Low-copy number plasmid vectors such as those based on pMB1 or ColE1 can be amplified by adding chloramphenicol to the bacterial culture. This results in a high yield of plasmid DNA (5–10-fold) even though the same amount of bacterial culture is used for plasmid isolation.
Chloramphenicol is a bacteriostatic antibiotic. It blocks the protein synthesis by reversibly binding to the 50S subunit (L10 protein) of the ribosome and inhibits peptidyl transferase activity during the translation process. Since replication of chromosomes requires active protein synthesis, chloramphenicol inhibits cell division. However, plasmids with relaxed replication control keep multiplying as their replication depends on long-lived host proteins. Thus, in the absence of active protein synthesis plasmid still continues to replicate, resulting in an increase in their copy number per bacterial cell.
Plasmids that require short-lived host protein for their replication such as pSC101 (requires RepA protein for replication) can not be amplified by adding Chloramphenicol in the culture.
