A new nanoparticle test for dangerous bacteria such as Escherichia coli O157:H7 is so sensitive it can detect a single bacterial cell within minutes. The food industry, medicine and the fight against bioterrorism could all eventually benefit from it, researchers say.
Even a few cells of the E. coli strain in food can be dangerous so it is important to be able to detect them in low numbers. Current tests, however, need a higher number of bacteria to be present before they can detect it, which can lead to long delays.
The new test, developed by Weihong Tan and colleagues from the University of Florida, US, could offer a sharper and faster way of detecting contamination. 鈥淚f you can give us one bacterial cell in a sample then we can detect it,鈥� says Lisa Hilliard, one of the team. 鈥淢ost people have to grow it and then detect it.鈥�
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The whole test can be carried out in just 20 minutes, compared with up to 48 hours for conventional tests. 鈥淥ther tests could be as sensitive but you would need to perform an enrichment first,鈥� says Mike Peck from the Institute of Food Research in the UK. 鈥淵ou would have to pop the food into a growth media for 24 hours,鈥� he explains.
Shelved beef
Waiting a day or two for the results can be costly and inconvenient, says Andrew Brabban, who works on E. coli at the Evergreen State College, Washington. 鈥淥ne of the basic problems at the moment is what is called 聭Test and Hold鈥�. Samples are held at US plants until they are shown to be free of O157:H7. This is obviously expensive for the industry, having large quantities of beef as shelved stock,鈥� he told 快猫短视频.
The new test consists of silica nanoparticles, each housing thousands of fluorescent dye molecules, and each attached to an antibody for a given bacterium.
The nanoparticles are added to a solution of the test sample, such as ground beef. If the bacterium sought is present the nanoparticles will quickly attach to it. The sample is then separated by weight in a centrifuge. The target bacteria, being heavier than the nanoparticles, will separate away from them. But those dye molecules already attached will fluoresce in this heavier sample, identifying the bacteria.
Diluted samples
The new test differs from other dye tests in that thousands of dye molecules will fluoresce if only one bacterium is present because they are all attached to the same antibody. Other tests contain only a few dye molecules for each test antibody used so that one bacterial cell will not cause enough fluorescence to be observed. Using samples so diluted that only one in four of them contained any bacteria at all, the researchers showed their test could detect a single bacterial cell.
Although the test was developed using E. coli O157:H7, it could be adapted to many different bacteria. The group are already looking at ways to identify more than one type of bacteria at a time, using different coloured dyes for each.
If this can be achieved quickly and accurately it could have great potential. 鈥淭he need to detect single cells is real,鈥� says Brabban. 鈥淎ny method that is fast, accurate and has [such a sensitive] detection level would certainly be very useful,鈥� he says.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0404806101)