, polysaccharides, lipids, and extracellular DNA. Understanding the function of individual EPS components within the matrix not only requires knowledge on the composition of the extracellular matrix, but also on the spatial distribution of said components. Molecular imaging like e.g. fluorescence microscopy have been used for the visualization of the extracellular matrix, but these target specific molecules. Untargeted approaches like FT-IR micro-spectroscopy would allow for a broader exploration. In this study FT-IR micro-spectroscopy analysis was implemented on sliced anaerobic granular sludge to explore the EPS distribution. Visualization of single wavenumber absorbance showed a higher polysaccharide content in the EPS at the granule perimeter, shifting to a higher protein concentration toward the centre. The boundary of this shift was approximately 150 {micro}m from the surface, which was in accordance with the layer of fermentative bacteria described in literature. The complexity in the polymer composition meant that many functional groups were overlapping, making FT-IR annotation challenging. To address this, principal component analysis and two-dimensional correlation spectroscopy analysis were included in the analysis. These methods enabled the identification of overlapping functional groups and correlations between functional groups. Positive correlations between protein and polysaccharide functional groups suggested the presence of glycoproteins, which has been regularly described in chemical EPS analysis studies. Additionally, correlations between sulfated compounds and protein/polysaccharide functional groups indicated potential co-localization in the extracellular matrix. Differences in positive correlations of sialic acids with polysaccharides suggest variations in polysaccharide compositions, possibly caused by differences in the microbial community.