Effect of pH on metal- and nanoparticle-microbe interactions.

摘要

Understanding metal-microbe interactions is essential in the study of biogeochemical transformations, microbial pathogenicity, and bioremediation applications. For this dissertation research we employed Burkholderia vietnamiensis PR1301 (PR1) as a model microbe to study the effects of pH on metal-microbe interactions. Initially PR1 was used to evaluate ZnO-nanoparticle (NP) toxicity with ZnCl2 as a reference toxicant using different cytotoxicity assays. These results demonstrated that ZnO-NP and ZnCl2 exhibit similar toxicities and both are more toxic at pH 7 than at pH 6. During these investigations we observed that PR1 produces membrane vesicles (MVs), which are 50 to 250 nm structures derived from the outer-membrane commonly produced by Gram-negative bacteria. At pH 7, when Zn2+ is 16-fold more toxic to PR1 than at pH 5, MV production was also two-fold greater, while at both pHs MV production was inversely related to Zn concentration. Most research to date has focused on the role of MVs in bacterial pathogenicity while their potential role in metal-microbe interactions has been largely overlooked. Due to their size, prevalence, and multifarious nature, the involvement of MVs in metal-microbe interactions was further investigated. First we demonstrated that MVs at physiological concentrations do not increase or decrease Zn2+ toxicity to PR1. Next, we demonstrated that MVs produced at pH 5 and 7 have different surface chemistries and that MVs from pH 7 are able to sorb greater quantities of Zn2+. Lastly, the formation and function of MVs at each pH was evaluated using molecular techniques, including proteomics. In addition to pH affecting MV production, pH also affected the nucleic acid, Fe and Zn concentration, and protein composition of MVs. There were 203 shared proteins at each pH and their identity indicates that, in addition to known MV functions, they might play a role in extracellular nutrient storage. Additionally, protein profiles predict that MVs produced at pH 5 would contain greater functional potential, including the ability to degrade organic contaminants. Overall, this research highlights not only the importance of pH in metal-microbe interactions, but also the involvement of MVs in metal-microbe interactions.