The term cost is used in plasmid biology to refer to the expense incurred on a bacterial cell for keeping a plasmid. It is also sometimes referred to as plasmid burden. This cost can be proportional to the amount of resources needed to synthesize the plasmid DNA and particularly the proteins expressed by plasmid-encoded genes. Plasmid cost can also be due to a toxic effect of one of the plasmid proteins on the cell (for example, due to interference with other cell components).
The primary method used to evaluate cost is measuring the difference in "fitness" (or simply stated, maximum growth rate) between cells that have the plasmid and those that do not.
One important detail to remember is that the cost of a plasmid can be dependent on the environment in which the bacteria lives. For example, in the presence of antibiotics, a plasmid that would confer a cost to its host in the absence of the drug, has now a huge benefit to the cell because the cell can grow or at least survive, while its plasmid-free counterparts cannot.
