Microbiology and Immunology

Lichen planus WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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000 ongoing cases identified worldwide in 2002. In conjunc-

tion with the World Health Organization, these countries have

committed to accelerating control efforts, including early

access to current drug therapy. The worldwide reduction and

control of leprosy stands as one of the major world health ini-

tiatives of modern times.

See alsoBacteria and bacterial infection; Mycobacterial infec-

tions, atypical

LETHAL DOSE, LD50•seeLABORATORY TECH-

NIQUES IN MICROBIOLOGY

LLichen planusICHEN PLANUS

Lichen planus is a skin rash characterized by small, flat-topped,

itchy purplish raised spots on the wrists, arms, or lower legs.

Although the evidence is not conclusive, many researchers

assert that Lichen planus is an autoimmune disease.

Lichen planus affects approximately one to two percent

of the population. Although there is no apparent correlation to

race or geographic region, it is interesting to note that the

majority of individuals affected are women, age 30 to 50

years. Lichen planus rashes may produce discoloration of the

skin, especially in darker skinned population groups. Lichen

planus lesions may develop on the genitals or in the mouth.

Within a few years, most of the spots disappear, even without

treatment.

Although not definitive, researchers assert Lichen

planus exhibits many of the characteristics of an autoimmune

disorder. Autoimmune diseases result when the immune sys-

temattacks the body’s own cells, causing tissue destruction.

Dermatologists argue that the condition may result from a

viral infection that is then aggravated by stress. Lichen planus

symptoms are similar to allergic reactions to arsenic, gold, and

bismuth. The spots are also similar to the type produced from

allergic reactions to certain chemicals used to develop film.

There is a correlation (a statistical relationship) between

allergic reactions to certain medications and the appearance of

a Lichen planus rash in the mouth. Oral lichen planus usually

forms white lines and spots that may appear in clusters. Only

a definitive biopsy can fully distinguish the rash from yeast

infections or canker sores. Dentists find that some patients

develop a Lichen planus rash following dental procedures.

Other reports indicate that a Lichen planus rash may appear as

an allergic-reaction like response to certain foods, candy, or

chewing gum.

Because the exact cause of Lichen planus is unknown,

there is no specific treatment for the rash. Treatment with var-

ious combinations of steroid creams, oral corticosteroids, and

oral antihistamines appears effective at relieving discomfort

caused by the rash. In more severe cases PUVA pho-

tochemotherapy, a procedure where cells are photosensitizing

and then exposed to ultraviolet light and antibiotics.

Lichen planus may also affect the growth of nails and,

if present on the scalp, may contribute to hair loss.

Lichen planus is not an infectious disease. Research

also indicates that it is not, as once argued, caused by any spe-

cific dietary deficiency.

See alsoAutoimmunity and autoimmune diseases; Viruses

and responses to viral infection; Yeast, infectious

LLichensICHENS

Lichens are an intimate symbiosis, in which two species live

together as a type of composite organism. Lichens are an obli-

gate mutualism between a fungus mycobiont and an alga or

blue-green bacterium phycobiont.

Each lichen mutualism is highly distinctive, and can be

identified on the basis of its size, shape, color, and biochem-

istry. Even though lichens are not true “species” in the con-

ventional meaning of the word, lichenologists have developed

systematic and taxonomic treatments of these mutualisms.

The fungal partner in the lichen mutualism gains impor-

tant benefits through access to photosynthetic products of the

alga or blue-green bacterium. The phycobiont profits from the

availability of a relatively moist and protected habitat, and

greater access to inorganic nutrients.

The most common fungiin lichens are usually species

of Ascomycetes,or a few Basidiomycetes.The usual algal part-

ners are either species of green algae Chlorophytaor blue-

green bacteriaof the family Cyanophyceae. In general, the

fungal partner cannot live without its phycobiont, but the algae

is often capable of living freely in moist soil or water. The

largest lichens can form a thallus up to 3 ft (1 m) long,

although most lichens are smaller than a few inches or cen-

timeters in length. Lichens can be very colorful, ranging from

bright reds and oranges, to yellows and greens, and white,

gray, and black hues.

Most lichens grow very slowly. Lichens in which the

phycobiont is a blue-green bacterium have the ability to fix

nitrogen gas into ammonia. Some lichens can commonly reach

ages of many centuries, especially species living in highly

stressful environments, such as alpine or arctic tundra.

Lichens can grow on diverse types of substrates. Some

species grow directly on rocks, some on bare soil, and others

on the bark of tree trunks and branches. Lichens often grow

under exposed conditions that are frequently subjected to peri-

ods of drought, and sometimes to extremes of hot and cold.

Lichen species vary greatly in their tolerance of severe envi-

ronmental conditions. Lichens generally respond to environ-

mental extremes by becoming dormant, and then quickly

becoming metabolically active again when they experience

more benign conditions.

Lichens are customarily divided into three growth forms,

although this taxonomy is one of convenience, and is not ulti-

mately founded on systematic relationships. Crustose lichens

form a thallus that is closely appressed to the surface upon

which they are growing. Foliose lichens are only joined to their

substrate by a portion of their thallus, and they are somewhat

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