Fluorescent identification and detection of Staphylococcus aureus with carboxymethyl chitosan/CdS quantum dots bioconjugates.

A fast and sensitive method based on fluorescent carboxymethyl chitosan/CdS quantum dots (CMCS-CdS QDs) composites was developed for specific detection of Staphylococcus aureus in food and the environment. Fluorescent CMCS-CdS QDs were prepared in aqueous solution through a green method. A human immunoglobulin (IgG) antibody was then bioconjugated to the QDs in the presence of 1-ethyl-3-(3)-dimethylaminopropyl carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) to make a novel type of mono-dispersed water-soluble fluorescent bioprobes. The fluorescent bioprobes were employed to identify S. aureus by incubating them with the bacteria for a certain time and observing the marked cells under fluorescence microscopy after removing free fluorescent QDs. Fluorescence microscopy images showed the S. aureus cells were successfully recognized by the bioprobes. Several other bacteria commonly found in environment such as Escherichia coli and Bacillus subtilis were also incubated with the bioprobes to test their specificity. It was found that the novel QDs-CMCS-IgG bioprobes had specific identification to S. aureus cells. Fluorescence measurement using a luminescence spectrometer could be applied to quantify S. aureus cells. The fluorescence intensity of the samples at 600 nm was proportional to the cell concentration in the range of 10(3)-10(7) cfu/ml, and the detection limit was as low as 900 cfu/ml. Considering the simplicity and cost-efficiency of this method, its application in the identification and quantification of bacteria in clinical, food and environmental samples is anticipated.