Plasmid-coded acquired resistance

Acquired resistance. This occurs when bacteria which were previously susceptible become resistant, usually, but not always, after exposure to the antibiotic concerned. Intrirrsic resistance is always chromosomally mediated, whereas acquired resistance may occirr by mutations in the chromosome or by the acquisition of genes coding for resistance ftom an external source normally via a plasmid or transposon. Both types are clinically important and can result in treatment failure, although acquired resistance is more of a threat in the spread of antibiotic resistance (Russell Chopra 1996). 

Acquired resistance implies a change in the DNA of the bacteria that results in the appearance of new characteristic features. Such resistance is achieved in two ways mutation of chromosomes in bacteria or acquisition of new pieces of DNA (plasmid) that code for a function of resistance. 

The solid nutrient medium in the petri dish contains the antibacterial drug ampiciliin This drug kills . coli however, the cells that harbor the plasmid readily grow in the presence of the drug because part of the plasmid contains a gene that codes for a drug resistance factor. A bacterium that has accepted a plasmid is said to have acquired ampidllin resistance. 

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