Microbiology and Immunology

Plaque WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Plant viruses may also be transmitted to a new plant host

via infected seeds from another plant. In the laboratory, viral

DNA can be introduced into the bacterium Agrobacterium

tumefaciens. When the bacterium infects a plant, the viral DNA

can be incorporated into the plant genome. Experimental infec-

tion of plants can be done by rubbing virus preparation into the

leaves of the plant. The virus can enter the plant through the

physical abrasion that is introduced.

As humans can mount an immune response against viral

infection, so plants have defense strategies. One strategy is the

presence of a tough cell wall on many plants that restricts the

entry of viruses unless the surface barrier of the plant is com-

promised, as by injury. Many plants also display a response

that is termed hypersensitivity. In this response the plant cells

in the vicinity of the infected cell die. This acts to limit the

spread of the virus, since the virus require living cells in which

to replicate.

Some plants have been shown to be capable of warn-

ing each other of the presence of a viral infection. This com-

munication is achieved by the airborne release of a specific

compound. This behavior is similar to the cell to cell signal-

ing found in bacterial populations, which is known as quo-

rum sensing.

See alsoViral genetics; Virology

PPlaqueLAQUE

Plaque is the diverse community of microorganisms, mainly

bacteria, which develops naturally on the surface of teeth. The

microbes are cocooned in a network of sugary polymers pro-

duced by the bacteria, and by host products, such as saliva,

epithelial and other host cells, and inorganic compounds such

as calcium. The surface-adherent, enmeshed community of

plaque represents a biofilm.

Plaque is important for two reasons, one beneficial and

the other detrimental. The beneficial aspect of dental plaque is

that the coverage of the tooth surface by microbes that are nor-

mally resident in the host can exclude the colonization of the

tooth by extraneous bacteria that might be harmful. This phe-

nomenon is known as competitive exclusion. However,

despite this benefit, the plaque can position acid-producing

bacteria near the tooth and protect those bacteria from

attempts to kill or remove them. Plaque can become extremely

hard, as the constituent inorganic components create a crys-

talline barrier. Protected inside the plaque, the acid-producing

bacteria can dissolve the tooth enamel, which can lead to the

production of a cavity.

A plaque is a complex community, consisting of hun-

dreds of species of bacteria. Plaque formation generally begins

with the adherence of certain bacteria, such as Streptococcus

sanguis, Streptococcus mutans, and Actinomyces viscosus.

Then, so-called secondary colonizers become established.

Examples include Fusobacterium nucleatumand Prevotella

intermedia. As the plaque matures, a varied variety of other

bacteria can colonize the tooth surface.

Maturation of the plaque is associated with a shift in the

type of bacteria that are predominant. Gram-positive bacteria

that can exist in the presence or absence of oxygen give way

to gram negative bacteria that require the absence of oxygen.

Depending on how the community evolves, the plaque

can become problematic in terms of a cavity. Even within the

plaque, there are variations in the structure and bacterial com-

position. Thus, even though one region of the plaque is rela-

tively benign is no guarantee that another region will house

detrimental bacteria.

The prevalence of acid-producing bacteria is related to

the diet. A diet that is elevated in the types of sugar typically

found in colas and candy bars will lower the pHin the plaque.

The lowered pH is harsh on all organisms except the acid-pro-

ducing bacteria. Most dentists assert that a diet that contains

less of these sugars, combined with good oral hygiene, will

greatly minimize the threat posed by plaque and will empha-

size the benefit of the plaque’s presence.

See alsoBacteria and bacterial infection; Biofilm formation

and dynamic behavior; Microbial flora of the oral cavity, den-

tal caries

PPlasmidsLASMIDS

Plasmidsare extra-chromosomal, covalently closed circular

(CCC) molecules of double stranded (ds) DNAthat are capa-

ble of autonomous replication. The prophages of certain bac-

terial phages and some dsRNA elements in yeastare also

called plasmids, but most commonly plasmids refer to the

extra-chromosomal CCC DNA in bacteria.

Plasmids are essential tools of genetic engineering.

They are used as vectors in molecular biologystudies.

Plasmids are widely distributed in nature. They are dis-

pensable to their host cell. They may contain genes for a vari-

ety of phenotypic traits, such as antibiotic resistance,

virulence, or metabolic activities. The products plasmids

encode may be useful in particular conditions of bacterial

growth. Replication of plasmid DNA is carried out by subsets

of enzymesused to duplicate the bacterial chromosome and

is under the control of plasmid’s own replicon. Some plas-

mids reach copy numbers as high as 700 per cell, whereas

others are maintained at the minimal level of 1 plasmid per

cell. One particular type of plasmid has the ability to transfer

copies of itself to other bacterial stains or species. These plas-

mids have a tra operon. Tra operon encodes the protein that is

the component of sex pili on the surface of the host bacteria.

Once the sex pili contact with the recipient cells, one strand

of the plasmid is transferred to the recipient cells. This plas-

mid can integrate into the host chromosomal DNA and trans-

fer part of the host DNA to the recipient cells during the next

DNA transfer process.

Ideally, plasmids as vectors should have three charac-

teristics. First, they should have a multiple cloning site

(MSC) which consists of multiple unique restriction enzyme

sites and allows the insertion of foreign DNA. Second, they

should have a relaxed replication control that allows suffi-

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