Low-Molecular-Mass Penicillin Binding Protein 6b (DacD) Is Required for Efficient GOB-18 Metallo-β-Lactamase Biogenesis in Salmonella enterica and Escherichia coli

Metallo-beta-lactamases (MBLs) are Zn2+-containing secretory enzymes of clinical relevance, whose final folding and metal ion assembly steps in Gram- negative bacteria occur after secretion of the apo form to the periplasmic space. In the search of periplasmic factors assisting MBL biogenesis, we found that dacD null (Delta dacD) mutants of Salmonella enterica and Escherichia coli expressing the pre-GOB-18 MBL gene from plasmids showed significantly reduced resistance to cefotaxime and concomitant lower accumulation of GOB-18 in the periplasm. This reduced accumulation of GOB-18 resulted from increased accessibility to proteolytic attack in the periplasm, suggesting that the lack of DacD negatively affects the stability of secreted apo MBL forms. Moreover, Delta dacD mutants of S. enterica and E. coli showed an altered ability to develop biofilm growth. DacD is a widely distributed low-molecular-mass (LMM) penicillin binding protein (PBP6b) endowed with low DD-carboxypeptidase activity whose functions are still obscure. Our results indicate roles for DacD in assisting biogenesis of particular secretory macromolecules in Gram-negative bacteria and represent to our knowledge the first reported phenotypes for bacterial mutants lacking this LMM PBP.