Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Cech, Thomas R.

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Both types of conversion take place in the presence and

the absence of oxygen. Algal involvement is an aerobic

process. The conversion of carbon dioxide to sugar is an

energy-requiring process that generates oxygen as a by-prod-

uct. This evolutionof oxygen also occurs in plants and is one

of the recognized vital benefits of trees to life on Earth.

The carbon available in the carbohydrate sugar mole-

cules is cycled further by microorganisms in a series of reac-

tions that form the so-called tricarboxylic acid (or TCA) cycle.

The breakdown of the carbohydrate serves to supply energy to

the microorganism. This process is also known as respiration.

In anaerobic environments, microorganisms can cycle the car-

bon compounds to yield energy in a process known as fer-

mentation.

Carbon dioxide can be converted to another gas called

methane (CH 4 ). This occurs in anaerobic environments, such

as deep compacted mud, and is accomplished by bacteria

known as methanogenic bacteria. The conversion, which

requires hydrogen, yields water and energy for the

methanogens. To complete the recycling pattern another group

of methane bacteria called methane-oxidizing bacteria or

methanotrophs (literally “methane eaters”) can convert

methane to carbon dioxide. This conversion, which is an aer-

obic (oxygen-requiring) process, also yields water and energy.

Methanotrophs tend to live at the boundary between aerobic

and anaerobic zones. There they have access to the methane

produced by the anaerobic methanogenic bacteria, but also

access to the oxygen needed for their conversion of the

methane.

Other microorganisms are able to participate in the

cycling of carbon. For example the green and purple sulfur

bacteria are able to use the energy they gain from the degra-

dation of a compound called hydrogen sulfide to degrade car-

bon compounds. Other bacteria such as Thiobacillus

ferrooxidansuses the energy gained from the removal of an

electron from iron-containing compounds to convert carbon.

The anerobic degradation of carbon is done only by

microorganisms. This degradation is a collaborative effort

involving numerous bacteria. Examples of the bacteria include

Bacteroides succinogenes, Clostridium butyricum, and

Syntrophomonas sp.This bacterial collaboration, which is

termed interspecies hydrogen transfer, is responsible for the

bulk of the carbon dioxide and methane that is released to the

atmosphere.

See alsoBacterial growth and division; Chemoautotrophic

and chemolithotrophic bacteria; Metabolism; Methane oxidiz-

ing and producing bacteria; Nitrogen cycle in microorganisms

CCaulobacterAULOBACTER

Caulobacter crescentusis a Gram-negative rod-like bacterium

that inhabits fresh water. It is noteworthy principally because

of the unusual nature of its division. Instead of dividing two

form two identical daughter cells as other bacteriado (a

process termed binary division), Caulobacter crescentus

undergoes what is termed symmetric division. The parent bac-

terium divides to yield two daughter cells that differ from one

another structurally and functionally.

When a bacterium divides, one cell is motile by virtue of

a single flagellum at one end. This daughter cell is called a

swarmer cell. The other cell does not have a flagellum. Instead,

at one end of the cell there is a stalk that terminates in an

attachment structure called a holdfast. This daughter cell is

called the stalk cell. The stalk is an outgrowth of the cell wall,

and serves to attach the bacterium to plants or to other microbes

in its natural environment (lakes, streams, and sea water).

Caulobacter crescentusexhibits a distinctive behavior.

The swarmer cell remains motile for 30 to 45 minutes. The cell

swims around and settles onto a new surface where the food

supply is suitable. After settling, the flagellum is shed and the

bacterium differentiates into a stalk cell. With each division

cycle the stalk becomes longer and can grow to be several

times as long as the body of the bacterium.

The regulation of geneexpression is different in the

swarmer and stalk cells. Replication of the genetic material

occurs immediately in the stalk cell but for reasons yet to be

determined is repressed in the swarmer cell. However, when a

swarmer cell differentiates into a stalk cell, replication of the

genetic material immediately commences. Thus, the transition

to a stalk cell is necessary before division into the daughter

swarmer and stalk cells can occur.

The genetics of the swarmer to stalk cell cycleare com-

plex, with at least 500 genes known to play a role in the struc-

tural transition. The regulation of these activities with respect

to time are of great interest to geneticists.

Caulobacter crescentuscan be grown in the laboratory

so that all the bacteria in the population undergoes division at

the same time. This type of growth is termed synchronous

growth. This has made the bacterium an ideal system to study

the various events in gene regulation necessary for growth and

division.

See alsoBacterial appendages; Bacterial surface layers; Cell

cycle (prokaryotic), genetic regulation of; Phenotypic variation

CDC•seeCENTERS FORDISEASECONTROL(CDC)

CCech, Thomas R.ECH, THOMASR.(1947- )

American biochemist

The work of Thomas R. Cech has revolutionized the way in

which scientists look at RNAand at proteins. Up to the time of

Cech’s discoveries in 1981 and 1982, it had been thought that

genetic coding, stored in the DNA of the nucleus, was

imprinted or transcribed onto RNA molecules. These RNA

molecules, it was believed, helped transfer the coding onto

proteins produced in the ribosomes. The DNA/RNA nexus

was thus the information center of the cell, while protein mol-

ecules in the form of enzymeswere the workhorses, catalyz-

ing the thousands of vital chemical reactions that occur in the

cell. Conventional wisdom held that the two functions were

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