Role of ionic strength in the retention and initial attachment of Pseudomonas putida to quartz sand

Biofilms in soil can offer solutions to many geotechnical problems. Controlling biofilm formation in soil requires a fundamental understanding of how bacterial cells interact with soil under relevant environmental conditions. Here, atomic force microscopy (AFM) was used to quantify the adhesion forces acting between a model soil microbe, Pseudomonas putida, and a model of quartz sand under a range of ionic strengths (ISs; 0.02M-0.52M) that bracket common environmental conditions present in soil. AFM forces were decoupled into specific and nonspecific components following the established Poisson statistical analysis method. The experimental nonspecific forces were compared to theoretical forces calculated using the soft-particle analysis of the Derjaguin, Landau, Verwie, and Overbeek theory of colloidal interactions. Their data indicated that both specific and nonspecific forces increased as the IS increased with specific forces being 37% higher on average than nonspecific forces. The AFM findings were validated by measuring the transport of P. putida in sand columns kinetically as a function of IS. Collected breakthrough curves were modeled using one-dimensional colloidal filtration theory. Their nanoscale AFM measurements of bacterial adhesion correlated positively with the bacterial macroscale sticking collision efficiencies with both increasing as the IS increased. Collectively, their results suggest that applying biofilms for bioclogging applications is suitable in sandy, hydrophobic environments with elevated ISs where specific forces such as hydrogen bonds or hydrophobic interactions may have a critical role in promoting bacterial adhesion.