"Highly sensitive and reproducible detection of bacteria and other biological agents will save lives, provide better health care, effectively fight terrorists and aid food safety," said Weihong Tan of the University of Florida.

Tan and colleagues employed the bioassay to detect E. coli O157:H7 bacteria in spiked samples of ground beef. The scientists say this strain is one of the most dangerous agents of food-borne diseases and can be fatal, particularly in children or the elderly. What's more, it's infectious, even when only tiny quantities are present.

To perform the test, the team doped silica nanoparticles with molecules of fluorescent RuBpy dye. Each 60-nm diameter nanoparticle contained thousands of dye molecules.

Then the scientists attached antibodies for antigens that occur on the surface of the bacteria to the nanoparticles. This enabled the particles to search out and stick to the E. coli O157:H7. Typically, thousands of nanoparticles bound to each bacterium. The Florida team could then detect the bacteria by looking for the fluorescence.

Traditionally, techniques for detecting trace amounts of bacteria rely on amplifying or enriching the target bacteria in the sample. But this is time-consuming and labour-intensive.

This fluorescent-bioconjugated silica nanoparticle technique solves the problem by amplifying the fluorescent signal from a single bacterium. That's because each nanoparticle contains thousands of dye molecules and thousands of the nanoparticles are likely to attach to each bacterium. Conventional techniques, in contrast, use only one or a few dye molecules attached to a single antibody.

By adding different antibodies to the nanoparticles, the team was able to detect other bacteria and spores. That means the technique could check for the presence of multiple contaminants simultaneously.

Now the scientists say they plan to research a more conventional protocol, handheld set-ups, multiple pathogen detection and a real diagnosis application.

The researchers reported their findings in PNAS.