Microbiology and Immunology

Microbial symbiosis WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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genic agent. For such studies, microorganisms offer the advan-

tage that they have short mean generation times, are easily cul-

tured in a small space under controlled conditions and have a

relatively uncomplicated structure.

Microorganisms, and particularly bacteria, were gener-

ally ignored by the early geneticists because of their small in

size and apparent lack of easily identifiable variable traits.

Therefore, a method of identifying variation and mutation in

microbes was fundamental for progress in microbial genetics.

As many of the mutations manifest themselves as metabolic

abnormalities, methods were developed by which microbial

mutants could be detected by selecting or testing for altered

phenotypes. Positive selectionis defined as the detection of

mutant cells and the rejection of unmutated cells. An example

of this is the selection of penicillinresistant mutants, achieved

by growing organisms in media containing penicillin such that

only resistant colonies grow. In contrast, negative selection

detects cells that cannot perform a certain function and is used

to select mutants that require one or more extra growth factors.

Replica plating is used for negative selection and involves two

identical prints of colonydistributions being made on plates

with and without the required nutrients. Those microbes that do

not grow on the plate lacking the nutrient can then be selected

from the identical plate, which does contain the nutrient.

The first attempts to use microbes for genetic studies

were made in the USA shortly before World War II, when

George W. Beadle (1903–1989) and Edward L. Tatum

(1909–1975) employed the fungus, Neurospora,to investigate

the genetics of tryptophan metabolismand nicotinic acid syn-

thesis. This work led to the development of the “one gene one

enzyme” hypothesis. Work with bacterial genetics, however,

was not really begun until the late 1940s. For a long time, bac-

teria were thought to lack sexual reproduction, which was

believed to be necessary for mixing genes from different indi-

vidual organisms—a process fundamental for useful genetic

studies. However, in 1947, Joshua Lederberg(1925– ) work-

ing with Edward Tatum demonstrated the exchange of genetic

factors in the bacterium, Escherichia coli.This process of

DNA transfer was termed conjugationand requires cell-to-cell

contact between two bacteria. It is controlled by genes carried

by plasmids, such as the fertility (F) factor, and typically

involves the transfer of the plasmid from donor torecipient cell.

Other genetic elements, however, including the donor cell

chromosome, can sometimes also be mobilized and trans-

ferred. Transfer to the host chromosome is rarely complete, but

can be used to map the order of genes on a bacterial genome.

Other means by which foreign genes can enter a bacte-

rial cell include transformation, transfection, and transduc-

tion. Of the three processes, transformation is probably the

most significant. Evidence of transformation in bacteria was

first obtained by the British scientist, Fred Griffith

(1881–1941) in the late 1920s working with Streptococcus

pneumoniaeand the process was later explained in the 1930s

by Oswald Avery (1877–1955) and his associates at the

Rockefeller Institute in New York. It was discovered that cer-

tain bacteria exhibit competence, a state in which cells are able

to take up free DNA released by other bacteria. This is the

process known as transformation, however, relatively few

microorganisms can be naturally transformed. Certain labora-

tory procedures were later developed that make it possible to

introduce DNA into bacteria, for example electroporation,

which modifies the bacterial membrane by treatment with an

electric field to facilitate DNA uptake. The latter two

processes, transfection and transduction, involve the participa-

tion of viruses for nucleic acid transfer. Transfection occurs

when bacteria are transformed with DNA extracted from a

bacterial virus rather than from another bacterium.

Transduction involves the transfer of host genes from one bac-

terium to another by means of viruses. In generalized trans-

duction, defective virus particles randomly incorporate

fragments of the cell DNA; virtually any gene of the donor can

be transferred, although the efficiency is low. In specialized

transduction, the DNA of a temperate virus excises incorrectly

and brings adjacent host genes along with it. Only genes close

to the integration point of the virus are transduced, and the

efficiency may be high.

After the discovery of DNA transfer in bacteria, bacte-

ria became objects of great interest to geneticists because their

rate of reproduction and mutation is higher than in larger

organisms; i.e., a mutation occurs in a gene about one time in

10,000,000 gene duplications, and one bacterium may produce

10,000,000,000 offspring in 48 hours. Conjugation, transfor-

mation, and transduction have been important methods for

mapping the genes on the chromosomesof bacteria. These

techniques, coupled with restriction enzyme analysis, cloning

DNA sequencing, have allowed for the detailed studies of the

bacterial chromosome. Although there are few rules governing

gene location, the genes encoding enzymes for many bio-

chemical pathways are often found tightly linked in operons in

prokaryotes. Large scale sequencing projects revealed the

complete DNA sequence of the genomes of several prokary-

otes, even before eukaryotic genomes were considered.

See alsoBacterial growth and division; Bacteriophage and

bacteriophage typing; Cell cycle (eukaryotic), genetic regula-

tion of; Cell cycle (prokaryotic), genetic regulation of; Fungal

genetics; Mutations and mutagenesis; Viral genetics; Viral

vectors in gene therapy

MMicrobial symbiosisICROBIAL SYMBIOSIS

Symbiosis is generally defined as a condition where two dis-

similar organisms live together in an intimate associate that

sees both organisms benefit. Microbial symbiosis tends to be

bit broader in definition, being defined as the co-existence of

two microorganisms.

Microbial symbiosis can be evident as several different

patterns of co-existence. One pattern is known as mutualism. In

this relationship, both organisms benefit. Another type of rela-

tionship is called commensalism. Here the relationship is ben-

eficial to one of the organisms and does no harm to the other.

Another relationship known as parasitism produces a

benefit to one organism at the expense of the other organism.

Parasitism is not considered to be a symbiosis between a

microorganism and the host.

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