Microbiology and Immunology

Slime molds WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Other symptoms resemble Parkinson’s disease: imbalance

when walking, slow and shuffling gait, trembling of the limbs,

involuntary movement, muscle tightness, and increasing men-

tal confusion. These symptoms culminate in coma, then death.

Diagnosis of sleeping sickness can be made by micro-

scopic examination of fluid from the site of the tsetse fly bite

or swollen lymph nodes for examination. A method to diag-

nose Rhodesian trypanosome involves culturing blood, bone

marrow, or spinal fluid. These cultures are injected into rats to

promote the development of blood-borne protozoan infection.

This infection can be detected in blood smears within one to

two weeks.

Medications effective against the Trypanosoma brucei

complex protozoa have significant potential for side effects.

Suramin, eflornithine, pentamidine, and several drugs which

contain arsenic (a chemical which is potentially poisonous)

are effective anti-trypanosomal agents. Each of these drugs

requires careful monitoring to ensure that they do not cause

serious complications such as a fatal hypersensitivity reaction,

kidney or liver damage, or inflammationof the brain. Trials are

underway to monitor the effectiveness of new medications for

treatment of trypanosomiasis.

Prevention of sleeping sickness requires avoiding con-

tact with the tsetse fly; insect repellents, mosquito netting, and

clothing that covers the limbs to the wrists and ankles are

mainstays. There are currently no immunizations available to

prevent sleeping sickness.

See alsoProtists

SLIME LAYER•seeGLYCOCALYX

SSlime moldsLIME MOLDS

Slime molds are organisms in two taxonomic groups, the cel-

lular slime molds (Phylum Acrasiomycota) and the plasmodial

slime molds (Phylum Myxomycota). Organisms in both

groups are eukaryotic (meaning that their cells have nuclei)

and are fungus-like in appearance during part of their life

cycle. For this reason, they were traditionally included in

mycologytextbooks. However, modern biologists consider

both groups to be only distantly related to the fungi. The two

groups of slime molds are considered separately below.

Species in the cellular slime moldgroup are micro-

scopic during most stages of their life cycle, when they exist

as haploid (having one copy of each chromosome in the

nucleus), single-celled amoebas. The amoebas typically feed

on bacteriaby engulfing them, in a process known as phago-

cytosis, and they reproduce by mitosis and fission. Sexual

reproduction occurs but is uncommon. Most of what we know

about this group is from study of the species Dictyostelium

discoideum.When there is a shortage of food, the individual

haploid amoebas of a cellular slime mold aggregate into a

mass of cells called a pseudoplasmodium. A pseudoplasmod-

ium typically contains many thousands of individual cells. In

contrast to the plasmodial slime molds, the individual cells in

a pseudoplasmodium maintain their own plasma membranes

during aggregation. The migrating amoebas often form beau-

tiful aggregation patterns, which change form over time.

After a pseudoplasmodium has formed, the amoebas

continue to aggregate until they form a mound on the ground

surface. Then, the mound elongates into a “slug.” The slug is

typically less than 0.04 in (1 mm) in length and migrates in

response to heat, light, and other environmental stimuli.

The slug then develops into a sporocarp, a fruiting body

with cells specialized for different functions. A sporocarp typ-

ically contains about 100,000 cells. The sporocarp of

Dictyosteliumis about 0.08 in (2 mm) tall and has cells in a

base, stalk, and ball-like cap. The cells in the cap develop into

asexual reproductive spores, which germinate to form new

amoebas. The different species of cellular slime molds are dis-

tinguished by sporocarp morphology.

Dictyostelium discoideumhas been favored by many

biologists as a model organism for studies of development,

biochemistry, and genetics. Aspects of its development are

analogous to that of higher organisms, in that a mass of undif-

ferentiated cells develops into a multicellular organism, with

different cells specialized for different functions. The devel-

opment of Dictyosteliumis much easier to study in the labora-

tory than is the development of higher organisms.

A food shortage induces aggregation in Dictyostelium.

In aggregation, individual amoebas near the center of a group

of amoebas secrete pulses of cAMP (cyclic adenosine-3’5’-

monophosphate). The cAMP binds to special receptors on the

plasma membranes of nearby amoebas, causing the cells to

move toward the cAMP source for about a minute. Then, these

amoebas stop moving and in turn secrete cAMP, to induce

other more distant amoebas to move toward the developing

aggregation. This process continues until a large, undifferenti-

ated mass of cells, the pseudoplasmodium, is formed.

Interestingly, cAMP is also found in higher organisms,

including humans. In Dictyosteliumand these higher organ-

isms, cAMP activates various biochemical pathways and is

synthesized in response to hormones, neurotransmitters, and

other stimuli.

The trypanosome that causes sleeping sickness is commonly

transferred to humans by mosquitoes.

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