Microbiology and Immunology

Sheathed bacteria WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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United States. For example, in Sweden the gonorrhea rate fell

by more than 95% from 1970 to 1989 after vigorous govern-

ment efforts to control sexually transmitted disease in Sweden.

Yet the role of government funding for community

health clinics, birth control, and public information campaigns

on sexually transmitted disease has long been controversial.

Public officials continue to debate the wisdom of funding pub-

lic distribution of condoms and other services that could affect

the transmission of sexually transmitted disease. Although sci-

ence has made great strides in understanding the causes and

cures of many sexually transmitted diseases, society has yet to

reach agreement on how best to attack them.

See alsoBacteria and bacterial infection; Immunization;

Immunogenetics; Public health, current issues; Virus replica-

tion; Viruses and responses to viral infection

SSheathed bacteriaHEATHED BACTERIA

Sheathed bacteriaare bacteria that grow as long filaments

whose exterior is covered by a layer known as a sheath. Within

the sheath, the bacteria can be capable of growth and division.

Examples of sheathed bacteria include Leptothrix discophora

(also known as “iron bacteria”), and Sphaerotilus natans.

Sheathed bacteria are common of the bacterial commu-

nities in water and in soil. In these environments, the sheath is

often coated with precipitates of elements in the water or soil

environments, such as oxides of iron and manganese. The ele-

ments are unstable in solution, and thus will readily come out

of solution when presented with an appropriate site.

The sheath that covers the bacteria can be of varied con-

struction. Much of the structural information has been gleaned

from the observation of thin slices of sample using the trans-

mission electron microscope. The sheath surrounding

Leptothrixspecies is glycocalyx-like in appearance. Often the

deposition of metals within the sheath network produces areas

where the material has crystallized. In contrast, the sheath of

Sphaerotilus natanspresents the “railroad track” appearance,

which is typical of a biological membrane consisting of two

layers of lipid molecules.

Electron microscopic studies of Leptothrixspecies have

shown that the bacterium is intimately connected with the over-

lying sheath. The connections consist of protuberances that are

found all over the surface of the bacterium. In contrast,

Sphaerotilus natansis not connected with the overlying sheath.

Both Leptothrixand Sphaerotilus natanscan exist inde-

pendently of the sheath. Bacteria in both genera have a life

cycle that includes a free-swimming form (called a swarmer

cell) that is not sheathed. The free-swimming forms have fla-

gella at one end of the bacteria that propels the cells along.

When encased in the sheath, the bacteria are referred to as

sheathed or resting bacteria.

Bacterial sheaths tend to be manufactured when the bac-

teria are in an aquatic or soil environment that contains high

amounts of organic matter. The sheath may serve to provide

protection to the bacteria in these environments, Also, the abil-

ity of metallic compounds to precipitate on the sheath may pro-

vide the bacteria with a ready supply of such inorganic nutri-

ents. For example, Leptothrixis able to utilize the manganese

contained in the manganese oxide precipitate on the sheath.

Sheaths may also help the bacteria survive over a wide

range of temperature and pH, by providing a relatively inert

barrier to the external environment.

See alsoBacterial appendages; Soil formation, involvement of

microorganisms

SShigellaHIGELLA

Shigellais a genus of Gram-negative bacteriathat is similar in

behavior and habitat to Escherichia coli. The bacterium is

named after its discoverer, Japanese scientist Kiyoshi Shiga.

The bacteria were discovered over 100 years ago.

Some strains of the bacteria can produce toxins, includ-

ing the so-called Shiga toxin, which is very similar to the

destructive verotoxin of Escherichia coliO157:H7. Indeed,

strain O157:H7 is now presumed to have arisen by virtue of a

genetic recombination between strains of Shigella and

Escherichia coliin the intestinal tract, which resulted in the

acquisition of the verotoxin by Escherichia coli.

The similarity between Shigellaand Escherichia coli

extends to the structure of the bacteria and their utilization of

certain compounds as nutrients. The similarity is so pro-

nounced that Shigella has been regarded as a strain of

Escherichia coli. However, this is now known not to be the

case. Shigelladoes not produce gas from the utilization of car-

bohydrates, while Escherichia colidoes.

Shigellais one of a group of bacteria, which includes

Escherichia coli, that inhabits the intestinal tract of humans

and other warm blooded animals. Most strains of the bac-

terium are innocuous. However, the strains that possess the

destructive toxins can do much damage to the intestinal wall

and other areas of the body.

There are a number of Shigellaspecies that are note-

worthy to humans. Shigella sonnei, which is also known as

group D Shigella, is the cause of almost 70 percent of the

reported cases of food-borne Shigellaillness in the United

States each year. Shigella flexneri, which is also called group

B Shigella, is responsible for virtually all the remaining cases

of food-borne illness. In underdeveloped countries of the

world, the bacterium Shigella dysenteriaetype 1 is epidemic

in its scope.

The illness that is caused by Shigellaspecies is called

shigellosis. The illness is classified as a bacillary dysentery.

An estimated 300,000 cases of shigellosis occurs in the United

States each year. Production of the toxins following the

ingested of Shigella-contaminated food produces the illness.

The illness is characterized by pain in the abdomen, cramps,

diarrhea that can become bloody as intestinal cells are dam-

aged, vomiting, and fever. These symptoms typically begin

from 12 hours to three days after consuming food that is con-

taminated with the microorganism. Contamination usually

results from the exposure of the food to feces-contaminated

water or from improper hygieneprior to the handling of the

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