THE "REAL STORY" BEHIND THE PURDUE~MIT1000 COLLABO

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ELS’s strengths and limitations
ELS should have a significant place in the arsenal of bacterial identification techniques: It can identify organisms without additional biochemical tests or genetic tools, thereby reducing costs. Moreover, ELS is faster than all other current identification techniques—in some cases working in mere seconds.

For example, E. coli cultures typically take about one day to grow. Conventional biochemical analyses and PCR can take four to seven days to confirm identification. ELS, on the other hand, can be used immediately after the bacterial culture is grown. The scattering patterns are automatically compared to a library of known patterns, allowing for near-instant identification. If, by chance, the scattering pattern does not have a match in the library, we can at least conclude that something of interest is growing on the E. coli-specific nutrient medium and investigate further.

Although some critics of ELS have suggested that it is limited because it requires a colony to work—and that the organism of interest be culturable—the same is true of virtually all microbial detection and identification techniques available today.

ELS technology has been tested on tens of thousands of bacterial colonies and on hundreds of strains and subspecies. The measurements are highly reproducible and very robust. The images on the facing page are sequential ELS patterns from a single rectangular plate that were collected using our fully automated collection system. Furthermore, identifying patterns for many bacteria have been stored in our database, allowing us to rapidly classify many different species. As an example, the image to the right shows 24 actual scatter patterns. The fingerprint of an organism is a deconvolution of many features extracted from these unique and magnificent scatter patterns.

Cost is another potential advantage of ELS. A mass-produced ELS system could consist of inexpensive, off-the-shelf hardware, such as red lasers and low-resolution digital cameras available at consumer electronics stores. The system also indirectly saves money by requiring no reagents and a small amount of bench space in the typical laboratory.

No technology is perfect. ELS is limited in that it currently operates only with bacterial colonies, and—in order to be measured—these colonies need to be more or less transparent and approximately the size of the laser-beam diameter. We are experimenting with smaller beam profiles, which would allow us to evaluate smaller colonies earlier in their development. The first profiles we studied were 24-hour colonies. More recently, we have been able to assess many organisms at 12 hours using a fully robotic system, which can make measurements at any time. We anticipate that this could be reduced even further with more advanced optics and imaging tools. Naturally, only organisms that are culturable can be measured with ELS. Culture time can contribute to the ELS pattern and must be considered in developing the required classification database.

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