**1. Introduction**

A large part of the population of developing countries is vulnerable to water borne diseases caused by pathogenic microbes present in the aquatic environment. Amongst the various enteric pathogens, *Escherichia coli* and *Staphylococcus aureus* are causal agents of various types of infections [1] and may lead to deterioration in the quality of drinking water in rural areas. The major causes behind this can be attributed to unawareness about personal hygiene practices and poor sanitation facilities. For disinfection of these microbes, composite nanoparticles-assisted photocatalysis has good potential for field application [2]. Currently, the simplicity and cost-effectiveness of sunlight- assisted photocatalysis using metal/metal oxide nanoparticles is gaining much attention for water treatment applications [3]. However, although the nanosized catalysts were successfully developed, certain issues such as short-shelf life of the nanoparticle systems due to catalyst poisoning, decreased active surface area of the nanoparticle systems by surface doping, and the possibility of leaching of reactive metal ions into the purified water have restricted their commercial exploitation. To deal with these limitations, core-shell structure nanoparticles were proposed. It is expected that nanoparticles with core-shell morphology will not only protect the metal catalysts, but also show promising results with regards to increased photocatalytic disinfection efficiency and extended shelf life of the material [4]. Metal@ZnO core-shell structure nanoparticles have been used for photocatalytic degradation of the organic dyes Rhodamine B and methyl orange in an aqueous solution [5,6]. *E. coli* has been extensively used as a good model micro-organism for studying photocatalytic disinfection but studies with *S. aureus* are mostly done with TiO2 and its doped variants [7,8]. To the best of our knowledge, no such disinfection with core-shell nanoparticles has been carried out with the latter micro-organism. Ag/TiO2 nanocomposites were previously explored for successful photocatalytic disinfection of *E. coli* [9]. Most of these reports have followed precipitation technique for coating metal oxide shell on noble metal (e.g., gold) nanoparticles. Although these materials have shown interesting photocatalytic properties, the high cost of gold is expected to hinder their practical application. Hence, Aguirre et al. tried to replace the gold core by a cheaper alternative, i.e., silver, and used this material for degradation of dyes [10]. Das and co-workers for the first time applied Ag@ZnO nanoparticles synthesized by a chemical precipitation technique for sunlight-assisted photocatalytic disinfection of the pathogenic bacterium *Vibreo cholerae* in synthetic as well as real water systems [11]. This could be a potential alternative to conventional disinfection techniques such as chlorination which are known to generate toxic byproducts [12]. However the conventional precipitation technique employed for the synthesis of metal@ZnO core-shell nanoparticles could not provide the well dispersed and porous materials required for catalytic applications [13]. Thus, it was necessary to investigate alternative synthetic protocols to obtain the monodispersed metal@ZnO core-shell nanoparticles and to check the potential of such materials for photocataytic applications. Recently, sonochemical techniques have been used extensively to obtain well dispersed and highly crystalline nanomaterials [4,14]. However, to the best of our knowledge, such techniques have never been exploited for the synthesis of metal@ZnO core-shell nanoparticles and examining their potential to disinfect bacterial pathogens. In the present paper, we report sunlight-assisted photocatalytic disinfection of two water borne pathogenic bacteria, *Escherichia coli* and *Staphylococcus aureus*, in saline solution (0.9%) and some real water systems using Ag@ZnO core-shell nanoparticles synthesized using an ultrasound assisted method.
