chapter  17
34 Pages

Bacterial Pathogens

Bacteria are single-celled microorganisms. They are easy to grow but production, isolation, harvesting, and storage of large quantities of these organisms is difficult. Bacteria come in various shapes including rods (bacilli), spheres (cocci), and commas or spirals (spirilla). Individual organisms range in size from less than one micron to tens of microns. They may ormaynotbe able tomoveon their own (i.e., motile). Bacteria canbeaerobic, that is, they can live and grow in the presence of oxygen, or they can be anaerobic and live without oxygen. In adverse conditions, some bacteria can enter a dormant state known as a spore. Spores

can remain dormant for decades and can survive under extreme temperatures and other adverse environmental conditions. Unlike fungi (Chapter 20), formation of spores is not related to reproduction and is done strictly as a protective mechanism. Upon reactivation, each spore produces a single active bacteria. Spores are normally spherical or oval and are only a fraction of the size of the active (i.e., vegetative) cell. Pathogens employed as biologicalweapons can beused for both lethal and incapacitating

purposes. They cause disease by invading tissues or by producing toxins (Chapter 16) that are detrimental to the infected individual. Pathogens can be selected to target a specific host (e.g., humans, cows, pigs) or they may pose a broad threat to both animals and to people. Pathogens deployed as antianimal biological weapons are generally used to produce

lethal effects in an agriculturally significant species such as cows, pigs, or chickens. Although these pathogens are selected to target a specific animal species, there is a possibility that the disease may migrate to humans. The diseases produced by these crossover pathogens may be difficult to diagnose for medical personnel not trained in exotic pathology. Other pathogens are selected to produce lethal effects in an agriculturally significant crop

species such aswheat, corn, or rice. There is little potential formigration of these pathogens to humans or animals. A final group of biological warfare pathogens are those used as simulants to model

the release of other, more hazardous agents. Pathogens employed as biological warfare simulants do not generally pose a significant risk to people, animals, or plants. However, individuals with respiratory illness or suppressed immune systems may be at risk should they be exposed to an infectious dose of the agent. Bacteria can be stored as either liquids (e.g., organisms concentrated growth media) or

powders (e.g., spores or freeze-dried mixtures of agent and growth media) and are easy to disperse. However, because they are living organisms and canbe killedduring thedispersal process, there are limitations to the methods that can be used. They can also be stored and

will be clandestine and only detected through epidemiological analysis of resulting disease patterns. Localized or small-scale attacks may take the form of “anthrax” letters. Even in these cases, without the inclusion of a threat, the attack may go unnoticed until the disease appears in exposed individuals (e.g., the initial 2001 anthrax attack at AmericanMedia Inc., which claimed the life of Robert Stevens). In general, unless a local reservoir (i.e., intermediate host thatmay ormay not be affected

by the bacteria) is established, pathogens are easily killed by unfavorable environmental factors such as fluctuations in temperature, humidity, food sources, or ultraviolet light. For this reason, their persistency is generally limited to days. However, bacterial spores are highly resistant to impacts from changes in environmental factors. Agents that can form spores can survive in this state for decades and then become active again under the proper conditions. In addition, pathogens can be freeze-dried and remain in a preserved state almost indefinitely. Freeze-dried pathogens are reactivated when exposed to moisture. It is possible that local insects can become both a reservoir and a vector for the pathogen.