The transfer of nutrients between cells, or cross-feeding, is a ubiquitous feature of microbial communities with emergent properties that influence our health and orchestrate global biogeochemical cycles. Cross-feeding inevitably involves the externalization of molecules. Some of these molecules directly serve as cross-fed nutrients, while others can facilitate cross-feeding. Altogether, externalized molecules that promote cross-feeding are diverse in structure, ranging from small molecules to macromolecules. The functions of these molecules are equally diverse, encompassing waste products, enzymes, toxins, signaling molecules, biofilm components, and nutrients of high value to most microbes, including the producer cell. As diverse as the externalized and transferred molecules are the cross-feeding relationships that can be derived from them. Many cross-feeding relationships can be summarized as cooperative but are also subject to exploitation. Even those relationships that appear to be cooperative exhibit some level of competition between partners. In this review, we summarize the major types of actively secreted, passively excreted, and directly transferred molecules that either form the basis of cross-feeding relationships or facilitate them. Drawing on examples from both natural and synthetic communities, we explore how the interplay between microbial physiology, environmental parameters, and the diverse functional attributes of extracellular molecules can influence cross-feeding dynamics. Though microbial cross-feeding interactions represent a burgeoning field of interest, we may have only begun to scratch the surface.

中文翻译：

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细胞外代谢为微生物交叉喂养奠定了基础

细胞之间营养物质的转移或交叉喂养是微生物群落的普遍特征，具有影响我们健康和协调全球生物地球化学循环的新兴特性。交叉喂养不可避免地涉及分子的外化。其中一些分子直接作为交叉喂养的营养物质，而另一些则可以促进交叉喂养。总而言之，促进交叉喂养的外化分子在结构上是多种多样的，从小分子到大分子。这些分子的功能同样多样，包括废物、酶、毒素、信号分子、生物膜成分和对大多数微生物（包括生产细胞）具有高价值的营养物质。与外化和转移的分子一样多样化的是可以从中得出的交叉喂养关系。许多交叉喂养关系可以概括为合作关系，但也受到剥削。即使是那些看似合作的关系也表现出合作伙伴之间某种程度的竞争。在这篇综述中，我们总结了主要类型的主动分泌、被动分泌和直接转移的分子，它们形成了交叉喂养关系的基础或促进了它们。借鉴自然和合成群落的例子，我们探索微生物生理学、环境参数和细胞外分子的不同功能属性之间的相互作用如何影响交叉喂养动态。虽然微生物交叉喂养的相互作用代表了一个新兴的兴趣领域，但我们可能才刚刚开始触及表面。