Combined pH ratiometry and fluorescence lectin-binding analysis (pH-FLBA) for microscopy-based analyses of biofilm pH and matrix carbohydrates.

Bacterial biofilms have a complex and heterogeneous three-dimensional architecture that is characterized by chemically and structurally distinct microenvironments. Confocal microscopy-based pH ratiometry and fluorescence lectin-binding analysis (FLBA) are well-established methods to characterize pH developments and the carbohydrate matrix architecture of biofilms at the microscale. Here, we developed a combined analysis, pH-FLBA, to concomitantly map biofilm pH and the distribution of matrix carbohydrates in bacterial biofilms while preserving the biofilm microarchitecture. As a proof of principle, the relationship between pH and the presence of galactose- and fucose-containing matrix components was investigated in dental biofilms grown with and without sucrose. The pH response to a sucrose challenge was monitored in different areas at the biofilm base using the ratiometric pH-sensitive dye C-SNARF-4. Thereafter, the fucose- and galactose-specific fluorescently labeled lectins Aleuria aurantia lectin (AAL) and Morus nigra agglutinin G (MNA-G) were used to visualize carbohydrate matrix components in the same biofilm areas and their immediate surroundings. Sucrose during growth significantly decreased biofilm pH (P < 0.05) and increased the amounts of both MNA-G- and AAL-targeted matrix carbohydrates (P < 0.05). Moreover, it modulated the biofilm composition towards a less diverse community dominated by streptococci, as determined by 16S rRNA gene sequencing. Altogether, these results suggest that the production of galactose- and fucose-containing matrix carbohydrates is related to streptococcal metabolism and, thereby, pH profiles in dental biofilms. In conclusion, pH-FLBA using lectins with different carbohydrate specificities is a useful method to investigate the association between biofilm pH and the complex carbohydrate architecture of bacterial biofilms.IMPORTANCEBiofilm pH is a key regulating factor in several biological and biochemical processes in environmental, industrial, and medical biofilms. At the microscale, microbial biofilms are characterized by steep pH gradients and an extracellular matrix rich in carbohydrate components with diffusion-modifying properties that contribute to bacterial acid-base metabolism. Here, we propose a combined analysis of pH ratiometry and fluorescence lectin-binding analysis, pH-FLBA, to concomitantly investigate the matrix architecture and pH developments in microbial biofilms, using complex saliva-derived biofilms as an example. Spatiotemporal changes in biofilm pH are monitored non-invasively over time by pH ratiometry, while FLBA with lectins of different carbohydrate specificities allows mapping the distribution of multiple relevant matrix components in the same biofilm areas. As the biofilm structure is preserved, pH-FLBA can be used to investigate the in situ relationship between the biofilm matrix architecture and biofilm pH in complex multispecies biofilms.

Fluorescence lectin binding analysis of carbohydrate components in dental biofilms grown in situ in the presence or absence of sucrose.

Carbohydrate components, such as glycoconjugates and polysaccharides, are constituents of the dental biofilm matrix that play an important role in biofilm stability and virulence. Exopolysaccharides in Streptococcus mutans biofilms have been characterized extensively, but comparably little is known about the matrix carbohydrates in complex, in situ-grown dental biofilms. The present study employed fluorescence lectin binding analysis (FLBA) to investigate the abundance and spatial distribution of glycoconjugates/polysaccharides in biofilms (n = 306) from 10 participants, grown in situ with (SUC) and without (H2O) exposure to sucrose. Biofilms were stained with 10 fluorescently labeled lectins with different carbohydrate specificities (AAL, ABA, ASA, HPA, LEA, MNA-G, MPA, PSA, VGA and WGA) and analyzed by confocal microscopy and digital image analysis. Microbial composition was determined by 16S rRNA gene sequencing. With the exception of ABA, all lectins targeted considerable matrix biovolumes, ranging from 19.3% to 194.0% of the microbial biovolume in the biofilms, which illustrates a remarkable variety of carbohydrate compounds in in situ-grown dental biofilms. MNA-G, AAL, and ASA, specific for galactose, fucose, and mannose, respectively, stained the largest biovolumes. AAL and ASA biovolumes were increased in SUC biofilms, but the difference was not significant due to considerable biological variation. SUC biofilms were enriched in streptococci and showed reduced abundances of Neisseria and Haemophilus spp., but no significant correlations between lectin-stained biovolumes and bacterial abundance were observed. In conclusion, FLBA demonstrates the presence of a voluminous biofilm matrix comprising a variety of different carbohydrate components in complex, in situ-grown dental biofilms.