A <i>pbpB1</i> mutation causing reduced β-lactam susceptibility in clinical <i>Listeria monocytogenes</i> 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 <i>Listeria monocytogenes</i> isolates are infrequently described but the mechanism of resistance is not known. A clinical <i>L. monocytogenes</i> 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 <i>L. monocytogenes</i> isolates with similar susceptibility profiles were found by searching the genome database of the German consultant laboratory for <i>Listeria</i> for <i>pbpB1 W428R</i>-positive isolates. Spontaneous suppressors of <i>L. monocytogenes</i> reference strain EGD-e with reduced β-lactam susceptibility were selected during ampicillin exposure and their genomes were sequenced. The same <i>pbpB1</i> 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 <i>pbpB1</i> mutations. Our work demonstrates that β-lactam susceptibility of <i>L. monocytogenes</i> can be reduced through specific substitutions in PBPB1. These mutations are potentially selected for during β-lactam treatment.