An evaluation of models for the effect of plasmid copy number on bacterial growth rate

The rates of growth of recombinant bacteria depend on their plasmid content. This is modelled by expressing the specific growth rate in terms of the number of copies of the plasmid per cell. Three models in common use have been tested with different Escherichia coli strains and one strain of Bacillus stearothermophilus containing different plasmids. While no particular model was decisively better than others for all data, that of Bentley & Quiroga (Biotechnol. Bioeng. 1993, 42: 222-234) was the best for specific growth rates which vary inversely with the plasmid copy number, and a modified form of the model of Satyagal & Agarwal (Biotechnol. Bioeng. 1989, 33: 1135-1144) was the best for growth rates which increase with the copy number. The differences appear to be linked to the plasmid replication mechanisms. Contrary to some claims, no model portrayed the experimentally observed inflection points.