For years, silver ion has been used in the water professions and medical fields as an antimicrobial agent to control the harmful microorganisms for years. In the past few years, emerging nanotechnologies have even created more silver-ion applications by using nanosilver as a bactericide in consumer goods. This increased attention on silver ion as an antimicrobial agent reveals an urgent demand for understanding how microorganisms respond to silver-ion toxicity, but the silver-resistance mechanisms of microorganisms are only partially known. Thus, a comprehensive understanding of the silver-resistance mechanisms of microorganisms is essential. This current research studied the putative silver-resistance/adaptation mechanisms of Gram-negative microorganisms in different physiological conditions. The central hypothesis of this research is that ¬the microorganisms could use their own heavy-metal-related genes to exclude silver ion either in the planktonic or the biofilm state, and the silver-resistance mechanisms would be decided by the physiologic states. To test the central hypothesis, this study selected two Gram-negative microorganisms, Escherichia coli and Pseudomonas aeruginosa, cultivated in either the batch or the biofilm-annular reactor. The microorganisms were challenged by silver ion and analyzed the genetic responses of these two microorganisms against silver-ion toxicity by using the whole-genome microarray and relative-quantitative real-time polymerase-chain reaction analyses.
The results of this research suggested that 1) The sessile-biofilm P. aeruginosa can adapt to the silver-ion toxicity and grow resiliently after being exposed to silver ion over a long period of time; 2) Copper-resistance genes are essential in silver-resistance/adaptation mechanisms of Gram-negative microorganisms; 3) Some other heavy-metal functional genes were involved in silver-resistance/adaptation mechanisms, but the involvements of these genes were determined by the physiological conditions and microbial species; 4) Microorganisms suffered severe oxidative stresses induced by silver ion because this study observed whole sets of the oxidant-defense-mechanism genes being up regulated (except the superoxide-dismutase genes in the sessile biofilm of both E. coli and P. aeruginosa) in response to silver-ion challenges; 5) E. coli can tailor expressions of resistance genes delicately to respond to silver-ion toxicity in different growth phases of the planktonic condition; and 6) The microorganisms would respond to the silver-ion toxicity differently on gene-expression levels, but they did share certain core resistance molecular functions.
The findings of this current study provided novel information on how Gram-negative microorganisms in different physiological conditions responded to the silver-ion toxicities. There were common genetic responses among them, but the different genetic responses varied by the different physiological conditions. The results strongly validated the central hypothesis of this research. More importantly, this research showed that the sessile-biofilm P. aeruginosa grew resiliently as it was exposed to silver ion over 51 days. This discovery put into doubt the current applications of using silver ion as a disinfectant in the water profession.