Microbiology and Immunology

Anthrax WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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with the bacterium. This can allow the organism to multiply

to large numbers that overwhelm the immune system. The

capsule also contains an antigenthat has been designated a

protective antigen. The antigen is protective, not to the host

being infected, but to the bacterium. The protective antigen

dissolves protein, which can allow the bacterium to “punch”

through the membrane surrounding cells of the host, such as

the epithelial cells that line the lung. One inside the cells, a

bacterium is safe from the host’s immune defenses. A second

toxic component, which is called lethal factor, destroys

immune cells of the host. Finally, a third toxic factor is

known as edema factor (named because it results in the accu-

mulation of fluid at the site of infection). Edema factor dis-

ables a molecule in the host called calmodulin, which is used

to regulate many chemical reactions in the body. The end

result of the activity of the toxic factors of Bacillus anthracis

is to quell the immune response and so, to allow the infection

to spread.

As the bacteriagain a foothold, toxins enter the blood-

stream and circulate throughout the body causing destruction

of blood cells and tissues. The damage can prove to be over-

whelming to treatment efforts and death occurs.

Anthrax infections are difficult to treat because the initial

symptoms are similar to other, less serious infections, such as

the flu. By the time the diagnosis is made, the infection can be

too advanced to treat. Avaccinefor anthrax does exist. But to

date, only those at high risk for infection (soldiers, workers in

meat processing plants, anthrax research scientists) have

received the vaccine, due to the possible serious side effects that

can occur. Work to establish a safer vaccine is underway. The

edema factor may be a potential target of a vaccine. Another

promising target is the protective antigen of the capsule. If the

action of this antigen could be blocked, the bacteria would not

be able to hide inside host cells, and so could be more effec-

tively dealt with by the immune response and with antibiotics.

See alsoAnthrax, terrorist use of as a biological weapon;

Bioterrorism

Light micrograph of Bacillus anthracis, showing the typical hair-like pattern of growth in a liquid.

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