ß-Lactam antibiotic targets and resistance mechanisms: from covalent inhibitors to substrates.

The ß-lactams are the most widely used antibacterial agents worldwide. These antibiotics, a group that includes the penicillins and cephalosporins, are covalent inhibitors that target bacterial penicillin-binding proteins and disrupt peptidoglycan synthesis. Bacteria can achieve resistance to ß-lactams in several ways, including the production of serine ß-lactamase enzymes. While ß-lactams also covalently interact with serine ß-lactamases, these enzymes are capable of deacylating this complex, treating the antibiotic as a substrate. In this tutorial-style review, we provide an overview of the ß-lactam antibiotics, focusing on their covalent interactions with their target proteins and resistance mechanisms. We begin by describing the structurally diverse range of ß-lactam antibiotics and ß-lactamase inhibitors that are currently used as therapeutics. Then, we introduce the penicillin-binding proteins, describing their functions and structures, and highlighting their interactions with ß-lactam antibiotics. We next describe the classes of serine ß-lactamases, exploring some of the mechanisms by which they achieve the ability to degrade ß-lactams. Finally, we introduce the l,d-transpeptidases, a group of bacterial enzymes involved in peptidoglycan synthesis which are also targeted by ß-lactam antibiotics. Although resistance mechanisms are now prevalent for all antibiotics in this class, past successes in antibiotic development have at least delayed this onset of resistance. The ß-lactams continue to be an essential tool for the treatment of infectious disease, and recent advances (e.g., ß-lactamase inhibitor development) will continue to support their future use.

Identification of a Novel Biosynthetic Gene Cluster in Aspergillus niger Using Comparative Genomics.

Previously, DNA microarrays analysis showed that, in co-culture with Bacillus subtilis, a biosynthetic gene cluster anchored with a nonribosomal peptides synthetase of Aspergillus niger is downregulated. Based on phylogenetic and synteny analyses, we show here that this gene cluster, NRRL3_00036-NRRL3_00042, comprises genes predicted to encode a nonribosomal peptides synthetase, a FAD-binding domain-containing protein, an uncharacterized protein, a transporter, a cytochrome P450 protein, a NAD(P)-binding domain-containing protein and a transcription factor. We overexpressed the in-cluster transcription factor gene NRRL3_00042. The overexpression strain, NRRL3_00042OE, displays reduced growth rate and production of a yellow pigment, which by mass spectrometric analysis corresponds to two compounds with masses of 409.1384 and 425.1331. We deleted the gene encoding the NRRL3_00036 nonribosomal peptides synthetase in the NRRL3_00042OE strain. The resulting strain reverted to the wild-type phenotype. These results suggest that the biosynthetic gene cluster anchored by the NRRL3_00036 nonribosomal peptides synthetase gene is regulated by the in-cluster transcriptional regulator gene NRRL3_00042, and that it is involved in the production of two previously uncharacterized compounds.