Amino acid supplementation decreases plasmid retention in Escherichia coli

The effect of amino acid supplementation on plasmid stability in Escherichia coli B/r was tested experimentally. Comparisons of experimental results to computer-predicted values were made using a detailed, structured single-cell model. The plasmid, pDW17 (a pBR322 derivative with a mutated tac promoter controlling the beta-lactamase gene), was used. In chemostat cultures, the amino acid supplemented cultures were always less stable than those grown in minimal medium. This effect was not a growth rate effect, as increasing growth rate improves stability for both cultures in minimal medium and in amino acid supplemented medium. The computer model also predicted a decrease in stability due to amino acid supplementation. The model also predicts that amino acid supplementation, combined with moderately strong plasmid-encoded protein expression, results in a depletion of low-molecular-weight organics compared with plasmid-free cells. In minimal medium the same level of plasmid-encoded protein synthesis results in a strong reduction in amino acid pools compared with plasmid-free cells. With amino acid supplementation the growth differential between plasmid-bearing and plasmid-free cells may be due to an "energy limitation," while in minimal medium the size of the growth rate differential may be due to a "building block" limitation.