Bacteria prefer to exist as surface biofilms in which the bacteria form organized colonies surrounded by a protective extracellular matrix that makes them very resistant to disinfection. The tenacity of biofilms can affect medical outcomes by infecting the surfaces of implants and medical devices that must be subsequently removed. They also have a profound effect on public health by transmitting waterborne and respiratory diseases through water distribution systems. For example, a recent study has shown that pathogenic strains of mycobacteria, commonly transmitted by contaminated water, are highly enriched in household showerheads and may be linked to a rise in pulmonary infections in the developed world. This study is examining the effect of small amounts of antibacterial nanoparticles on the attachment of mycobacteria to ceramic surfaces prior to the development of mature biofilms and consequent resistance to disinfection. The fundamental information obtained can be used to develop strategies for the design and manufacture of biofilm resistant, ceramic water purification membranes to be used in water treatment plants in the developed world and ceramic point-of-use filters designed for households in the less developed world.
TECHNICAL DETAILS: A unique combination of ceramics processing science and microbiology is being used to study the effect of antibacterial silver nanoparticles on the attachment of mycobacteria to ceramic surfaces. A controlled deposition method is used to vary the concentration and spatial distribution of the silver nanoparticles on the ceramic surfaces. Then the surfaces are exposed to flowing water containing different strains of mycobacteria and the effect of the silver nanoparticles on the attachment of the bacteria is observed and quantified using confocal optical microscopy, electron microscopy and image analysis. The information is then used to develop biofilm resistant ceramic surfaces. Additionally, the project participants are mentoring undergraduate students participating in service learning experiences that provide product development and engineering services to non-governmental organizations that make low-cost ceramic water filters in poor communities in the less developed world.
This project studied the effect of silver nanoparticles on three species of environmental mycobacteria in the free swimming planktonic state and the surface-attached state called biofilm. Environmental mycobacteria have previously been found in drinking water distribution systems and some species, including one used in this study, are known to be opportunistic human pathogens. In terms of intellectual merit, this study showed that silver nanoparticles were effective at killing free swimming bacteria and bacteria in the early stages of biofilm formation. However, there was an unexpected large variation in the effectiveness of silver nanoparticles against the three species of mycobacteria studied. The fast growing mycobacterium smegmatis, commnoly used in laboratory experiments, was very sensitive to silver nanoparticles. Mycobacterium avium, an opprtunistic pathogen, was less sensitive to silver nanoparticles as free swimming bacteria and also in the biofilm state. Finally, mycobacterium marinum was much less sensitive to silver nanparticles in the same concentration range. It is hypothesized that the observed differences in sensitivity may be due to the degree to which these mycobacteria species are exposed to silver ions in their unique natural microenvironment. This exposure may confer differing degrees of resistance to the silver nanoparticles used in this study. Continuing this theme, some wild type mycobacterium smegmatis were exposed to non-lethal doses of silver nanoparticles to see how easily they became resistant to silver nanoparticles. At silver nanoparticle concentrations for which only a few colonies survived , the bacteria were recovered and cultured in normal media resulting in new strains of silver nanoparticle resistant mutants. When the silver nanoparticle resistant mutant strains were reexposed to silver nanoparticles they were found to be much less sensitive. In fact the doses of silver nanoparticles that completely killed the initial wild type strain of mycobacterium smegmatis had little effect on the growth of the silver resistant mutant strains. This broader impacts of this result is the implicit suggestion that while silver nanoparticles will be a promising and durable antibacterial surface protection strategy, care should be exercised to limit the exposure of wild type mycobacteria to silver nanoparticles so that they do not develop antibacterial resistance. A second aspect involving the broader impacts of this work involved a collaboration with the Braddock Carnegie Arts Program in the Ceramics Studio at the Braddock Carnegie Library. The Ceramic Studio has a unique facility for the manufacture of low-cost ceramic water filters that are being used to remove waterborne bacterial pathogens from drinking water in low- income communities around the world that suffer from extreme hunger. This collaboration allowed undergraduate students working on this program to perform service learning research projects concerning the low-cost ceramic water filters. The projects were aimed at improving the sustainability of the filter manufacturing process taking place in low-income communities by reducing the need for wood. The first project involved removing saw dust from the clay formulation used to make the pots and replacing it with ground agricultural waste. It was found that manufacturing with ground corn husk could achieve the same filtering rate as the saw dust and future work is needed to determine the efficacy of this new material against bacteria. The second project focused on making water filters using a ceramic cement called geopolymer. This would allow the manufacturing to take place without the need to fire the filters at high temperature in wood burning kilns. New geopolymer formulations were developed and tested and found to give filtration rates similar to the current low-cost ceramic filters.
