The lipopeptide daptomycin is used as an antibiotic to treat severe infections with grampositive pathogens, such as methicillin resistant Staphylococcus aureus (MRSA) and drugresistant enterococci. Its precise mechanism of action is incompletely understood, and a specific molecular target has not been identified. Here we show that Ca 2+-daptomycin specifically interacts with undecaprenyl-coupled cell envelope precursors in the presence of the anionic phospholipid phosphatidylglycerol, forming a tripartite complex. We use microbiological and biochemical assays, in combination with fluorescence and optical sectioning microscopy of intact staphylococcal cells and model membrane systems. Binding primarily occurs at the staphylococcal septum and interrupts cell wall biosynthesis. This is followed by delocalisation of components of the peptidoglycan biosynthesis machinery and massive membrane rearrangements, which may account for the pleiotropic cellular events previously reported. The identification of carrier-bound cell wall precursors as specific targets explains the specificity of daptomycin for bacterial cells. Our work reconciles apparently inconsistent previous results, and supports a concise model for the mode of action of daptomycin.
The rapid rise of multidrug-resistant (MDR) bacteria has once again caused bacterial infections to become a global health concern. Antimicrobial peptides (AMPs), also known as host defense peptides (HDPs), offer a viable solution to these pathogens due to their diverse mechanisms of actions, which include direct killing as well as immunomodulatory properties (e.g., anti-inflammatory activity). HDPs may hence provide a more robust treatment of bacterial infections. In this review, the advent of and the mechanisms that lead to antibiotic resistance will be described. HDP mechanisms of antibacterial and immunomodulatory action will be presented, with specific examples of how the HDP aurein 2.2 and a few of its derivatives, namely peptide 73 and cG4L73, function. Finally, resistance that may arise from a broader use of HDPs in a clinical setting and methods to improve biocompatibility will be briefly discussed.
Bile salts such as cholate are steroid compounds with a C carboxylic side chain and occur ubiquitously in vertebrates. Upon their excretion into soils and waters, bile salts can serve as growth substrates for diverse bacteria. sp. strain Chol11 degrades 7-hydroxy bile salts via 3-keto-7-deoxy-Δ metabolites by the dehydration of the 7-hydroxyl group catalyzed by the 7α-hydroxysteroid dehydratase Hsh2. This reaction has not been observed in the well-studied 9-10-seco degradation pathway used by other steroid-degrading bacteria indicating that strain Chol11 uses an alternative pathway. A reciprocal BLASTp analysis showed that known side chain degradation genes from other cholate-degrading bacteria ( Chol1, CNB-2, and RHA1) were not found in the genome of strain Chol11. The characterization of a transposon mutant of strain Chol11 showing altered growth with cholate identified a novel steroid-24-oyl-coenzyme A ligase named SclA. The unmarked deletion of resulted in a strong growth rate decrease with cholate, while growth with steroids with C side chains or without side chains was not affected. Intermediates with a 7-deoxy-3-keto-Δ structure, such as 3,12-dioxo-4,6-choldienoic acid (DOCDA), were shown to be likely physiological substrates of SclA. Furthermore, a novel coenzyme A (CoA)-dependent DOCDA degradation metabolite with an additional double bond in the side chain was identified. These results support the hypothesis that sp. strain Chol11 harbors an alternative pathway for cholate degradation, in which side chain degradation is initiated by the CoA ligase SclA and proceeds via reaction steps catalyzed by so-far-unknown enzymes different from those of other steroid-degrading bacteria. This study provides further evidence of the diversity of metabolic pathways for the degradation of steroid compounds in environmental bacteria. The knowledge about these pathways contributes to the understanding of the CO-releasing part of the global C cycle. Furthermore, it is useful for investigating the fate of pharmaceutical steroids in the environment, some of which may act as endocrine disruptors.
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