Device sounds out salmonella-infected eggs.

Picture thousands of large, white Grade AA chicken eggs rolling gently down a conveyor belt. As they roll by, a device zaps each one with sound waves and scans it for the telltale sign of Salmonella.

Bingo. An infected egg is yanked from the assembly line.

This fictional scenari may one day become reality if a team of New Mexico researchers perfects an acoustic device capable of detecting eggs contaminated with Salmonella enteritidis, a bacterium responsible for many outbreaks of food poisoning in the United States. Raw or undercooked eggs sometimes prove the culprit in cases of Salmonella-linked human illness (SN: 8/18/90, p.109).

Physicist Roger G. Johnston and his colleagues at the Los Alamos (N.M.) National Laboratory began their study with the knowledge that all objects will vibrate at certain natural resonance frequencies. Los Alamos researchers have used this knowledge in the past to design a means of distinguishing between artillery shells filled with explosives and those filled with noxious chemicals.

For this experiment, Johnston's team bought scores of white Grade AA chicken eggs from te grocery store. The researchers gently placed each egg between two small transducers. One transducer converts an electrical signal into sound waves that pass through the egg. The second transducer picks up the resulting vibration and converts it back into an electrical signal.

When the scientists tested 36 eggs in pristine condition, they discovered a single resonance at about 830 hertz, which they believe to be the natural resonance frequency f uninfected eggs. They cultured the eggs right after their acoustic experiments and found no sign of Salmonella or any other bacterium.

Next, the scientists wanted to find out whether infected eggs would show a different acoustic resonance. First, they dipped a syringe into a liquid teeming with S. enteritidis. Then they gently inserted the needle through the eggshell and into the egg white.

The researchers incubated the resulting 17 Salmonella-infected eggs for a day and then tested them with the acoustic device. These eggs also showed the characteristic vibration at about 830 hertz. However, four of the infected eggs (24 percent0 showed a second resonance--this one at a higher frequency.

The team believes that as Salmonella organisms make their way through the egg white and into the yolk, the breach in the yolk membrane changes the egg's acoustic properties and thus produces the second vibration.

Johnston suggests chicken farmers may one day use the marker resonance to screen eggs for Salmonella infection. A paper describing the team's preliminary data will appear in the May Biotechnology Progress.

The researchers hope to nab closer to 100 percent of the infected eggs by fine-tuning their device. Yet even a 24 percent success rate is better than the system currently in use. About the only way to spot a bad egg now is to crack it open and culture the yolk, a process that takes about a day--and spoils the egg.