VMC 211: General Veterinary Microbiology (1+1)

PLASMIDS

• Plasmids are double stranded, closed circular DNA molecules capable of autonomous replication. One exception to circularity is the linear plasmid present in Borrelia burgdorferi. Depending on number copies present in the cell plasmids are called low copy number and high copy number plasmids.
• Episomes are plasmids that can integrate into bacterial chromosome. F factor is an example of episome.
• Size of plasmids: 1-200 kb
• Conjugative plasmid: A plasmid that can mediate its own transfer to a new strain.
• Non conjugative plasmid: A plasmid which cannot mediate its own transfer to a new strain.
• Cryptic plasmids: They have no identifiable function other than self replication.
• Resistance plasmids: They have ability to impart resistance to drugs. R factor provides resistance to many drugs. A single R factor may carry traits for resistance to as many as seven or more chemotherapeutic agents. R factors in the normal flora of human and animals may be transferred to pathogenic organisms leading to sudden appearance of multiply resistant strains.R plamids play a major role in the development of resistance to antibiotics.