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Antibiotic resistance genes are carried on plasmids

In 1968 when I arrived on the Stanford faculty as an Assistant Professor, I set out to isolate and characterize the plasmids of infectious drug resistance with the hope of answering these questions. Work carried out in my laboratory and elsewhere soon showed that antibiotic resistance plasmids were molecules of circular DNA.

Breaking open bacterial cells released these DNA circles as seen in this electron photo micrograph taken some years ago at Stanford by Dr. Jack Griffith. These circles could then be isolated and characterized by biochemical methods. Studies showed that plasmids contain a DNA segment called the replication region which allows the plasmid to propagate itself independently of the machinery that reproduces the chromosomal DNA.

Plasmid Ancillary Genes include:

  • Antibiotic-resistance genes

  • Antibiotics production genes

  • Heavy Metal resistance genes

  • Virulence genes

  • Tumorigenicity (in plants)

  • Fertility (transfer) genes

  • Toxin production

  • Restriction / Modification

  • Metabolism of hydrocarbons

In different plasmids the replications regions are linked to different ancillary genes, encoding traits that ordinarily are not essential to the bacterial host but which can, in some environments, provide a biological advantage to bacteria carrying the plasmid. Antibiotic resistance is one of these traits. In some plasmids the replication region is attached to genes that produce antibiotics or toxins. In others, the genes that enable to plasmid to be transferred between bacteria are other genes the plasmid has picked up from host cells.

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