Bacterial Agents

"...little living animalcules, very prettily a-moving..."
~Anton Van Leeuwenhoek

Bacteria are one-celled, microscopic (1µm-10µm), free-living organisms. Different from eukaryotic cells that are the building blocks of animal organisms, bacteria are prokaryotic cells. Prokaryotic comes from Greek and means 'before nucleus.' These cells have no membrane around the nucleus, and rather than having DNA in chromosome form, prokaryotic cells' single strand of DNA float freely in the cytoplasm in a loop known as a plasmid. Additionally, bacteria have no mitochondria, a tube-like organelle that generates power for an eukaryotic cell, or chloroplast, an organelle found in planet cells or algae responsible for photosynthesis. Bacteria have cell walls made of peptidoglycan, a polymer, and some bacteria have flagella, a filament of protein protruding from the bacteria that renders it mobile.

As possible biological agents, bacteria are defined by its virulence or ability to induce disease, incubation period, mode of transmission, and case fatality rates.

Some bacteria can form spores when it finds itself in a hostile environment. The bacterium, through a process called sporulation, replicates its genetic material and then surrounds it with a thick coating. In spore form, the bacterium's water is released and metabolism ceases; it can survive temperature extremes, radiation, and lack of air, water, and nutrients for extended periods of time to be revived when nutrients are abundant again. A spore's virtual indestructibility renders it ideal as biological weapons agent.

History of Bacteria

Scientists suspect that life on earth evolved from bacteria. At first, man believed disease to be either punishment from the divine, sorcery from enemies, or the invasion of evil spirits. The ancient Greek physician Hippocrates wrote Corpus Hipporaticum, which combined 50-70 volumes and formulated a new theory of disease. He believed the body to be composed of four humors: yellow bile, black bile, phlegm, and blood. Disease was caused by an imbalance of these humors. Fever, for example, was believed to be an excess of yellow bile, and a cold bath would increase the cold and wet phlegm humor to re-balance the body. The theory that imbalance of bodily humors caused disease was also popularized by the 2nd Century Greek physician Galen. Aristotle formulated the theory of spontaneous generation, the spontaneous process through which a decaying flesh generated small animals. Another theory of disease, the miasma theory, postulated that a poisonous vapor with particles from decaying and rotting biomatter, mold, or dust caused diseases like cholera and the Black Death.

The first recorded descriptions of bacteria were in letters from Anton Van Leeuwonhoek to the Royal Society in the 1670s- descriptions of what he observed in his single-lensed microscope. The word 'bacteria' derives from the Greek word for 'staff' to describe the rod-like shape of bacilli. In 1857, Louis Pasteur, after he observed that yeast and bacteria fermented wine, put forth the theory that disease was caused by microorganisms. In the 1870s, German scientist Robert Koch proved the disease theory by experimenting with anthrax in mice. An intense competition between Koch and Pasteur, reflecting the political rivalries of late 19th Century Europe between Germany and France, propelled forward advances in the bacteriology and epidemiology fields. In 1905, Koch won the Nobel Prize for his work, and Pasteur won his in 1907. In 1909, Paul Ehrlich discovered that Atoxyl, an arsenic compound, disabled the syphilis bacteria, and in 1910, produced the drug called Salvarsan. Salvarsan inspired Alexander Fleming to pursue experiments that led to the discovery of penicillin, the first antibiotic drug, in 1928.

The history of using bacterial agents as biological weapons stretched back to ancient history. The Greek historian Herodotus from the 5th Century BCE wrote of Scythian archers who lived in the Black Sea region and used poisoned arrows. The poison was composed of the decomposed bodies of venomous adders, human blood, and dung that were mixed together and left to putrefy. Scientists suggested that such a concoction would contain gangrene (Clostridium perfingins) and tetanus (Clostridium tentani). In the 14th Century, the Tartar army flung bodies of plague victims over the walls of Kaffa, the modern city of Feodosia in Ukraine. While plague did break out in Kaffa, it was more likely that flea-infested rats brought plague from the Tarter camp into Kaffa. During World War I, Germany began its bioweapons program infecting Romanian sheep with anthrax before they were transported to Russia. During World War II, the Japanese bioweapons program known as Unit 731 poisoned Chinese wells with cholera and typhus bacteria, and tested deadly diseases on Prisoners of War. Since then, biological agents have developed globally as a weapon of destruction.

