Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Viruses and responses to viral infection

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influenza viruses; on other enveloped viruses, the spikes are

extremely difficult to see. The spikes help the virus invade

host cells. The influenza virus, for instance, has two types of

spikes. One type, composed of hemagglutininprotein (HA),

fuses with the host cell membrane, allowing the virus particle

to enter the cell. The other type of spike, composed of the pro-

tein neuraminidase (NA), helps the newly formed virus parti-

cles to bud out from the host cell membrane.

The capsid of viruses is relatively simple in structure,

owing to the few genes that the virus contains to encode the

capsid. Most viral capsids consist of a few repeating protein

subunits. The capsid serves two functions: it protects the viral

genetic material and it helps the virus introduce itself into the

host cell. Many viruses are extremely specific, targeting only

certain cells within the plant or animal body. HIV, for instance,

targets a specific immune cell, the T helper cell. The cold virus

targets respiratory cells, leaving the other cells in the body

alone. How does a virus “know” which cells to target? The

viral capsid has special receptors that match receptors on their

targeted host cells. When the virus encounters the correct

receptors on a host cell, it “docks” with this host cell and

begins the process of infection and replication.

Most viruses are rod-shaped or roughly sphere-shaped.

Rod-shaped viruses include tobacco mosaic virusand the

filoviruses. Although they look like rods under a microscope,

these viral capsids are actually composed of protein molecules

arranged in a helix. Other viruses are shaped somewhat like

spheres, although many viruses are not actual spheres. The

capsid of the adenovirus, which infects the respiratory tract of

animals, consists of 20 triangular faces. This shape is called an

icosahedron. HIV is a true sphere, as is the influenza virus.

Some viruses are neither rod- nor sphere-shaped. The

poxviruses are rectangular, looking somewhat like bricks.

Parapoxviruses are ovoid. Bacteriophages are the most unusu-

ally shaped of all viruses. Abacteriophageconsists of a head

region attached to a sheath. Protruding from the sheath are tail

fibers that dock with the host bacterium. Bacteriophage struc-

ture is eminently suited to the way it infects cells. Instead of

the entire virus entering the bacterium, the bacteriophage

injects its genetic material into the cell, leaving an empty cap-

sid on the surface of the bacterium.

Viruses are obligate intracellular parasites, meaning

that in order to replicate, they need to be inside a host cell.

Viruses lack the machinery and enzymesnecessary to repro-

duce; the only synthetic activity they perform on their own is

to synthesize their capsids.

The infection cycle of most viruses follows a basic pat-

tern. Bacteriophages are unusual in that they can infect a bac-

terium in two ways (although other viruses may replicate in

these two ways as well). In the lytic cycle of replication, the

bacteriophage destroys the bacterium it infects. In the lyso-

genic cycle, however, the bacteriophage coexists with its bac-

terial host and remains inside the bacterium throughout its life,

reproducing only when the bacterium itself reproduces.

An example of a bacteriophage that undergoes lytic

replication inside a bacterial host is the T4 bacteriophage,

which infects E. coli.T4 begins the infection cycle by docking

with an E. colibacterium. The tail fibers of the bacteriophage

make contact with the cell wall of the bacterium, and the bac-

teriophage then injects its genetic material into the bacterium.

Inside the bacterium, the viral genes are transcribed. One of

the first products produced from the viral genes is an enzyme

that destroys the bacterium’s own genetic material. Now the

virus can proceed in its replication unhampered by the bacter-

ial genes. Parts of new bacteriophages are produced and

assembled. The bacterium then bursts, and the new bacterio-

phages are freed to infect other bacteria. This entire process

takes only 20–30 minutes.

In the lysogenic cycle, the bacteriophage reproduces its

genetic material but does not destroy the host’s genetic mate-

rial. The bacteriophage called lambda, another E. coli-infect-

ing virus, is an example of a bacteriophage that undergoes

lysogenic replication within a bacterial host. After the viral

DNA has been injected into the bacterial host, it assumes a cir-

cular shape. At this point the replication cycle can become

either lytic or lysogenic. In a lysogenic cycle the circular DNA

attaches to the host cell genome at a specific place. This com-

bination host-viral genome is called a prophage. Most of the

viral genes within the prophage are repressed by a special

repressor protein, so they do not encode the production of new

bacteriophages. However, each time the bacterium divides, the

viral genes are replicated along with the host genes. The bac-

terial progeny are thus lysogenically infected with viral genes.

Interestingly, bacteria that contain prophages can be

destroyed when the viral DNA is suddenly triggered to

undergo lytic replication. Radiation and chemicals are often

the triggers that initiate lytic replication. Another interesting

aspect of prophages is the role they play in human diseases.

The bacteria that cause diphtheriaand botulismboth harbor

viruses. The viral genes encode powerful toxins that have dev-

astating effects on the human body. Without the infecting

viruses, these bacteria may well be innocuous. It is the pres-

ence of viruses that makes these bacterial diseases so lethal.

Scientists have classified viruses according to the type

of genetic material they contain. Broad categories of viruses

include double-stranded DNA viruses, single-stranded DNA

viruses, double-stranded RNA viruses, and single stranded

RNA viruses. For the description of virus types that follows,

however, these categories are not used. Rather, viruses are

described by the type of disease they cause.

Poxviruses are the most complex kind of viruses

known. They have large amounts of genetic material and fib-

rils anchored to the outside of the viral capsid that assist in

attachment to the host cell. Poxviruses contain a double strand

of DNA.

Viruses cause a variety of human diseases, including

smallpox and cowpox. Because of worldwide vaccination

efforts, smallpox has virtually disappeared from the world,

with the last known case appearing in Somalia in 1977. The

only places on Earth where smallpox virus currently exists are

two labs: the Centers for Disease Controlin Atlanta and the

Research Institute for Viral Preparation in Moscow. Prior to

the eradication efforts begun by the World Health Organization

in 1966, smallpox was one of the most devastating of human

diseases. In 1707, for instance, an outbreak of smallpox killed

18,000 of Iceland’s 50,000 residents. In Boston in 1721,

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