What Are We Looking For?

The spectrum of disease concerns shown in figure 1 ranges from disease states potentially caused by bioterrorism or biological warfare agents to emerging diseases that are naturally occurring—exemplified by the agents responsible for Severe Acute Respiratory Syndrome (SARS) and Avian Flu.

Figure 1 – The spectrum of disease concerns ranges from traditional bioterrorism threats to emerging and re-emerging infectious diseases such as SARS and the West Nile Virus.

Biological agents—usually viruses, bacteria or toxins—are the cause of the majority of the disease states that concern us. A brief survey of basic biology will be helpful in understanding the issues associated with this problem.

Bacteria are living organisms. They contain genetic material and reproduce both sexually (exchanging genetic material) and asexually, (simply splitting in two).1 Bacteria can be beneficial to humans, as are the billions of bacteria that inhabit our guts and aid in digestion. 2 Or, they can be quite lethal, regardless of the route of entry or location, as evidenced by the deaths caused by Bacillus anthracis during the anthrax attacks.

Bacteria are extremely small. The average bacterium is about one to five micrometers long and about one micrometer wide. 3 (A micrometer is about one one-millionth of a meter. The period at the end of this sentence is about 500 micrometers wide.)

Figure 2 – Bacillus anthracis, the bacteria responsible for the infectious disease anthrax. Source: Public Health Image Library

Antibiotics can kill bacteria in a variety of ways. They can attack the cell wall surrounding the bacteria, act on membranes inside the bacteria, disrupt the production of proteins inside the bacteria, or interfere with the manufacture of nucleic acids needed for genetic material inside the bacteria.4

Bacteria can cause illness directly, in which case antibiotics may be successful in killing the bacteria before much harm is done to the infected individual.5 Bacteria also can produce toxins that can cause illness or death.6 The toxins are essentially by-products from bacterial growth and are not susceptible to the antibiotics. Killing the bacteria before too much toxin is produced is often the only approach to preventing toxic effects, as antitoxins do not exist for many of the bacterial toxins. This is the case for anthrax, for example.

Toxins also can be derived from other living organisms and used directly in an attack. This is actually an example of chemical warfare. Ricin is a toxin derived from the castor plant. No antitoxin exists, and, because a toxin is not a living organism, antibiotics are of no use against it. Many toxins exist and can be aerosolized and used in a fine spray as a weapon. Toxins used as weapons would be of extremely small size—orders of magnitude smaller than a bacterium.

Figure 3 – A computer model of ricin, the toxic protein made from castor beans. Source: http://www.nbc-med.org.

Viruses “live” on the edge of being animate and inanimate. They are nothing more than genetic material surrounded by a protective coat of protein.7 They have no metabolic functions and are incapable of reproducing on their own. However, when they infect a cell, they take over the cell’s reproductive machinery and cause it to manufacture new viruses.8 Viruses can be rather benign, as is the cold virus, or quite deadly, as is the human immunodeficiency virus (HIV). Because viruses are not alive in the sense that bacteria are, antibiotics are of no use against them. Some antiviral drugs exist, but are usually rather specific to selected viruses. Vaccination is the usual way we protect humans against viruses, although vaccines do not exist for all viruses.

Viruses are exceedingly small—much smaller than bacteria. A single virus particle is about 65 nanometers 9—a nanometer being one one-billionth of a meter.

Figure 4 – Smallpox is caused by the variola virus shown above. The virus emerged in human populations thousands of years ago. Source: http://microbes.historique.net/smallpox.html.

Other possible agents exist for a biological attack, but in general, bacteria, toxins, and viruses are the most likely candidates. They also are the most likely agents of a naturally occurring biological incident. The amount of toxin or the number of bacteria or number of virus particles needed to make a person ill varies as to the type of toxin/organism being considered and the health of the individual in question. It can be quite small, however. Indeed, many of the biological agents of concern are much more lethal than even our most lethal chemical warfare agents. (See figure 5.)

Figure 5 – Comparative toxicity of effective doses of biological agents, toxins and chemical agents.

Also, it is important to note the distinction between something being infectious and something being communicable. A person can become infected with a given virus—say HIV, which we all know to be lethal—but will not be contagious and transmit that virus to other humans by just walking around in the course of daily activities. So HIV is infectious but not contagious, while smallpox is both infectious and contagious.

Thus, when discussing a system for sensing the presence of potentially lethal agents, a system has to be able to detect microorganisms and toxic molecules of extremely small size and possibly in extremely small concentrations.

1. Neil A. Campbell and Jane B. Reece, Biology. 6th edition. (San Francisco, CA: Benjamin Cummings, 2002), 340-341.
2. It is interesting to note, however, that the E. coli bacteria that inhabit our gut can be lethal if they make their way to other parts of our bodies.
3. Campbell and Reece, 329.
4. Ibid., 529.
5. Ibid., 344-345.
6. Campbell and Reece, 541.
7. Campbell and Reece, 329.
8. Ibid., 330.
9. Ibid., 329.