Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Viral genetics

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Cattle and sheep are also susceptible to microbiological

ailments. Foot and mouth disease is a prominent example.

This contagious and fatal disease can sweep through cattle and

sheep populations, causing financial ruin for ranchers.

Moreover, there is now evidence that bovine spongiform

encephalopathy, a disease caused by an infectious agent

termed a prion, may be transmissible to humans, where it is

manifest as the always lethal brain deterioration called

Creutzfeld-Jacob disease.

See alsoZoonoses

VViable but nonculturable bacteriaIABLE BUT NONCULTURABLE BACTERIA

Viable but nonculturable bacteriaare bacteria that are alive,

but which are not growing or dividing. Their metabolic activ-

ity is almost nonexistent.

This state was recognized initially by microbial ecolo-

gists examining bacterial populations in natural sediments.

Measurements of the total bacterial count, which counts both

living and dead bacteria, are often far higher than the count of

the living bacteria. At certain times of year, generally when

nutrients are plentiful, the total and living numbers match more

closely. These observations are not the result of seasonal “die-

off,” but reflect the adoption of an almost dormant mode of

existence by a sizable proportion of some bacterial populations.

A viable but nonculturable bacteria cannot be cultured

on conventional laboratory growth media but can be demon-

strated to be alive by other means, such as the uptake and

metabolismof radioactively labeled nutrients. Additionally,

the microscopic examination of populations shows the bacte-

ria to be intact. When bacteria die they often lyse, due to the

release of enzymesthat disrupt the interior and the cell wall of

the bacteria.

The viable but nonculturable state is reversible.

Bacterial that do not form spores can enter the state when con-

ditions become lethal for their continued growth. The state is

a means of bacterial survival to stresses that include elevated

salt concentration, depletion of nutrients, depletion of oxygen,

and exposure to certain wavelengths of light. When the stress

is removed, bacteria can revive and resume normal growth.

The shift to the nonculturable state triggers the expres-

sion of some 40 genes in bacteria. As well, the composition of

the cell wall changes, becoming enhanced in fatty acid con-

stituents, and the genetic material becomes coiled more tightly.

The entry of a bacterium into the nonculturable state

varies from days to months. Younger bacterial cells are capa-

ble of a more rapid transition than are older cells. In general,

however, the transition to a nonculturable state seems to be in

response to a more gradual change in the environment than

other bacterial stress responses, (e.g., spore formation, heat

shock response).

In contrast to the prolonged entry into the quiescent

phase, the exit from the viable but nonculturable state is quite

rapid (within hours for Vibrio vulnifucus). Other bacteria, such

as Legionella pneumophila, the causative agent of

Legionnaires’ disease, revive much more slowly. The adoption

of this mode of survival by disease-causing bacteria further

complicates strategies to detect and eradicate them.

See alsoBacteria and bacterial infection; Bacterial adaptation

VIRAL EPIDEMICS•seeEPIDEMICS, VIRAL

VViral geneticsIRAL GENETICS

Viral genetics, the study of the genetic mechanisms that oper-

ate during the life cycle of viruses, utilizes biophysical, bio-

logical, and genetic analyses to study the viral genome and its

variation. The virus genome consists of only one type of

nucleic acid, which could be a single or double stranded DNA

or RNA. Single stranded RNA viruses could contain positive-

sense (+RNA), which serves directly as mRNA or negative-

sense RNA (–RNA) that must use an RNA polymerase to

synthesize a complementary positive strand to serve as

mRNA. Viruses are obligate parasitesthat are completely

dependant on the host cell for the replication and transcription

of their genomes as well as the translationof the mRNA tran-

scripts into proteins. Viral proteins usually have a structural

function, making up a shell around the genome, but may con-

tain some enzymesthat are necessary for the virus replication

and life cycle in the host cell. Both bacterial virus (bacterio-

phages) and animal viruses play an important role as tools in

molecular and cellular biology research.

Viruses are classified in two families depending on

whether they have RNA or DNA genomes and whether these

genomes are double or single stranded. Further subdivision into

types takes into account whether the genome consists of a sin-

gle RNA molecule or many molecules as in the case of seg-

mented viruses. Four types of bacteriophages are widely used in

biochemical and genetic research. These are the T phages, the

temperate phages typified by bacteriophagelambda, the small

DNA phages like M13, and the RNA phages. Animal viruses are

subdivided in many classes and types. Class I viruses contain a

single molecule of double stranded DNA and are exemplified

by adenovirus, simian virus 40 (SV40), herpesviruses and

human papilloma viruses. Class II viruses are also called par-

voviruses and are made of single stranded DNA that is copied

in to double stranded DNA before transcription in the host cell.

Class III viruses are double stranded RNA viruses that have seg-

mented genomes which means that they contain 10–12 separate

double stranded RNA molecules. The negative strands serve as

template for mRNA synthesis. Class IV viruses, typified by

poliovirus, have single plus strand genomic RNA that serves as

the mRNA. Class V viruses contain a single negative strand

RNA which serves as the template for the production of mRNA

by specific virus enzymes. Class VI viruses are also known as

retrovirusesand contain double stranded RNA genome. These

viruses have an enzyme called reverse transcriptase that can

both copy minus strand DNA from genomic RNA catalyze the

synthesis of a complementary plus DNA strand. The resulting

double stranded DNA is integrated in the host chromosome and

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