Lecture on Bacterial Plasmid by Dr. Avizit Sarker

Microbiological context of Bacterial Plasmid

1. Bacterial Plasmid
Dr. Avizit Sarker
MBBS (DMC), MD (Microbiology), BCS (Health)
Department of Microbiology
Dhaka Medical College

2. History
•ThetermplasmidwasfirstintroducedbytheAmerican
molecularbiologistJoshuaLederbergin1952.
•HewasanAmericanmolecularbiologistknownforhis
workingenetics,artificialintelligenceandspace
exploration.
•HewontheNobelPrizeinPhysiologyorMedicinein
1958fordiscoveringthatbacteriacanmateand
exchangegenes.HesharedtheprizewithEdward
TatumandGeorgeBeadlewhowonfortheirwork
withgenetics.

3. Plasmid
•Aplasmidisasmall,circular,double-stranded
DNAmoleculewithinacellthatisphysically
separatedfromachromosomalDNAandcan
replicateindependently.
•Plasmidsarepresentinbacteria,archaea&
eukaryoticorganisms.

4. Properties of Plasmid
1.Small, circular, double-stranded DNA molecule can exist
independently of host chromosomes.
2.They have their own replication origins and are autonomously
replicating stably inherited.
3.It has relatively few genes. The size of the plasmid varies from
1-10 Kilo-base pair (kbp).
4.Plasmid can be transmitted from one bacterium to another.
5.Can be essential for specific environments: virulence,
antibiotics resistance, use of unusual nutrients, production of
bacteriocins.

5. Structure of Plasmid
•Origin of replication (replicon) -The origin of replication (ori) refers to a
specific location in the strand at which replication begins. For plasmids,
this location is largely composed of A-T base pairs that are easier to
separate during replication.
•Antibiotic resistance gene -The antibiotic resistance gene is one of the
main components of plasmids. These genes play an important role in drug
resistance (to one or more antibiotics) thus making treatment of some
diseases more challenging.
•Multiple cloning sites -Short segment of DNA which contains several
restriction sites allowing for the easy insertion of DNA.
•Promoter region -This is the component of plasmids thatisinvolved in
recruiting transcriptional machinery.

6. •Primer binding site -This is a short sequence of DNA on a single strand
that is typically used as an initiation point for the purposes of PCR
amplification or DNA sequencing.
•Insert -Gene, promoter or other DNA fragments cloned into the MCS for
further study.

7. Classification of Plasmid
A.Based on ability to perform conjugation
1.Conjugative plasmids
Conjugativeplasmidscontainasetoftransferortra
geneswhichpromotesexualconjugationbetween
differentcells.Inthecomplexprocessofconjugation,
plasmidmaybetransferredfromonebacteriumto
anotherviasexpiliencodedbysomeofthetragenes.
2. Non-conjugative plasmids
Non-conjugative plasmids are incapable of initiating
conjugation, hence they can be transferred only with
the assistance of conjugative plasmids.

8. B. Based on compatibility
1. Compatible -Microbes can contains different types of plasmids if they are compatible
to each other.
2. Incompatible -Can not co-exists in a microorganism.

9. C.Basedonfunction
1.FertilityorF-plasmids-Containstragenes.Theyarecapableofconjugationandexpresssexpili.
2.Resistance(R)plasmids-
Structure-
a)Resistancetransferfactor:carryingallgenesnecessaryforconjugaltransfer.
b)r-determinants:containingallormostoftheantibioticsresistancegenes.
Propertiesofrfactors-
•Responsibleofdrugresistance.
•Theycanbetransferrednotonlytothecellsofthesamespeciesbutalsotoother
species.
•Resistancetometalions.
•Resistancetocertainbacterialvirusbycodingforrestrictionendonucleasethat
degradetheDNAoftheinfectingbacteriophage.

10. 3. Col plasmids -contains genes that code for bacteriocins.
4. Degradative plasmids -they enable the host in various metabolic activities.
•Digestion of unusual substance, like toluene, salicylate etc.
•Urease synthesis.
•Nitrogen fixation.
5. Virulence plasmids -they code for certain virulence factors and toxins that
help in bacterial pathogenesis.
•Heat labile & heat stable toxin of E. coli.
•Siderophore production.
•Adherence antigens.

11. Bacterial cells depending on F factors :
•1. F+ cell

12. •2. Hfrcells (High frequency of recombination)
•Cells that contain F plasmid free in the cytoplasm (F+ cells) have no
unusual character apart from the ability to produce pili to transfer F to
F-cells by conjugation.
•In very small proportion of F+ cells, F factors become inserted into
bacterial chromosome.
•These type of cells are called Hfrcells.

13. •3. F’
(Prime)
cell

14. Phenotypic effects of plasmids
A. Resistance effect
1. Resistance to antibiotic.
2. Resistant to heavy metal -Arsenic, Mercury, Cobalt etc.
3. Resistant to anion -Aresnite, Telurite.
4. Resistant to UV ray.
B. Metabolic
1.Antibiotic production by Streptomyces.
2.Metabolism of CHO, Protein, Gelatine, Casine.
3.Nitrogen fixation.
4.Citrate utilization.
5.H2S production.
6.Leucine biosynthesis.

15. C. Pathogenic effect
1. Antibiotic resistance & bacteriocin production
2. Toxin production
3. Colonization antigen
4. Tumorigenicity
D. Conjugal effect
1. Sex pili formation
2. Surface exclusion
3. Fertility inhibition

16. Application of Plasmid
Plasmid as a vector
•A vector is a DNA molecule used to carry genes from organism to organism.
Plasmid have an origin of replication, multiple cloning site, a selectable
marker gene and used in genetic engineering.
Protein production
•Human insulin production by recombinant DNA technology.
DNA vaccine
•Plasmid DNA is the best for promising DNA vaccine & gene therapies
against many infections, acquired & genetic disease including HIV-AIDS,
Malaria, different types of cancer, enteric pathogens & influenza.

18. Question
•1. Classification of plasmid.
•2. What is R factor? Properties of R factor.
•3. Application of plasmid in genetic engineering.