The signal molecule for beta-lactamase induction in Enterobacter cloacae is the anhydromuramyl-pentapeptide

Abstract

Beta-lactamase induction in Enterobacter cloacae, which is linked to peptidoglycan recycling, was investigated by high-performance liquid chromatographic analysis of cell wall fragments in genetically defined cells of Escherichia coli. After treatment of cells with beta-lactams, we detected an increase in a D-tripeptide (disaccharide-tripeptide, N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-mes o-diaminopimelic acid), aD-tetrapeptide (disaccharide-tetrapeptide, N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-mes o-diaminopimelic acid-D-alanine), and aD-pentapeptide (disaccharide-pentapeptide, N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-mes o-diaminopimelic acid-D-alanyl-D-alanine)levels in the periplasms of bacterial cells. Furthermore, only the accumulation of aD-pentapeptide correlates with the beta-lactamase-inducing capacity of the beta-lactam antibiotic. The transmembrane protein AmpG transports all three aD-peptides into the cytoplasm, where they are degraded into the corresponding monosaccharide peptides. In the absence of AmpD the constitutive overproduction of beta-lactamase is accompanied by an accumulation of aM-tripeptide (monosaccharide-tripeptide, anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic acid) and aM-pentapeptide (L1,6-anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic acid-D-alanyl-D-alanine), but not aM-tetrapeptide (anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic acid-D-alanine), in the cytoplasm. Only the amount of aM-pentapeptide is increased upon treatment with imipenem. These findings indicate that aD-pentapeptide is the main periplasmic muropeptide, which is converted into the cytoplasmic signal molecule for beta-lactamase induction, the aM-pentapeptide.