Biodetection and biosurveillance capabilities are widely considered to be a critical element of the U.S. national and local emergency-response capabilities. The anthrax incidents of 2001 that contaminated offices on Capitol Hill and elsewhere were an important illustration of the human and economic toll that can be exacted by the dissemination of biological agents. The threat is formidable. A key question is: “What are the capabilities that U.S. responders need to identify dangerous biological agents so that effective steps can be taken to avoid or mitigate the negative effects?” The answer is not simple.
Standard microbiological and biochemical techniques that are routinely used in medicine to diagnose infections take several days and are not appropriate for field detection. Moreover, the various “standoff” spectral techniques developed to date simply do not have the resolution needed to discriminate biological agents from other elements in the environment, especially naturally occurring microorganisms. The level of discrimination needed to distinguish a dangerous biological agent from clutter requires a direct detection element that actually “touches” the agent. This means that the sample must be presented to the detector in liquid form – making it a “wet” assay. There is a rapid and inexpensive test of this type being used in some communities, but it detects only the presence of proteins, biological structural elements, and provides no discrimination between “good” and “bad.”
In recent years, immunoassays have been used for the field detection of a limited number of biological threats. These tests use antibodies – which the human body uses to identify and eliminate substances that are foreign to human systems – that recognize a physical feature on the surface of the “bug.” The benefit of using this method of identifying a potential threat is that it is not only rapid and relatively inexpensive, but also requires little or no sample preparation to identify the target in a complex background. One important drawback, though, is that the surface features may not discriminate between a bug that is truly harmful and a near neighbor. In fact, the positive control for a popular anthrax test is the vaccine strain.
Recent, Reliable, and Relatively Rapid
More recently, molecular tests, such as PCRs (polymerase chain reactions), have been adopted by the U.S. Postal Service and other agencies and organizations for the screening of potential biological threats. These tests are based on recognition of one, or a few, genetic elements that are unique to a specific threat agent. The human DNA, or genetic code, determines every physical characteristic of a living organism and thus can provide excellent discriminating power.
These tests also are relatively rapid – taking about 30 minutes or so to obtain reliable results. On the downside, current devices look at only one, or a few, genetic elements – which usually are not enough to definitively distinguish a true threat from closely related organisms. In addition, enzymes – i.e., catalytic proteins – are critical, but often finicky, elements of these systems that require fastidious front-end sample preparation. Add this complication to the need for detection elements that can “see” fluorescent tags, and the result is usually a somewhat complex and expensive device as well as reagents that are not ideal for field use.
Improved molecular tests are on the horizon, however. One major improvement is the ability to multiplex – i.e., to identify numerous distinguishing features of a threat agent. Several formats are being used to do this, including the use of bead-based assays and/or microarrays. The multiplexing capabilities of bead-based assays are still somewhat limited, though, and the instrumentation is still too complex and cumbersome for field use.
Microarrays are somewhat like chessboards, with particular genetic elements located in each square. The physical separation of these individual elements permits the simultaneous high-fidelity discrimination of literally thousands of genetic elements. This means that tests can be developed for high-confidence identification and numerous biological threats simultaneously. The same tests can also look both for virulence indicators (how the bugs might be harmful to humans) and for antibiotic sensitivity (how humans can hurt the bug), thus improving the response capabilities available.
Multiplexed assays already are being routinely used in the laboratory, and the science is now rapidly moving toward fieldable systems. The challenge facing first responders and laboratory researchers, therefore, is to find an optimum technology mix that is not only both rapid and simple, but also inexpensive enough to serve the first-responder and first-receiver communities at all levels of government.