Control of biofilm formation in water using molecularly capped silver nanoparticles

Abstract

Control of biofouling and its negative effects on process performance of water systems is a serious operational challenge in all of the water sectors. Molecularly capped silver nanoparticles (Ag-MCNPs) were used as a pretreatment strategy for controlling biofilm development in aqueous suspensions using the model organism Pseudomonas aeruginosa. Biofilm control was tested in a two-step procedure: planktonic P. aeruginosa was exposed to the Ag-MCNPs and then the adherent biofilm formed by the surviving cells was monitored by applying a model biofilm-formation assay. Under specific conditions, Ag-MCNPs retarded biofilm formation, even when high percentage of planktonic P. aeruginosa cells survived the treatment. For example, Ag-MCNPs (10 μg mL⁻¹) retarded biofilm formation (>60%), when 50 percent of the planktonic P. aeruginosa cells survived the treatment. Moreover, stable low value of relative biomass has been formed in the presence of fixed Ag-MCNPs concentrations at various biofilm incubation times. Our results showed that Ag-MCNPs pretreated cells were able to produce EPS although they succeeded to form relatively low adherent biofilm. These pretreated cells appear well preserved and undamaged under TEM HPH/freeze micrographs, yet the intra cellular material seems to be pushed towards the peripheral parts of the cell, possibly indicating a survival strategy to the presence of Ag-MCNPs. The lower value of relative biomass formed in the presence of Ag-MCNPs could be associated with molecular mechanisms related to biofilm formation or continuous release of silver ions in the sample. However, further research is required to examine these factors.

Introduction

Biofilm are surface-adherent communities of microorganisms embedded in their own microbial-originated matrix of protective and adhesive extracellular polymeric substances (EPSs), mainly polysaccharides, lipids and proteins (Bishop, 2007, Denkhaus et al., 2007). The EPS may also contain components from the surrounding environment as DNA (Steinberger and Holden, 2005). The biofilm habitat represents a primary mode of growth and living for most microorganisms on earth (Flemming, 2002); wherever there is water, a surface support and available nutrients, a biofilm will form (Bishop, 2007). In the water industry, biofilm formation is beneficial when it is the fundamental basis of treatment technologies such as biofiltration and biodegradation (Denkhaus et al., 2007, Flemming, 2002). Biofouling, however, is the undesired deposition of microorganisms and their EPSs on various surfaces (Flemming, 2002). Avoiding biofouling and its negative effects on process performance of water systems is a serious operational challenge in all of the water sectors, including pipes, water-distribution systems, filtration processes, cooling facilities and power plants.

Current approaches to preventing or removing biofouling use disinfectants as a pretreatment or apply frequent chemical cleaning (CIP) after biofilm has formed. However, microorganisms in biofilm develop a tolerance to the cleaning procedures, and a mature biofilm is persistent and difficult to completely eradicate (Kierek-Pearson and Karatan, 2005, Flemming, 2002). Moreover, periodic cleaning is costly and can damage process surfaces. Oxidative disinfectants may enhance the formation of biodegradable organic substances that can be utilized by microorganisms, thereby promoting biofilm formation. Furthermore, the use of oxidative disinfectants results in elevated levels of harmful disinfection by-products and can damage surfaces such as polyamide desalination membranes (Momba et al., 2000). Due to these limitations, alternatives are being sought to control biofouling in water systems such as distribution pipelines or membrane-filtration processes. In this study, a pretreatment technique for controlling potential biofouling based on the application of molecularly capped silver nanoparticles (Ag-MCNPs) suspended in water is presented.

The antimicrobial properties of silver compounds and silver in its ionic form (Ag⁺) have been applied in a wide range of applications including water systems (Kim et al., 2004, Silvestry-Rodriguez et al., 2008). Exposure to silver is considered safe for humans; yet, a standard of 0.1 mg/L of soluble silver is allowed in drinking water (WHO, 2004). Moreover, most strains of bacteria have not yet developed resistance to Ag⁺ (Silver, 2003). Turner and colleagues (Harrison et al., 2004) found Ag⁺ to be the most toxic of 17 different metals tested for the eradication of biofilm while screening various microorganisms, among them, Pseudomonas aeruginosa. Recent studies have demonstrated superior performance of silver nanoparticles (Ag-NPs) over Ag⁺ in controlling the growth and activity of various microorganisms (Lok et al., 2006), and Ag-NPs have been shown to inactivate planktonic Escherichia coli in aqueous suspensions (Dror-Ehre et al., 2009). Many researchers in the field refer to Ag-NPs as a new class of materials that can be used in a broad range of applications as an antibiotic (Choi et al., 2008, Fernandez et al., 2008, Morones et al., 2005, Nel et al., 2006), or even in consumer products such as food containers, films and face creams, or as important textile finishing biocides (Chopra, 2007, Kramer et al., 2006). Moreover, effective prevention of P. aeruginosa biofilm formation has been achieved in catheters coated with a matrix of Ag-NPs (Roe et al., 2008) or Ag-releasing rubber (de Prijck et al., 2007). With regard to membrane-filtration processes for water treatment, hybrid nanofiltration and ultrafiltration membranes with immobilized Ag-NPs have shown good anti-biofouling ability (Lee et al., 2007, Zodrow et al., 2009). These applications of Ag-NPs involve the creation of antibacterial surfaces by immobilized or surface-supported NPs. However, an aqueous suspension of NPs can also be used as a pretreatment in water systems prior to the main treatment units, such as membrane filtration.

The pretreatment with Ag-MCNPs presented in this study was aimed at controlling or retarding biofilm formation on surfaces in water; it was not meant as a treatment for the eradication of existing or mature biofilm or as a disinfection process. The effect of pretreatment with Ag-MCNPs on the development of biofilm in an aqueous suspension was determined using the model organism P. aeruginosa and a model biofilm-formation assay.