Microbiology and Immunology

Desiccation WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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The reasons for the varied degrees of severity and symp-

toms that the viral infection can elicit are still unclear. Not sur-

prisingly, there is currently no cure for dengue, nor is there a

vaccine. Treatment for those who are afflicted is palliative, that

is, intended to ease the symptoms of the disease. Upon recov-

ery, immunityto the particular antigenic type of the virus is in

place for life. However, an infection with one antigenic type of

dengue virus is not protective against the other three antigenic

types. Currently, the only preventive measure that can be taken

is to eradicate the mosquito vector of the virus.

See alsoEpidemics, viral; Zoonoses

DEOXYRIBONUCLEIC ACID•seeDNA

(DEOXYRIBONUCLEIC ACID)

DDesiccationESICCATION

Desiccation is the removal of water from a biological system.

Usually this is accomplished by exposure to dry heat. Most

biological systems are adversely affected by the loss of water.

Microorganismsare no exception to this, except for those that

have evolved defensive measures to escape the loss of viabil-

ity typically associated with water loss.

Desiccation also results from the freezing of water, such

as in the polar regions on Earth. Water is present at these

regions, but is unavailable.

Microorganisms depend on water for their structure and

function. Cell membranes are organized with the water-loving

portions of the membrane lipids positioned towards the exte-

rior and the water-hating portions pointing inward. The loss of

water can throw this structure into disarray. Furthermore, the

interior of microorganisms such as bacteriais almost entirely

comprised of water. Extremely rapid freezing of the water can

be a useful means of preserving bacteria and other microor-

ganisms. However, the gradual loss of water will produce

lethal changes in the chemistry of the interior cytoplasmof

cells, collapse of the interior structure, and an alteration in the

three-dimensional structure of enzymes. These drastic

changes caused by desiccation are irreversible.

In the laboratory, desiccation techniques are used to

help ensure that glassware is free of viable microbes.

Typically, the glassware is placed in a large dry-heat oven and

heated at 160° to 170° C [320° to 338° F] for up to two hours.

The effectiveness of sterilizationdepends on the penetration of

heat into a biological sample.

Some microorganisms have evolved means of coping

with desiccation. The formation of a spore by bacteria such as

Bacillus and Clostridium allows the genetic material to sur-

vive the removal of water. Cysts produced by some protozoans

can also resist the destruction of desiccation for long periods

of time. Bacterial biofilms might not be totally dehydrated if

they are thick enough. Bacteria buried deep within the biofilm

might still be capable of growth.

The fact that some microbes on Earth can resist desic-

cation and then resuscitate when moisture becomes available

holds out the possibility of life on other bodies in our solar

system, particularly Mars. The snow at the poles of Mars is

proof that water is present. If liquid water becomes transiently

available, then similar resuscitation of dormant Martian

microorganisms could likewise occur.

See alsoCryoprotection

DETECTION OF MUTANTS•seeLABORATORY

TECHNIQUES IN MICROBIOLOGY

DDiatomsIATOMS

Algae are a diverse group of simple, nucleated, plant-like

aquatic organisms that are primary producers. Primary pro-

ducers are able to utilize photosynthesisto create organic

molecules from sunlight, water, and carbon dioxide.

Ecologically vital, algae account for roughly half of photosyn-

thetic production of organic material on Earth in both fresh-

water and marine environments. Algae exist either as single

cells or as multicellular organizations. Diatoms are micro-

scopic, single-celled algae that have intricate glass-like outer

cell walls partially composed of silicon. Different species of

diatom can be identified based upon the structure of these

walls. Many diatom species are planktonic, suspended in the

water column moving at the mercy of water currents. Others

remain attached to submerged surfaces. One bucketful of

water may contain millions of diatoms. Their abundance

makes them important food sources in aquatic ecosystems.

When diatoms die, their cell walls are left behind and sink to

the bottom of bodies of water. Massive accumulations of

diatom-rich sediments compact and solidify over long periods

of time to form rock rich in fossilized diatoms that is mined for

use in abrasives and filters.

Diatoms belong to the taxonomic phylum Bacillario-

phyta. There are approximately 10,000 known diatom species.

Of all algae phyla, diatom species are the most numerous. The

diatoms are single-celled, eukaryotic organisms, having

genetic information sequestered into subcellular compart-

ments called nuclei. This characteristic distinguishes the group

from other single-celled photosynthetic aquatic organisms,

like the blue-green algaethat do not possess nuclei and are

more closely related to bacteria. Diatoms also are distinct

because they secrete complex outer cell walls, sometimes

called skeletons. The skeleton of a diatom is properly referred

to as a frustule.

Diatom frustules are composed of pure hydrated silica

within a layer of organic, carbon containing material.

Frustules are really comprised of two parts: an upper and

lower frustule. The larger upper portion of the frustule is

called the epitheca. The smaller lower piece is the hypotheca.

The epitheca fits over the hypotheca as the lid fits over a shoe-

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