Comparison of the phenotypes of the lpxA and lpxD mutants of Escherichia coli.

We compared the phenotype of two thermosensitive Escherichia coli mutants defective in lipid A biosynthesis, i.e. SM101 (lpxA) and CDH23-213 (lpxD). More than 40% of the periplasmic 27-kDa marker enzyme beta-lactamase was released from SM101 at 28 degrees C. At this temperature, the mutant still grew with a generation time (67 min), not much longer than that of the parent control strain (57 min). CDH23-213 released beta-lactamase only at higher temperatures. SM101 and CDH23-213 were both unable to grow in hypo-osmotic conditions. Derivatives of SM101 and CDH23-213 with mdoA::Tn10 had identical phenotypes (including thermosensitivity and defective outer membrane permeability barrier to hydrophobic probes) to those of SM101 and CDH23-213, indicating that the potential loss of membrane-derived oligosaccharides (MDO) did not explain these phenotype properties. A method for the estimation of lipid A synthesis rate was developed.