|
|
|
|
|
|
|
Acta Medica Medianae
|
BACTERIAL PLASMIDS
Biljana
Miljkovic-Selimovic, Tatjana Babic, Branislava Kocic, Predrag
Stojanovic, Ljiljana Ristic and Marina Dinic Public Health Institute Nis, Clinical Center Nis
Plasmids,
extrachromosomal DNA, were identified in bacteria pertaining to
family of Enterobacteriacae for the very first time. After that,
they were discovered in almost every single observed strain. The
structure of plasmids is made of circular double chain DNA molecules
which are replicated autonomously in a host cell. Their length may
vary from few up to several hundred kilobase (kb). Among the
bacteria, plasmids are mostly transferred horizontally by
conjugation process. Plasmid replication process can be divided into
three stages: initiation, elongation, and termination. The process
involves DNA helicase I, DNA gyrase, DNA polymerase III,
endonuclease, and ligase. Plasmids contain genes essential for
plasmid function and their preservation in a host cell (the
beginning and the control of replication). Some of them possess
genes which control plasmid stability. There is a common opinion
that plasmids are unnecessary for a growth of bacterial population
and their vital functions; thus, in many cases they can be taken up
or kicked out with no lethal effects to a plasmid host cell.
However, there are numerous biological functions of bacteria related
to plasmids. Plasmids identification and classification are based
upon their genetic features which are presented permanently in all
of them, and these are: abilities to preserve themselves in a host
cell and to control a replication process. In this way, plasmids
classification among incompatibility groups is performed. The method
of replicon typing, which is based on genotype and not on phenotype
characteristics, has the same results as incompatibility grouping.
Acta Medica Medianae 2007;46(4):61-65. Key words: bacteria, plasmid |