Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Stanley, Wendell Meredith

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SSporulationPORULATION

Sporulation is the formation of nearly dormant forms of bacte-

ria. In a limited number of bacteria, spores can preserve the

genetic material of the bacteria when conditions are inhos-

pitable and lethal for the normal (vegetative) form of the bac-

teria. The commitment of a bacterium to the sporulation

process sets in motion a series of events that transform the cell.

Sporulation ultimately provides for a multilayered

structure can be maintained for a very long time. Relative to

the norm life span of the microorganism, spores are designed

to protect a bacterium from heat, dryness, and excess radiation

for a long time. Endospores of Bacillus subtilishave been

recovered from objects that are thousands of years old.

Furthermore, these are capable of resuscitation into an actively

growing and dividing cell. Spores have been recovered from

amber that is more than 250 million years old.

Given that resuscitation is possible, sporulation does not

result in a completely inert structure. The interior of a spore

contains genetic material, cytoplasm, and the necessary

enzymesand other materials to sustain activity. But, this activ-

ity occurs at an extremely slow rate; some 10 million times

slower than the metabolic rate of a growing bacterium.

The sporulation process has been well studied in

Bacillus subtilis. The process is stimulated by starvation.

Typically, sporulation is a “last resort,” when other options fail

(e.g., movement to seek new food, production of enzymes to

degrade surrounding material, production of antimicrobial

agents to wipe out other microbes competing for the food

source, etc.). The genetic grounding for the commitment to

form a spore is a protein called SpoA. This protein functions

to promote the transcriptionof genes that are required for the

conversion of the actively growing bacterium to a spore. The

formation of an active SpoA protein is controlled by a series

of reactions that are themselves responsive to the environ-

mental conditions. Thus, the activation of SpoA comes only

after a number of checkpoints have been passed. In this way a

bacterium has a number of opportunities to opt out of the

sporulation process. Once committed to sporulation, the

process is irreversible.

A similar series of reactions has been identified as a

means of regulating the degree of host damage caused by a

Bordetella pertussis, the bacterium that causes pertussis, as

well as in the response of the yeastSaccharomyces cervesiae

to osmotic pressure.

Sporulation begins with the duplication of the bacterial

genome. The second copy and some of the cytoplasm is then

enveloped in an in-growth of the membrane that surrounds the

bacterium. The result is essentially a little spherical cell inside

the larger bacterium. The little cell is referred to as the “daugh-

ter cell” and the original bacterium is now called the “mother

cell.” Another membrane layer is laid down around the daugh-

ter cell. Between these two membranes lies a layer of pepti-

doglycanmaterial, the same rigid material that forms the

stress-bearing network in the bacterial cell wall. Finally, a coat

of proteins is layered around the outside of the daughter cell.

The result is a nearly impregnable sphere.

The above spore is technically termed an endospore,

because the formation of the membrane-enclosed daughter

cell occurs inside the mother cell. In a so-called exospore, the

duplicated DNAmigrates next to a region on the inner surface

of the cell membrane and then a bud forms. As the bud pro-

trudes further outward, the DNA is drawn inside the bud.

Examples of endospore forming bacteria include those in the

genera Bacillusand Clostridium. Endospore forming bacteria

include Methylosinus, Cyanobacteria, and Microsporidia.

When still in the mother cell, the location of the spore

(e.g., in the center, near one end or at one pole) is often a dis-

tinctive feature for a particular species of bacteria, and can be

used as a feature to identify the bacteria.

As the mother cell dies and degrades, the spore will be

freed. When conditions become more hospital, the metabolic

machinery within the spore will sense the change and a reverse

process will be initiated to transform the spore into a vegeta-

tive cell.

The type of sporulation described here is different from

the sporulation process that occurs in many kinds of fungiand

in the bacteria called Actinomyces. The latter spores are essen-

tially seeds, and are used in the normal reproduction cycle of

the microorganisms. Bacterial sporulation is an emergency

protective and survival strategy.

See alsoAsexual generation and reproduction; Bacterial adap-

tation; Bacterial growth and division; Bacterial kingdoms;

Bacterial membranes and cell wall; Cell cycle (prokaryotic),

genetic regulation of; Desiccation; Extraterrestrial microbiol-

ogy; Extremophiles; Fossilization of bacteria; Genetic identi-

fication of microorganisms; Genetic regulation of prokaryotic

cells; Life, origin of; Radiation resistant bacteria

STANLEY, WENDELLMEREDITH

(1904-1971)Stanley, Wendell Meredith

American biochemist

Wendell Meredith Stanley was a biochemist who was the first

to isolate, purify, and characterize the crystalline form of a

virus. During World War II, he led a team of scientists in

developing a vaccinefor viral influenza. His efforts have

paved the way for understanding the molecular basis of hered-

ity and formed the foundation for the new scientific field of

molecular biology. For his work in crystallizing the tobacco

mosaic virus, Stanley shared the 1946 Nobel Prize in chem-

istry with John Howard Northrop and James B. Sumner.

Stanley was born in the small community of Ridgeville,

Indiana. His parents, James and Claire Plessinger Stanley,

were publishers of a local newspaper. As a boy, Stanley helped

the business by collecting news, setting type, and delivering

papers. After graduating from high school he enrolled in

Earlham College, a liberal arts school in Richmond, Indiana,

where he majored in chemistry and mathematics. He played

football as an undergraduate, and in his senior year, he became

team captain and was chosen to play end on the Indiana All-

State team. In June of 1926, Stanley graduated with a Bachelor

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