The influence of ionic strength on the adhesive bond stiffness of oral streptococci possessing different surface appendages as probed using AFM and QCM-D

Bacterial adhesion to surfaces poses threats to human-health, not always associated with adhering organisms, but often with their detachment causing contamination elsewhere. Bacterial adhesion mechanisms may not be valid for their detachment, known to proceed according to a visco-elastic mechanism. Here we aimed to investigate influences of ionic strength on the adhesive bond stiffness of two spherically shaped Streptococcus salivarius strains with different lengths of fibrillar surface appendages. The response of a Quartz-Crystal-Microbalance-with-Dissipation (QCM-D) upon streptococcal adhesion and changes in the ionic strength of the surrounding fluid indicated that the bond stiffness of S. salivarius HB7, possessing a dense layer of 91 nm long fibrils, was unaffected by ionic strength. Atomic-force-microscopic (AFM) imaging in PeakForce-QNM mode showed a small decrease in bond stiffness from 1200 to 880 kPa upon decreasing ionic strength from 57 to 5.7 mM, while Total-Internal-Reflection-Microscopy suggested a complete collapse of fibrils. S. salivarius HBV51, possessing a less dense layer of shorter (63 nm) fibrils, demonstrated a strong decrease in bond stiffness both from QCM-D and AFM upon decreasing the ionic strength, and a partial collapse of fibrils. Probably, the more hydrophobic and less negatively charged long fibrils on S. salivarius HB7 collapse side-on to the cell surface, while the more hydrophilic and negatively charged fibrils of S. salivarius HBV51 remain partially stretched. In summary, we demonstrate how a combination of different methods can yield a description of the structural changes occurring in the interfacial region between adhering, fibrillated streptococci and a substratum surface upon changing the ionic strengt

Effects of soil matric suction on retention and percolation of Bacillus subtilis in intact soil cores

Bacillus subtilis endospores (resistant to rifampicin) irrigated on the surface of intact soil cores (20 cm diameter x 8 cm length) which were equilibrated under selected suctions, i.e. 0, 0.5, 2, 5, 10 kPa, were then percolated by saturated water flow. The bacterial retention and percolation percentage were significantly correlated with the suctions. The higher retention with higher suction was explained by micropore storage, attachment to static air-water interface (AWI), and irreversible adsorption to soil particles. The bacterial percolation was mainly controlled by initial replacement of pore water storage, and following reversible detachment process. Another sensitivity experiment with four replicates using lincomycin-resistant B. subtilis at 0 and 0.5 kPa suctions revealed that small increase (0 to 0.5 kPa) in soil matric suction incurred a substantial higher level of bacterial retention. Based on our experimental results, soil matric suction was proposed as a comprehensive parameter to monitor bacterial transport and fate for animal waste disposal (irrigation) and subsurface bioremediation