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Project Progress

We used whole bacterial cells to address the question of whether EPS provides armor against cell lysis. A newly developed EPS-lacking mutant strain of Pseudomonas aeruginosa (Figure 1), and the wild-type, which is a robust EPS producer (Figure 1), were used to conduct cell viability studies in suspensions of oxide particles with cells. Wild type cell viability was greater than that of the mutant type. Furthermore, viability was oxide-dependent, increasing as γ-Al₂O₃ > amorphous SiO₂ (Figure 2). Thus, our results confirm our hypothesis that EPS acts as a shield against the toxicity of certain minerals. We also examined the cell + oxide particle suspensions using High Resolution Transmission Electron Microscopy (HRTEM) to visualize the location of the particles with respect to the cell surface. The amorphous silica particles were located in the extra-cellular region of both wild type cells that possess EPS as well as washed mutant type cells that lack EPS, suggesting that the similar negative charge on both SiO₂ particles and the cell surfaces resulted in electrostatic repulsion and prevented cell-particle contact. In contrast, γ-Al₂O₃ particles of size < 80 nm were seen inside the washed mutant type cells, but were seen only in the extracellular space for the wild type cells. These results indicated that the positively-charged γ-Al₂O₃ particles were electrostatically attracted to the oppositely-charged cell surface. The particles could not penetrate in the case of the wild type due to the presence of the EPS, whereas the smaller particles < 80 nm could enter the mutant type cells that lack EPS. We are also currently investigating the effect of reactive oxygen species at TiO₂ surfaces on cell viability. Our results support our hypotheses that EPS acts an armor against mineral toxicity, and that not all minerals are toxic or benign, but that toxicity depends on mineral surface properties such as surface charge, surface free radicals, etc.

We are currently planning to conduct silicification experiments for both wild and mutant type cells in order to examine differences in morphology for preservation differences.