Diarrheal diseases caused by a range of enteropathogenic bacteria represent a major health threat in developing countries. According to the data from the World Health Organization, there are almost two million deaths per year (1.7-2.5 million deaths) caused by diarrhea. Emerging and known enteropathogenic bacteria are, among others, Escherichia coli (E. coli) O157:H7 and Salmonella typhimurium (S. typhimurium). E. coli O157:H7 has been recognized as a major intestinal flora for a long time since it was detected as the pathogen that causes foodborne disease outbreaks in the USA [1, 2]. In the past decades, E. coli O157:H7 has received tremendous attention, because it becomes an important pathogenic cause for several severe illnesses in human beings such as gastrointestinal disease and bloody diarrhea which is a root cause of hemolytic uremic syndrome . In addition, the incidence of S. typhimurium outbreaks is on the rise. In 2010, the outbreak of S. typhimurium resulted in 85 infections according to the data from the Centers for Disease Control and Prevention (CDC, USA). Most recently, a multi-state outbreak in the USA was associated with contaminated ground beef.
With the health risks of enteropathogenic bacteria, various methods have been developed for their analysis. Traditional culture-based methods for assay of E. coli O157:H7 and S. typhimurium, however, are time-consuming and are unable to meet the needs of real-time bacteria detection. Other detection technologies such as immunoassays [4, 5] and polymerase chain reaction-based assay  require either long time spans or specialized instrumentation. Therefore, it is critical to develop a fast and simple method to detect enteropathogenic bacteria for the diagnosis and treatment.
Aptamer is single-stranded nucleic acid (DNA or RNA) ligand that usually possesses high affinity and results in a significant conformation change upon binding with a wide range of targets. Aptamers are generally selected from the pools containing randomly created sequences through an in vitro systematic evolution of ligands by exponential enrichment (SELEX) . Compared to antibody-based biosensors, aptamer-based biosensors (aptasensor) [8, 9] possess unprecedented advantages with high productivity, affinity, selectivity, and stability.
Recently, several aptasensor using gold nanoparticles (AuNPs) [10, 11] that act as signal transducer element of the biosensor  have been developed. The application of single-stranded DNA-modified AuNPs for the highly selective colorimetric detection has been conducted, in which it can result in an aggregation of AuNPs with red to pinkish/purple color change in the presence of target molecules in solution . Wei et al. reported a simple and sensitive aptamer-based colorimetric sensor of thrombin using unmodified AuNPs . The thrombin aptamer (TBA) was used to form aptamer-AuNPs. Introduction of thrombin leads to the conformation change of aptamer and increases the repulsion between TBA and AuNPs causing salt-induced aggregation.
Nevertheless, most of AuNPs-based aptasensors were developed to detect proteins and small molecules [15, 16]. To the best of our knowledge, no work exists in AuNPs-based aptasensors for bacteria detection. Herein, we reported the development of aptamer-based biosensors (aptasensors) based on label-free aptamers and AuNPs for the detection of E. coli O157:H7 and S. typhimurium, with the aim of establishing a preliminary method to evaluate the utility of aptamer-AuNPs assay for enteropathogenic bacteria. Two species of bacteria (E. coli O157:H7 and S. typhimurium) were examined in this work.