Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Bacteriophage and bacteriophage typing

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tidoglycanlayer of Gram-positive and Gram-negative bacte-

ria. By preventing the assembly of the peptidoglycan, peni-

cillin antibiotics destroy the ability of the peptidoglycan to

bear the stress of osmotic pressure that acts on a bacterium.

The bacterium ultimately explodes. Other antibiotics are lethal

because they prevent the manufacture of DNA or protein.

Unlike bacteriocidal methods such as the use of heat, bacteria

are able to acquire resistance to antibiotics. Indeed, such

resistance by clinically important bacteria is a major problem

in hospitals.

Bacteriostatic agents prevent the growth of bacteria.

Refrigeration can be bacteriostatic for those bacteria that can-

not reproduce at such low temperatures. Sometimes a bacte-

riostatic state is advantageous as it allows for the long-term

storage of bacteria. Ultra-low temperature freezing and

lyophilization (the controlled removal of water from a sample)

are means of preserving bacteria. Another bacteriocidal tech-

nique is the storage of bacteria in a solution that lacks nutri-

ents, but which can keep the bacteria alive. Various buffers

kept at refrigeration temperatures can keep bacteria alive for

weeks.

See alsoBacterial growth and division; Disinfection and dis-

infectants; Laboratory techniques in microbiology

BACTERIOLOGY• seeBACTERIA AND BACTERIAL

INFECTION

BACTERIOPHAGE AND BACTERIOPHAGE

TYPINGBacteriophage and bacteriophage typing

A bacteriophage, or phage, is a virus that infects a bacterial

cell, taking over the host cell’s genetic material, reproducing

itself, and eventually destroying the bacterium. The word

phage comes from the Greek word phagein, meaning “to eat.”

Bacteriophages have two main components, protein coat and a

nucleic acid core of DNAor RNA. Most DNA phages have dou-

ble-stranded DNA, whereas phage RNA may be double or sin-

gle-stranded. The electron microscopeshows that phages vary

in size and shape. Filamentous or threadlike phages, discov-

ered in 1963, are among the smallest viruses known.

Scientists have extensively studied the phages that infect

Escherichia coli(E.coli), bacteriathat are abundant in the

human intestine. Some of these phages, such as the T4 phage,

consist of a capsid or head, often polyhedral in shape, that con-

tains DNA, and an elongated tail consisting of a hollow core,

a sheath around it, and six distal fibers attached to a base plate.

When T4 attacks a bacterial cell, proteins at the end of the tail

fibers and base plate attach to proteins located on the bacterial

wall. Once the phage grabs hold, its DNA enters the bacterium

while its protein coat is left outside.

Double stranded DNA phages reproduce in their host

cells in two different ways: the lytic cycle and the lysogenic

cycle. The lytic cycle kills the host bacterial cell. During the

lytic cycle in E.coli, for example, the phage infects the bacte-

rial cell, and the host cell commences to transcribe and trans-

late the viral genes. One of the first genes that it translates

encodes an enzyme that chops up the E.coliDNA. The host

now follows instructions solely from phage DNA which com-

mands the host to synthesize phages. At the end of the lytic

cycle, the phage directs the host cell to produce the enzyme,

lysozyme, that digests the bacterial cell wall. As a result, water

enters the cell by osmosis and the cell swells and bursts. The

destroyed or lysed cell releases up to 200 phage particles ready

to infect nearby cells. On the other hand, the lysogenic cycle

does not kill the bacterial host cell. Instead, the phage DNA is

incorporated into the host cell’s chromosome where it is then

called a prophage. Every time the host cell divides, it repli-

cates the prophage DNA along with its own. As a result, the

two daughter cells each contain a copy of the prophage, and

the virus has reproduced without harming the host cell. Under

certain conditions, however, the prophage can give rise to

active phages that bring about the lytic cycle.

In 1915, the English bacteriologist Frederick Twort

(1877–1950) first discovered bacteriophages. While attempt-

ing to grow Staphylococcus aureus, the bacteria that most

often cause boils in humans, he observed that some bacteria in

his laboratory plates became transparent and died. Twort iso-

lated the substance that was killing the bacteria and hypothe-

sized that the agent was a virus. In 1917, the French-Canadian

scientist Felix H. d’Hérelleindependently discovered bacterio-

phages as well. The significance of this discovery was not

appreciated, however, until about thirty years later when sci-

entists conducted further bacteriophage research. One promi-

nent scientist in the field was Salvador E. Luria (1912–1991),

an Italian-American biologist especially interested in how x

rays cause mutationsin bacteriophages. Luria was also the

first scientist to obtain clear images of a bacteriophage using

an electron microscope. Salvador Luria emigrated to the

United States from Italy and soon met Max Delbruck

(1906–1981), a German-American molecular biologist. In the

1940s, Delbruck worked out the lytic mechanism by which

some bacteriophages replicate. Together, Luria, Delbruck and

the group of researchers that joined them studied the genetic

changes that occur when viruses infect bacteria. Until 1952,

scientists did not know which part of the virus, the protein or

the DNA, carried the information regarding viral replication.

It was then that scientists performed a series of experiments

using bacteriophages. These experiments proved DNA to be

the molecule that transmits the genetic information. (In 1953,

the Watsonand Crickmodel of DNA explained how DNA

encodes information and replicates). For their discoveries con-

cerning the structure and replication of viruses, Luria,

Delbruck, and Hersheyshared the Nobel Prize for physiology

or medicine in 1969. In 1952, two American biologists,

Norton Zinder and Joshua Lederberg at the University of

Wisconsin, discovered that a phage can incorporate its genes

into the bacterial chromosome. The phage genes are then

transmitted from one generation to the next when the bac-

terium reproduces. In 1980, the English biochemist, Frederick

Sanger, was awarded a Nobel Prize for determining the

nucleotide sequence in DNA using bacteriophages.

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