Evaluation of Stringency Elicited by Antibiotics on Plasmid Copy Number of pBR322 Per Genome in DH5α Escherichia coli Cells

Stringency is produced in response to stress in bacteria. The stringent response results in the global transcriptional down-regulation of genes required for rapid growth and upregulation of the genes for amino acid biosynthesis. Past reports have demonstrated that heat shock-inducing antibiotics such as kanamycin and streptomycin induce a stringent response, while cold shock-inducing antibiotics such as chloramphenicol and erythromycin induce a relaxed response. Also, sub-inhibitory levels of chloramphenicol increase the replication of pBR322, a common Escherichia coli plasmid cloning vector. Since the replication of pBR322 is stimulated under the relaxed condition yet inhibited under the stringent condition, this study attempted to address whether antibiotics which elicit the stringent or the relaxed response would cause a decrease or an increase in the ratio of plasmid copy numbers to genomic DNA numbers (PCN), respectively. Kanamycin, streptomycin, chloramphenicol and erythromycin were used individually for the treatment in E. coli DH5 α. Turbidity and PCN were determined at four time points; a control without antibiotics was set up. It was expected that kanamycin and streptomycin would produce lower PCN than chloramphenicol and erythromycin. However, the results indicated that antibiotics that elicit the stringent or relaxed response did not lead to a decreased or an increased PCN, respectively. No correlation was observed between the types of antibiotics and the PCN. A partial stringency, the recovery of the plasmid DNA and the natures of antibiotics are possible explanations.