A pbpB1 mutation causing reduced β-lactam susceptibility in clinical Listeria monocytogenes isolates

Listeriosis is a severe foodborne infection and associated with high mortality. Treatment is based on ampicillin, amoxicillin or penicillin, often combined with gentamicin, but meropenem is also used occasionally. β-lactam resistant Listeria monocytogenes isolates are infrequently described but the mechanism of resistance is not known. A clinical L. monocytogenes isolate with reduced β-lactam susceptibility was collected from a German listeriosis patient. Resistance profiling, whole genome sequencing, comparative genomics and genetic experiments were used to identify the causative DNA polymorphism. A W428R substitution near the active site of penicillin binding protein B1 (PBPB1) was identified as the cause of reduced ampicillin, amoxicillin and meropenem susceptibility. Further clinical L. monocytogenes isolates with similar susceptibility profiles were found by searching the genome database of the German consultant laboratory for Listeria for pbpB1 W428R-positive isolates. Spontaneous suppressors of L. monocytogenes reference strain EGD-e with reduced β-lactam susceptibility were selected during ampicillin exposure and their genomes were sequenced. The same pbpB1 mutation emerged in strain EGD-e during cultivation in the presence of ampicillin. Further experiments showed that meropenem also promotes the development of resistant suppressor mutants with pbpB1 mutations. Our work demonstrates that β-lactam susceptibility of L. monocytogenes can be reduced through specific substitutions in PBPB1. These mutations are potentially selected for during β-lactam treatment.