Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Conjugation

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compost piles are occasionally agitated or “turned over” to

mix the contents, allow oxygen to diffuse throughout the

material (efficient decomposition requires the presence of

oxygen) and to disperse some of the heat. The ideal blend of

microorganisms can be established and maintained by the

addition of waste material to the compost pile so as to not let

the pile become enriched in carbon or nitrogen. A proper ratio

is about 30 parts carbon to one part nitrogen by weight.

Thermophilic bacteria present at this stage of decompo-

sition include Bacillus stearothermophilusand bacteria of the

genus Thermus. A variety of thermophilic fungi are present as

well. These include Rhizomucor pusills, Chaetomium ther-

mophile, Humicola insolens, Humicola lanuginosus, Thermo-

ascus aurabtiacus, and Aspergillus fumigatus.

Thermophilic activity decomposes protein, fat, and car-

bohydrates such as the cellulose that makes up plants and

grass. As this phase of decomposition ends, the temperature

drops and once again the lower-temperature microbes become

dominant. The decomposition of the complex materials by the

thermophilic organisms provides additional nutrients for the

continued decomposition by the mesophilic populations.

Microbiological composting is becoming increasingly

important as space for waste disposal becomes limited. Some

30% of yard and household waste in the United States is

compostable. An average household can decompose about

700 pounds of material per year. If such waste is added to

landfills intact, the subsequent decomposition produces

methane gas and acidic run-off, both of which are environ-

mentally undesirable.

See alsoChemoautotrophic and chemolithotrophic bacteria;

Economic uses and benefits of microorganisms; Soil forma-

tion, involvement of microorganisms

COMPOUND MICROSCOPE•seeMICROSCOPE

AND MICROSCOPY

CONDITIONAL LETHAL MUTANT•see

MICROBIAL GENETICS

CONFIRMED TESTS•seeLABORATORY TECHNIQUES

IN MICROBIOLOGY

CONFOCAL MICROSCOPY•seeMICROSCOPE AND

MICROSCOPY

CConjugationONJUGATION

Conjugation is a mechanism whereby a bacterium can transfer

genetic material to an adjacent bacterium. The genetic transfer

requires contact between the two bacteria. This contact is

mediated by the bacterial appendage called a pilus.

Conjugation allows bacteria to increase their genetic

diversity. Thus, an advantageous genetic trait present in a bac-

terium is capable of transfer to other bacteria. Without conjuga-

tion, the normal bacterial division process does not allow for the

sharing of genetic information and, except for mutationsthat

occur, does not allow for the development of genetic diversity.

A pilus is a hollow tube constructed of a particular pro-

tein. One end is anchored to the surface of a bacterium. The

other end is capable of binding to specific proteins on the sur-

face of another bacterium. A pilus can then act as a portal from

the cytoplasmof one bacterium to the cytoplasm of the other

bacterium. How the underlying membrane layers form chan-

nels to the bacterial cytoplasm is still unclear, although chan-

nel formation may involve what is termed a mating pair

formation (mpf) apparatus on the bacterial surface.

Nonetheless, once a channel has been formed, transfer

of deoxyribonucleic acid (DNA) from one bacterium (the

donor) to the other bacterium (the recipient) can occur.

Conjugation requires a set of F (fertility) genes.

Transfer of DNA from the genome of a bacterium can occur if

the F set of genes is integrated in the bacterial chromosome.

These F genes enter the pilus and literally drag the trailing

genome along behind. Often the pilus will break before the

transfer of the complete genome can occur. Thus, genes that

are located in the vicinity of the F genes will tend to be suc-

cessfully transferred in conjugation more often than genes

located far away from the F genes. This process was originally

discovered in Escherichia coli. Strains that exhibit a higher

than usual tendency to transfer genomic DNA are known as

High Frequency of Recombination(Hfr) strains.

Conjugation also involves transfer of DNA that is

located on a plasmid. A plasmid that contains the F genes is

called the F episome or F plasmid. Other genes on the episome

will be transferred very efficiently, since the entire episome

can typically be transferred before conjugation is terminated

by pilus breakage. If one of the genes codes for a disease caus-

ing factor or antibiotic resistancedeterminant, then episomal

conjugation can be a powerful means of spreading the genetic

trait through a bacterial population. Indeed, conjugation is the

principle means by which bacterial antibiotic resistance is

spread.

Finally, conjugation can involve the transfer of only a

plasmid containing the F genes. This type of conjugation is

also an efficient means of spreading genetic information to

other bacteria. In this case, as more bacteria acquire the F

genes, the proportion of the population that is capable of

genetic transfer via conjugation increases.

Joshua Lederbergdiscovered the process of conjuga-

tion in 1945. He experimented with so-called nutritional

mutants(bacteria that required the addition of a specific nutri-

ent to the growth medium). By incubating the nutritional

mutants in the presence of bacteria that did not require the

nutrient to be added, Lederberg demonstrated that the muta-

tion could be eliminated. Subsequently, another bacteriologist,

William Hayes, demonstrated that the acquisition of genetic

information occurred in a one-way manner (e.g., information

was passing from one bacterium into another), and that the

basis for the information transfer was genetic (i.e., mutants

were isolated in which the transfer did not occur).

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