Distribution of extracellular adhesins in environmental biofilms and flocs: Reimagining the microbial structure

Extracellular cellular adhesins facilitate microbial aggregation; however, most of the information about extracellular adhesins is based on pure culture studies. In this study, we characterized the hydrophobic characteristics and distribution of the extracellular adhesins in environmental biofilms and flocs. The hydrophobic characteristics of the extracellular adhesins were studied by sonicating the microbial aggregates to disperse the cells and by fractionating them using the microbial adhesion to the hydrocarbon method. Furthermore, we probed environmental biofilms and flocs using immunohistochemistry coupled with confocal laser scanning microscopy for reimaging the microbial aggregates based on extracellular adhesins. Small flocs have a relatively dispersed distribution of extracellular adhesins (flagella, fimbriae, pili, and amyloid adhesins). The stratified distribution of extracellular adhesins was observed in environmental biofilms. It was observed that the pili and amyloid adhesins were predominantly present in the core of biofilms, whereas flagella and fimbriae were present in the outer layer of the microbial aggregates. The dispersion of microbial aggregates is one of the limiting factors that challenge the sustainable application of wastewater treatment processes. Greater attention to the components of extracellular protein (such as the adhesins) is required to understand the aggregation of dispersible environmental microbial aggregates.