Taxonomy of Bacteria

The taxonomy of bacteria separates bacteria into three shapes: bacilli or rod-shaped, cocci or spherical, and spirilla or curved walls. Further, bacteria are either Gram-positive or Gram-negative. In the late 19th Century, Danish bacteriologist Hans Christian Gram discovered this method of bacterial identification. When stained with purple crystal violet stain, Gram-positive bacteria trap the stain with their outer layer of peptidoglycan. Gram-negative bacteria's lipopolysaccharide and protein layer prevents the purple stain from reaching the peptidoglycan layer, but acetone acts as a pink counterstain that does penetrate the lipopolysaccharide and protein layer. Oxygen use is another distinction between bacteria: anaerobic bacteria thrive in the absence of oxygen and aerobic bacteria use oxygen in their metabolic processes. A bacterium is also distinguished by its ability to form spore and whether it has a flagellum to facilitate movement.

Bacteria and the Human Body

The human body is covered with bacteria colonies. Biofilm or the normal microflora such as dental plaque is one example. The presence of bacteria protects the human body from other destructive organisms, and creates nutrients, such as Vitamin K, required by the human body. Probiotics or helpful bacteria facilitate the digestion processes and crowd out harmful bacteria growth. Bacteria like Candida albicans and E. coli are part of the normal human flora but can cause disease under certain conditions.

The human body boasts many natural barriers or non-specific immune responses against bacterial infection. The epidermis or skin, stomach and gut mucus, and hydrochloric acid produced by the stomach and other human organs stem possible bacterial invasion. Tear ducts and the urinary tract also allow the body to flush out unwanted bacteria. Once in the bloodstream, the bacteria face complements, which are a group of inactive proteins that are activated when triggered by bacterial infection. These proteins attach to the bacteria and punch holes into the cell walls of the invader until the bacteria bursts. In addition, these proteins call for phagocytes to assist in the immune response.

Phagocytes are white blood cells that ingest or engulf bacteria. Neutrophils travel through the bloodstream and either stick to foreign bacteria and fungi to immobilize them, or swallow the bacteria and then eliminate them with chemicals. There are 3,000 to 6,000 neutrophils per milliliter of blood. Another type of phagocytes, macrophages, also engulf bacteria, foreign cells, and dead and damaged cells in a process called phagocytosis. Unlike neutrophils, macrophages are organ-specific and do not circulate freely in the bloodstream. Basophils, which typically respond to allergic reactions, and Eosinophils, which mostly attacks invading parasites, are also phagocytes that help the body resist against bacterial invasion. In addition to phagocytes, natural killer (NK) cells, another form of white blood cells, originate in the bone marrow and attach to bacteria. They release enzymes that target the cell membranes without the need of antibodies. NK cells also produce cytokines- signaling proteins and peptides that spread news throughout the immune system of a bacterial invasion.

When the body encounters a bacterium it recognized either through previous infection or vaccination, a specific immune response is triggered. Acquired specific immunity functions through the recognition of antigen, the protein specific to each bacterium. The immune system recognizes 'self' from 'non-self' thanks to the major histocompatibility complex (MHC, class I, II, and III) displayed on the cell surface. There are two types of specific immune response: cellular and humoral. The cellular response produces more white blood cells to respond to the bacterial invasion while the humoral response involves the creation of antibodies to fight the bacteria.

Dendritic cells engulf bacteria, and they move from the tissue to lymph nodes with the antigen of the bacteria. They 'present' the antigen to the T-lymphocytes. Lymphocytes are either B 'killer' cells that are produced in the bone marrow or T cells that are produced in the thymus gland. Of the T cells there are two forms: killer or cytotoxic T cells (CD8+), which kills invading bacteria, and helper T cells (CD4+), which learns to recognize antigens and helps B cells. Scientists believe a third T cell, suppressor T cells, also assists in the immune process to stop cytotoxic T-cells when enough antibodies have been produced. B cells, when attached to specific antigen, transform into plasma cells that produce antibodies (immunoglobulins, or Ig) specific to each antigen. Antibodies are Y shaped molecules, and there are five groups: IgA, IgG, IgM, IgE, and IgD. Antibodies neutralize toxins, help the immune system recognize antigens, and attack the bacterial infection itself. Once having recognized an antigen, the B cells will always recognize the antigen should infection recur in the future. A vaccine uses weakened bacteria to trigger an immune response from B cells to protect against future invasions.

Bacteria as Biological Weapons

Despite the widespread use of antibiotics, bacterial biological weapons agents remain a formidable challenge to global security. Antibiotic resistant strains of bacteria, aerosolized, combined with a health infrastructure that rarely handles cases of atypical diseases such as plague or anthrax could present an attractive target for a biological weapons attack.