MICROBIAL MOLECULAR BIOLOGY AND GENETICS
Conjugation, the transfer of DNA between bacteria involving direct contact, depends on the presence of an “extra” piece of circular DNA known as a plasmid. Plasmids play many important
roles in the lives of bacteria. They also have proved invaluable to
microbiologists and molecular geneticists in constructing and transferring new genetic combinations and in cloning genes.
Plasmids are small double-stranded DNA molecules, usually circular, that can exist independently of host chromosomes
and are present in many bacteria (they are also present in some
yeasts and other fungi). They have their own replication origins
and are autonomously replicating and stably inherited. A replicon is a DNA molecule or sequence that has a replication origin and is capable of being replicated. Plasmids and bacterial
chromosomes are separate replicons. Plasmids have relatively
few genes, generally less than 30. Their genetic information is
not essential to the host, and bacteria that lack them usually
function normally. Single-copy plasmids produce only one
copy per host cell. Multicopy plasmids may be present at concentrations of 40 or more per cell.
Characteristically, plasmids can be eliminated from host
cells in a process known as curing.Curing may occur spontaneously or be induced by treatments that inhibit plasmid replication while not affecting host cell reproduction. The inhibited plasmids are slowly diluted out of the growing bacterial population.
Some commonly used curing treatments are acridine mutagens,
UV and ionizing radiation, thymine starvation, and growth above optimal temperatures.
Plasmids may be classified in terms of their mode of existence and spread. An episome is a plasmid that can exist either with or without being integrated into the host’s chromosome. Some plasmids,conjugative plasmids,have genes for pili and can transfer copies of themselves to other bacteria during conjugation.
A plasmid called the fertility or F factor plays a major role in
conjugation in E. coli and was the first to be described. The F factor is about 100 kilobases long and bears genes
responsible for cell attachment and plasmid transfer between specific bacterial strains during conjugation. Most of the information
required for plasmid transfer is located in the tra operon, which
contains at least 28 genes. Many of these direct the formation of
sex pili that attach the F cell (the donor cell containing an F plasmid) to an F cell.
Other gene products aid DNA transfer.
The F factor also has several segments called insertion sequence that assist plasmid integration into the host
cell chromosome. Thus the F factor is an episome that can exist outside the bacterial chromosome or be integrated into it.
Virulence Plasmids and Disease
It is becoming increasingly evident that many bacteria are pathogenic because of their plasmids. These plasmids can carry genes for
toxins, render the bacterium better able to establish itself in the
host, or aid in resistance to host defenses. E.coli provides the beststudied example of virulence plasmids. Several strains of E. coli cause
diarrhea. The enterotoxigenic strains responsible for traveler’s diarrhea
can produce two toxins:a heat-labile toxin (LT), which is a large protein
very similar in structure and mechanism of action to cholera toxin, and a heat-stable toxin (ST), a low molecular weight
polypeptide. Both toxin genes are plasmid borne, and sometimes they are
even carried by the same plasmid. The ST toxin gene is located on a
transposon. Enterotoxigenic strains of E. coli also must be able to colonize the epithelium of the small intestine to cause diarrhea. This is made
possible by the presence of special adhesive fimbriae encoded by genes
on another plasmid. A second type of pathogenic E. coli invades the intestinal epithelium and causes a form of diarrhea very similar to the
dysentery resulting from a Shigella infection. This E.coli strain and Shigella contain virulence plasmids that code for special cell wall antigens and other factors enabling them to enter and destroy epithelial cells.
Some E. coli strains can invade the blood and organs of a host, causing a generalized infection. These pathogens often have ColV plasmids
and produce colicin V. The ColV plasmid carries genes for two virulence
determinants. One product increases bacterial resistance to host defense
mechanisms involving complement . The
other plasmid gene directs the synthesis of a hydroxamate that enables E.coli
to accumulate iron more efficiently from its surrounding. Since iron is not readily available in the animal host, but is essential
for bacterial growth, this is an important factor in pathogenicity.
Several other pathogens carry virulence plasmids. Some Staphylococcus aureusstrains produce an exfoliative toxin that is plasmid borne.
The toxin causes the skin to loosen and often peel off in sheets, leading
to the disease staphylococcal scalded skin syndrome .
Other plasmid-borne toxins are the tetanus toxin of Clostridium tetani
and the anthrax toxin of Bacillus anthracis.
Plasmids often confer antibiotic resistance on the bacteria that
contain them. R factors or plasmids typically have genes that
code for enzymes capable of destroying or modifying antibiotics.
They are not usually integrated into the host chromosome. Genes
coding for resistance to antibiotics such as ampicillin, chloramphenicol, and kanamycin have been found in plasmids. Some R
plasmids have only a single resistance gene, whereas others can
have as many as eight. Often the resistance genes are within a
transposon , and thus it is possible for bacterial strains to rapidly develop multiple resistance plasmids.
Because many R factors also are conjugative plasmids, they
can spread throughout a population, although not as rapidly as
the F factor. Often, nonconjugative R factors also move between
bacteria during plasmid promoted conjugation. Thus a whole
population can become resistant to antibiotics. The fact that
some of these plasmids are readily transferred between species
further promotes the spread of resistance. When the host consumes large quantities of antibiotics,E. coli and other bacteria
with R factors are selected for and become more prevalent. The
R factors can then be transferred to more pathogenic genera such
as Salmonella or Shigella,causing even greater public health
problems by forming channels in the plasma membrane, thus increasing its
permeability. They also may degrade DNA and RNA or attack
peptidoglycan and weaken the cell wall. Col plasmids contain
genes for the synthesis of bacteriocins known as colicins, which
are directed against E. coli. Similar plasmids carry genes for bacteriocins against other species. For example, Col plasmids produce cloacins that kill Enterobacter species. Clearly the host is
unaffected by the bacteriocin it produces. Some Col plasmids are
conjugative and also can carry resistance genes.
Other Types of Plasmids
Several other important types of plasmids have been discovered.
Some plasmids, called virulence plasmids,make their hosts
more pathogenic because the bacterium is better able to resist
host defense or to produce toxins. For example, enterotoxigenic
strains of E. coli cause traveler’s diarrhea because of a plasmid
that codes for an enterotoxin . Metabolic plasmids
carry genes for enzymes that degrade substances such as aromatic
compounds (toluene), pesticides (2,4-dichlorophenoxyacetic
acid), and sugars (lactose). Metabolic plasmids even carry the
genes required for some strains of Rhizobium to induce legume nodulation and carry out nitrogen fixation.
Please refer this link:
www.boundless.com › Microbiology › Microbial Genetics › Plasmids
Cited By Kamal Singh Khadka
Msc Microbiology, TU
Assistant Professor At Pokhara University(PU), Regional College Of Science &Technology(RECOST), PNC, LA, NA