Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Adenoviruses

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can be very useful in, for example, evaluating the effect of an

antibacterial agent on the survival of bacteria.

Acridine orange is utilized in the specialized type of

light microscopic technique called fluorescence microscopy.

In addition, fluorescence of DNA or RNA can allow cells in a

sample to be differentiated using the technique of flow cytom-

etry. This sort of information allows detailed analysis of the

DNA replication cycle in microorganisms such as yeast.

See alsoLaboratory techniques in microbiology

AActinomycesCTINOMYCES

Actinomycesis a genus of bacteria. The bacteria that grouped

in this genus share several characteristics. The bacteria are

rod-like in shape. Under the light microscope, Actinomyces

appear fungus-like. They are thin and joined together to form

branching networks. Bacteria of this genus retain the primary

stain in the Gram stain reaction, and so are classified as being

Gram positive. Actinomycetesare not able to form the dormant

form known as a spore. Finally, the bacteria are able to grow

in the absence of oxygen.

Members of the genus Actinomycesare normal residents

of the mouth, throat, and intestinal tract. But they are capable

of causing infections both in humans and in cattle if they are

able to enter other regions. This can occur as the result of an

accident such as a cut or abrasion.

An infection known as Actinomycosisis characterized

by the formation of an abscess—a process “walling off” the

site of infection as the body responds to the infection—and by

swelling. Pus can also be present. The pus, which is composed

of dead bacteria, is granular, because of the presence of gran-

ules of sulfur that are made by the bacteria.

The diagnosis of an Actinomycesinfection can be chal-

lenging, as the symptoms and appearance of the infection is

reminiscent of a tumor or of a tuberculosislesion. A well-

established infection can produce a great deal of tissue dam-

age. Additionally, the slow growth of the bacteria can make

the treatment of infection with antibiotics very difficult,

because antibiotics rely on bacterial growthin order to exert

their lethal effect.

The culturing of Actinomycesin the laboratory is also

challenging. The bacteria do not grow on nonselective media,

but instead require the use of specialized and nutritionally

complex selective media. Furthermore, incubation needs to be

in the absence of oxygen. The growth of the bacteria is quite

slow. Solid growth medium may need to be incubated for up

to 14 days to achieve visible growth. In contrast, a bacterium

like Escherichia coliyields visible colonies after overnight

growth on a variety of nonselective media. The colonies of

Actinomyces are often described as looking like bread

crumbs.

Currently, identification methods such as polymerase

chain reaction(PCR), chromatography to detect unique cell

wall constituents, and antibody-based assays do always per-

form effectively with Actinomyces.

See alsoAnaerobes and anaerobic infections; Microbial flora

of the oral cavity, dental caries

ACTIVE TRANSPORT•seeCELL MEMBRANE TRANS-

PORT

AAdenovirusesDENOVIRUSES

Adenoviruses are viruseswhich have twenty sides. As such

they are called icosahedrons. The outer surface, the capsid, is

made of particles of a protein. The protein is arranged in

groups of six (hexagons) except at the twenty points where

the sides meet (each is called an apex), where the particles

are in a pentagon arrangement. A so-called penton fibre,

which resembles a stick with a ball at the end, protrudes from

each apex.

Adenoviruses contain deoxyribonucleic acid(DNA) as

their genetic material. The DNAencodes 20 to 30 proteins, 15

of which are proteins that form the structure of the virus par-

ticle. Similar to other viruses, adenoviruses invade a host cell

and use the host genetic machinery to manufacture new virus

particles. The new viruses are released from the host cell.

Children suffer from adenovirus infections much more

so than adults.

The viruses of this group infect the membranes that

line the respiratory tract, the eyes, the intestines, and the uri-

nary tract. The adenoviruses that infect humans usually

cause mild maladies, including respiratory and intestinal ill-

nesses and conjunctivitis (an inflammationof eye membrane,

which is also commonly called “pink eye”). A more severe

eye malady called keratoconjunctivitis can more widely

infect the eye. The eye infectionsare very contagious and are

typically a source of transmission of adenovirus from one

person to another. Children can also develop a sore throat,

runny nose, cough and flu-like illness. Bronchitis, an inflam-

mation of the membranes lining the air passages in the lungs,

can also result from adenovirus infection, as can an inflam-

mation of the stomach called gastroenteritis. Urinary tract

infections can cause pain and burning upon urination and

blood in the urine. In dogs, adenovirus type 2 causes what is

known as kennel cough. But curiously, the virus also protects

dogs against hepatitis.

In the setting of the laboratory, some of the human

strains of adenovirus can transform cells being grown in cell

culture. Transformed cells are altered in their regulation of

growth, such that the unrestricted growth characteristic of can-

cers occurs.

Adenoviruses have been known since the mid-1950s.

They were first isolated from infected tonsils and adenoidal

tissue in 1953. Within the next several years they had been

obtained from cells involved in respiratory infections. In 1956,

the multiple antigenic forms of the virus that had been discov-

ered were classified as adenovirus. Then, in 1962, laboratory

studies demonstrated that an adenovirus caused tumors in

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