Loss of the lipopolysaccharide (LPS) inner core increases the electrocompetence of Escherichia coli

Abstract

Mutations that shorten the lipopolysaccharide (LPS) inEscherichia coliwere found to significantly increase the number of transformants after electroporation. The loss of the LPS outer core increased the number of transformants with plasmid pAmCyan (3.3 kb) from 5.0 x 10(5)colony-forming units (CFU)/mu g in the wild-typeE. coliBW25113 to 3.3 x 10(7)CFU/mu g in a Delta waaGbackground, a 66.2-fold increase in efficiency. Truncation of the inner core improved this even further, with the Delta waaFmutant exhibiting the best transformation efficiencies obtained, i.e., a 454.7-fold increase in the number of colonies over the wild-type strain. Similar results were obtained when a larger plasmid (pDA1; 11.3 kb) was used, with the Delta waaFmutant once more giving the best transformation rates, i.e., a 73.7-fold increase. Subsequent tests proved that the enhanced transformabilities of these mutants were not due to a better survival or their surface charge properties, nor from preferential binding of these strains to the plasmid. Using N-phenyl-1-naphthylamine (NPN), we confirmed that the outer membranes of these mutant strains were more permeable. We also found that they leaked more ATP (3.4- and 2.0-fold higher for the Delta waaFand Delta waaGmutants, respectively, than wild-typeE. coliBW25113), suggesting that the inner membrane stability is also reduced, helping to explain how the DNA enters these cells more easily.