Foodborne pathogens have been a cause of a large number of diseases worldwide and more so in developing countries. This has a major economic impact. It is important to contain them, and to do so, early detection is very crucial. Detection and diagnostics relied on culture-based methods to begin with and have developed in the recent past parallel to the developments towards immunological methods such as enzyme-linked immunosorbent assays (ELISA) and molecular biology-based methods such as polymerase chain reaction (PCR). The aim has always been to find a rapid, sensitive, specific and cost-effective method. Ranging from culturing of microbes to the futuristic biosensor technology, the methods have had this common goal. This review summarizes the recent trends and brings together methods that have been developed over the years.
A novel assay was developed for the detection of Bacillus thuringiensis (BT) spores. The assay is based on the fluorescence observed after binding an aptamer-quantum dot conjugate to BT spores. The in vitro selection and amplification technique called SELEX (Systematic Evolution of Ligands by EXponential enrichment) was used in order to identify the DNA aptamer sequence specific for BT. The 60 base aptamer was then coupled to fluorescent zinc sulfide-capped, cadmium selenide quantum dots (QD). The assay is semi-quantitative, specific and can detect BT at concentrations of about 1,000 colony forming units/ml.
The effect of a 20-min exposure to antibody-quantum dot (Ab-QD) conjugates on colony counts of Escherichia coli was assessed by the spread-plate method and compared with exposure to unconjugated QDs having only amine or carboxyl groups on their surfaces. Under these conditions, Ab-QD conjugates generally exhibited >90% reduction in colony-forming units as compared to untreated E. coli and E. coli treated with unconjugated QDs after incubation for as long as 41 h. The antibacterial effect of Ab-QD conjugates vs. unconjugated QDs on Salmonella enterica subsp. enterica serovar Typhimurium was also assessed by means of a disk-diffusion technique which demonstrated greater growth inhibition (approximately 3 mm greater) by Ab-QD conjugate-impregnated disks than by unconjugated-QD-only-impregnated disks at a 10-microg disk load. At a 25-microg disk load, both treatment groups exhibited nearly equal growth inhibition.
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