Fingerprinting Bacteria

Before they can effectively combat the bacteria that may infect a patient, threaten a swiftly spreading epidemic, or contaminate a municipal water sup ply, doctors and scientists need quick and positive identification of the invading organisms. But traditional laboratory tests that single out and classify bacterial troublemakers are complex, time-consuming and sometimes inconclusive. Often, before the results are in, the disease has spread or the patient has died. In the future, though, bacteria may lose their cloak of anonymity more quickly. Scientists have discovered that each species and strain has a distinctive "fingerprint" that can be used for virtu ally immediate identification.

In tests involving 32 different strains of bacteria, Cornell University Biologist Martin Alexander and General Electric Chemist John Gould have found that each excretes metabolic wastes that are chemically distinct. When the waste products of a single strain are passed through a laboratory chromatograph. a device that separates chemical compounds, they produce a distinctive graph with characteristic peaks and valleys. Thus the graphs or chromatograms of unidentified bacteria can be com pared with those of known bacteria and--like fingerprints--be used to establish their exact identity.

Segregated Compounds. To prepare a bacterial chromatogram, Scientists Alexander and Gould use a pure strain of bacteria, allow them to grow for several hours in a nutrient solution, then extract the metabolic products that have been excreted. These are injected into a chromatograph, where they are converted by heat into gaseous form and fed into a column containing a packing material and an organic liquid such as Carbowax--a chemical that has a different attraction for the molecules of each chemical compound. Thus every compound that passes through the column is slowed to a degree.

Because of their varying speeds, the compounds that constitute the metabolic products are segregated. As each compound emerges--in order of its speed through the column--it is sensed by an ionization detector and recorded on a graph as a distinct peak. Within minutes, all of the compounds have passed through the chromatograph, each forming its own peak on the graph. Since the metabolic products of each strain of bacteria contain different chem ical compounds, each chromatogram forms an easily identifiable profile.

Stored Chromatograms. When Alexander and Gould have substantially increased their collection of such bacteria identifications, they hope to store it in a computer, creating a central file similar to the FBI's store of human fingerprints in Washington. Then, when a chromatogram of unidentified bacteria is prepared, it can be fed into the computer, matched electronically with the appropriate chromatogram in the computer's memory, and quickly identified. By using the combination of bacterial fingerprints and computers, says Gould, "detection and identification, which now take days or weeks by classical methods, could be done in hours."

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