| Abstract
| - Atomic force microscopy (AFM) was used to probe theeffects of pH, ionic strength, and the presence of bacterialsurface polymers on interaction forces between individual,negatively charged bacteria and silicon nitride. Bacterialsurface polymers dominated interactions between bacteriaand AFM silicon nitride tips. The measured forces wererepresented well by an electrosteric repulsion modelaccounting for repulsion between the tip and bacterialpolymers but were much larger in magnitude and extendedover longer distances (100's of nanometers) than predictedby DLVO theory. The equilibrium length (Lo) of the polymerswas allowed to vary with solution chemistry to account forintramolecular electrostatic interactions between individualpolymer units. The effects of the variables pH and ionicstrength on bacterial interaction forces were investigatedindependently. Pseudomonas putida KT2442 was studiedin 1 mM MOPS buffer at pH values of 2.2, 4.75, 7.00, and 8.67.Burkholderia cepacia G4 was studied in 1 mM MOPSbuffer at pH values of 2.2, 4.75, and 6.87. Then, the pH washeld constant at 4.5 or 4.75, and the ionic strength wasstudied in 0.01, 1, or 100 mM MOPS buffer (for each microbe).For KT2442 (in 1 mM MOPS buffer), Lo increased from230 to 750 nm as pH increased from 4.75 to 8.67. For G4(in 1 mM MOPS buffer), Lo increased from 350 nm at pH 2.2to 1040 nm at pH 7.0. Varying the ionic strength between0.01 and 100 mM did not affect the equilibrium length of thepolymers nearly as much as pH. Partially removingpolysaccharides from the bacterial surfaces resulted inlower repulsive forces that decayed much more rapidly. Themagnitude of the measured forces in these experimentsand the equilibrium lengths predicted by the electrostericmodel are comparable to other force measurementsand size estimates on polymers and polysaccharides.
|
