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.
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.
Applications of ELS in the
food industry
According to the U.S. Centers for
Disease Control and Prevention (CDC),
it can take up to two weeks to identify a
pathogen after a person initially reports
having become sick after eating contam-
inated food. Conventional physiological
and serological methods require two
to seven days to identify the pathogen
because of the multiple steps involved
in sample preparation and the need (in
most cases) to establish a pure culture to
work from. During this time, pathogens
can spread among the public. Immuno-
logically compromised individuals are at
particular risk because they face greater
health consequences from an infection.
Here we show optical signatures of 24 colonies of different bacterial strains collected using ELS.
Profiles are highly reproducible and require only one to two seconds per colony to obtain. There
are a variety of profiles, depending on the size, shape, height and composition of the colony. Agar
density, color and water content can also contribute to variations.
