Bacteria can migrate in groups of flagella‐driven cells over semisolid surfaces. This coordinated form of motility is called swarming behavior. Swarming is associated with enhanced virulence and antibiotic resistance of various human pathogens and may be considered as favorable adaptation to the diverse challenges that microbes face in rapidly changing environments. Consequently, the differentiation of motile swarmer cells is tightly regulated and involves multi‐layered signaling networks. Controlling swarming behavior is of major interest for the development of novel anti‐infective strategies. In addition, compounds that block swarming represent important tools for more detailed insights into the molecular mechanisms of the coordination of bacterial population behavior. Over the past decades, there has been major progress in the discovery of small‐molecule modulators and mechanisms that allow selective inhibition of swarming behavior. Herein, an overview of the achievements in the field and future directions and challenges will be presented.
Highlights d AvbD is the key player in the biosynthesis of the siderophore avaroferrin d Multispecificity leads to three native products: avaroferrin, putrebactin, and bisucaberin d Enzyme kinetics discriminate the two native substrates in favor of bisucaberin d The cellular substrate pool shifts siderophore production toward avaroferrin
Macrocyclic and linear hydroxamate siderophores produced by NRPS-independent siderophore (NIS; NRPS=nonribosomal peptide synthetase) synthetases are important in the bacterial competition for iron, as virulence factors, and as drugs for medical use in humans. Despite their importance, the mechanistic details of NIS synthetases have so far remained obscure. Using synthetic substrate analogues as tools allowed for an interrogation of the mechanism of the two closely related NIS synthetases AvbD and DesD. While AvbD produces macrocyclic homo- and heterodimers as native products, DesD is responsible for the synthesis of trimeric desferrioxamines. These enzymes comprise two adjacent binding sites with different substrate selectivities, which direct oligomerization and macrocyclization steps. Exploiting this difference, synthetic substrates were used to invert the native affinities for the sites resulting in switching from trimerization to dimerization reactions for DesD. Based on this work, a comprehensive model explaining the mechanistic details of the reactions and the differences between trimerizing and dimerizing enzymes was developed. Finally, a DesD mutant demonstrated the tuneability of the enzyme's substrate selectivity by only minor changes in the protein sequence. This finding confirms the affinity-directed mechanism responsible for the iterativity of oligomerization and macrocyclization steps.
Bacteria compete for ferric iron by producing siderophores, and some microbes engage in piracy by scavenging siderophores of their competitors. The macrocyclic hydroxamate siderophore avaroferrin of Shewanella algae inhibits swarming of Vibrio alginolyticus by evading this piracy. Avaroferrin, as well as related putrebactin and bisucaberin, are produced by the IucC-like synthetases AvbD, PubC, and BibC. Here, we have established that they are capable of synthesizing not only their native product but also other siderophores. Exploiting this relaxed substrate specificity by synthetic precursors generated 15 different ring-size engineered macrocycles ranging from 18- to 28-membered rings, indicating unprecedented biosynthetic flexibility of the enzymes. Two of the novel siderophores could be obtained in larger quantities by precursor-directed biosynthesis in S. algae. Both inhibited swarming motility of Vibrio and, similar to avaroferrin, the most active one exhibited a heterodimeric architecture. Our results demonstrate the impact of minor structural changes on biological activity, which may trigger the evolution of siderophore diversity.
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