The impact of silver nanoparticles of marine biofilms

Nanoparticles are particles with at least one dimension between 1-100nm, the use of silver nanoparticles in industry has increased over recent years due to the bactericidal capacity, low cost and ease of handling. Silver nanoparticles (AgNPs) are used in products such as cosmetics, plastics and clothing. Due to the increase in their use the potential for their release and accumulation into the environment has also increased. The environmental risk of AgNPs is not yet understood, some studies suggest that there is limited environmental risk, however due to the antibacterial properties of AgNPs there is potential for them to have detrimental effects on natural bacterial communities. Bacteria are primarily found in the marine environment as biofilms, and understanding the effect of AgNPs on bacteria is extremely important because biofilms are primary producers in many ecosystems and also they are responsible for environmental processes such as biogeochemical cycling. Furthermore there may be adverse effects as a result of AgNP exposure on wastewater microorganisms; this may result in the decrease in the effectiveness of contaminant removal in biological treatment processes.

The aim of the study was to assess the impact of AgNPs on waste water biofilms microbial community structure. The author hypothesised that the impact of AgNPs would be dependent on the strain of bacteria and also the impact would differ between planktonic cells and those in a biofilm. To assess their hypotheses, both original wastewater biofilms and isolated planktonic pure culture bacteria from the biofilms were exposed to AgNPs. Possible protective mechanisms in the biofilm were investigated, such as physical exclusion due to the effects of EPS. The role of community interactions was also studied using an artificially mixed community to verify any effect on the community interaction. To analyse community shift after AgNP exposure PCR-DGGE was used. Three terms were proposed to suggest the response of the microbes after AgNP exposure. Tolerance was where the sample survived under the AgNP treatment, susceptibility and sensitivity were used to describe their ability to react to AgNP exposure.

The study found that bacterial biofilms are highly tolerant to AgNP exposure, after 24 hours of 200 mg/l exposure the heterotrophic plate counts were insignificant. However if the Extracellular polymeric substances were removed the bacteria became highly susceptible under the same conditions. Furthermore if bacteria from the biofilm were treated as planktonic cells they were highly sensitive to AgNP exposure with most cells dying after 1 hour of 1mg/l exposure. These results suggest that the EPS and the microbial community interactions play an important role in effect the way AgNP exposure effect the bacterial cells. The study also found that slower growing strains WWBF-3 (CFB group bacterium) and WWBF-5 (Microbacterium oxydans) were more tolerant to AgNPs than the faster growing strains, previous studies have also documented the fact slower growing bacteria are more resistant to antibiotics. Although the reason has not been highlighted in the paper, it can be speculated the reason for this is that if the antibiotics are a β-lactam antibiotics, which only lyse growing cells, a slower growing community would have less chance for cells to be lysed and hence the lower effect of antibiotics on these strains.

In the paper it is outlined that during the study the AgNPs were found to be sorbed to biofilm matrix, indicating a removal of NPs from wastewater into the biofilm. This finding I feel is extremely important, as biofilms are a primary food source in many ecosystems incorporations of NPs in the biofilm may result in transfer of NPs through the food chain.

A review of : Sheng, Z., Lui, Y. 2011. Effects of silver nanoparticles on wastewater biofilms. Water Research. 45 (18), 6039-6050.