Adhesion of Pseudomonas fluorescens (ATCC 17552) to nonpolarized and polarized thin films of gold

Abstract

The adhesion of Pseudomonas fluorescens (ATCC 17552) to nonpolarized and negatively polarized thin films of gold was studied in situ by contrast microscopy using a thin-film electrochemical flow cell. The influence of the electrochemical potential was evaluated at two different ionic strengths (0.01 and 0.1 M NaCl; pH 7) under controlled flow. Adhesion to nonpolarized gold surfaces readily increased with the time of exposition at both ionic-strength values. At negative potentials (-0.2 and -0.5 V [Ag/AgCl-KCl saturated [sat.]]), on the other hand, bacterial adhesion was strongly inhibited. At 0.01 M NaCl, the inhibition was almost total at both negative potentials, whereas at 0.1 M NaCl the inhibition was proportional to the magnitude of the potential, being almost total at -0.5 V. The existence of reversible adhesion was investigated by carrying out experiments under stagnant conditions. Reversible adhesion was observed only at potential values very close to the potential of zero charge of the gold surface (0.0 V [Ag/AgCl-KCl sat.]) at a high ionic strength (0.1 M NaCl). Theoretical calculations of the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy for the bacteria-gold interaction were in good agreement with experimental results at low ionic strength (0.01 M). At high ionic strength (0.1 M), deviations from DLVO behavior related to the participation of specific interactions were observed, when surfaces were polarized to negative potentials.

FIG. 1

Schematic diagram of the thin film electrochemical flow cell designed for microscopic observations. Bacterial suspension is pumped into the chamber through stainless steel liquid inlets (L in and L out) at a controlled flow. The thin-film gold WE is placed facing down and microscopic observations are made from the back. A platinum wire CE circumvents the light path on the bottom of the chamber to ensure a uniform current distribution. The reference electrode (RE) is connected through a salt bridge to improve conductivity.

FIG. 2

Variations of the number of adhered bacteria to the gold surface at various surface potentials, with the time of exposition to a flowing bacterial suspension in 0.01 M NaCl, pH 7. ●, 0.2 V (E_oc); ⊗, −0.2 V; ○, −0.5 V. An Ag/AgCl-KCl sat. electrode was taken as a reference. The flow rate was 0.7 ml min⁻¹.

FIG. 3

Variations of the number of adhered bacteria to the gold surface at various surface potentials, with the time of exposition to a flowing bacterial suspension in 0.1 M NaCl, pH 7. ●, 0.2 V (E_oc); ⊗, −0.2 V; ○, −0.5 V. An Ag/AgCl-KCl sat. electrode was taken as a reference. The flow rate was 0.7 ml min⁻¹.

FIG. 4

DLVO interaction energy G_DLVO(h) curves for bacterial cells and gold as a function of separation distance (H) at various surface potentials in 0.01 M NaCl. —, −0.57 V; –––, −0.27 V; ······, 0.13 V. The Hamaker constants for the gold surface, the bacterial surface, and the surrounding water were 62.5 kT (1), 15.3 kT (17), and 9.34 kT (17), respectively. The zeta potential for the bacterial surface was −0.0315 V.

FIG. 5

DLVO interaction energy G_DLVO(h) curves for bacterial cells and gold as a function of separation distance (H), at various surface potentials in 0.1 M NaCl. —, −0.57 V; –––, −0.27 V; ······, 0.13 V. The Hamaker constants for the gold surface, the bacterial surface, and the surrounding water were 62.5 kT (1), 15.3 kT (17), and 9.34 kT (17), respectively. The zeta potential for the bacterial surface was −0.0155 V.

FIG. 6

Cumulative image showing reversible adhesion movements during polarization of gold to 0.00 V (Ag/AgCl-KCl sat.) in a bacterial suspension in 0.1 M NaCl. Sequential images were captured at a rate of 12 pictures s⁻¹ during a 5-s interval. Each image was subtracted from the previous one, and the minimal gray value from every differential image was selected to construct the cumulative image. The box indicates the points where bacteria sorbed at the surface and broke away, during its translation movement. (a to d) Footprints indicating that a bacterial cell sorbed, rotated, and broke away during the sampling time.

FIG. 7

DLVO interaction energy G_DLVO(h) curves for bacterial cells and gold as a function of separation distance (H), at a surface potential of −0.07 V (versus the PZC) in 0.1 M NaCl. The Hamaker constants for the gold surface, the bacterial surface, and the surrounding water were 62.5 kT (1), 15.3 kT (17), and 9.34 kT (17), respectively. The zeta potential for the bacterial surface was −0.0155 V.