Journal of Basic Microbiology 2011, 51, 183 – 190 183 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jbm-journal.com Research Paper Bacterial tolerance to silver nanoparticles (SNPs): Aeromonas punctata isolated from sewage environment S. Sudheer Khan, E. Bharath Kumar, Amitava Mukherjee and N. Chandrasekaran Nanobio-Medicine Laboratory, School of Bio-Sciences and Technology, VIT University, Vellore, India Use of silver nanoparticles (SNPs) is increasing in a large number of consumer products. Thus, the possible build-up of the nanoparticles in the environment is becoming a major concern. Aeromonas punctata isolated from sewage showed tolerance to 200 μg/ml SNPs. The growth kinetics data for A. punctata treated with nanoparticles were similar to those in the absence of nanoparticles. There was a reduction in the amount of exopolysaccharides (EPS) in bacterial culture supernatant after nanoparticle-supernatant interaction. EPS capping of the nanopartic- les was confirmed by UV-visible, XRD and comparative FTIR analysis. The EPS-capped SNPs showed less toxicity to Escherichia coli, Staphylococcus aureus and Micrococcus luteus compared to the uncapped ones. The study suggests capping of nanoparticles by bacterially produced EPS as a probable physiological defense mechanism. Keywords: SNPs / Aeromonas punctata / EPS / Encapsulation / Bacterial tolerance / Toxicity reduction Received: February 19, 2010; accepted: July 08, 2010 DOI 10.1002/jobm.201000067 Introduction * Silver nanoparticles (SNPs) are potential candidates of strong antimicrobial activity and are used in significant amounts in consumer products, in the food industry for storage, packaging, and processing [1], in textiles [2], in medical applications for wound care products and therapeutic devices [3], and in diagnostics and drug delivery [4, 5]. But increasing concentrations of SNPs with varied physical and surface properties could pose a threat to human and environmental health [6]. Impellitteri et al. 2009 [7] revealed that the SNPs impregnated in clothes and washing systems can easily leak into wastewater during washing, thus potentially disrupting helpful bacteria used in wastewater treatment facilities or en- dangering aquatic organisms in lakes and streams. In vitro and in vivo toxicity studies in mammalian species proved that SNPs have the capability to enter cells and cause cellular damage [8]. They have the po- tential to cause chromosomal aberrations and DNA Correspondence: Prof. Dr. N. Chandrasekaran, Nano Bio-Medicine Laboratory, School of Bio-Sciences and Technology, VIT University, Vellore-632014, India E-mail: nchandrasekaran@vit.ac.in; nchandra40@hotmail.com Phone: +91 416 2202624 damage and are capable of inducing proliferation arrest in cell lines [9]. Indiscriminate use of nano-sized silver materials in commercial products leads to their release into the environment and ultimately harms microbial commu- nities in the ecosystem [10]. The release of SNPs enter- ing sewage treatment plants was estimated to be 270 t/ year [11]. Due to this, sensitive bacteria get disturbed, besides harming some beneficial forms as well. Ulti- mately, the released SNPs destabilize the functioning of ecosystems [11]. On the other hand, some resistant bacteria exist due to their adaptive nature [12]. The hypothesis of the paper was to study the principle un- derlying bacterial tolerance to SNPs and its possible mechanism. Materials and methods Materials All chemicals and media were obtained from Himedia Laboratories Ltd., Mumbai, India. The bacterium was isolated from sewage (Vellore, India). Manufactured SNPs were obtained from Sigma Aldrich, USA. The nanoparticles were dispersed using an ultrasonic pro- cessor at a frequency of 132 kHz (Crest, USA).