Conjugative, broad host-range plasmids of the L/M complex have been associated with antibiotic resistance since the 1970s. They are found in Gram-negative bacterial genera that cause human infections and persist in hospital environments. It is crucial that these plasmids are typed accurately so that their clinical and global dissemination can be traced in epidemiological studies. The L/M complex has previously been divided into L, M1 and M2 subtypes. However, those types do not encompass all diversity seen in the group. Here, we have examined 148 complete L/M plasmid sequences in order to understand the diversity of the complex and trace the evolution of distinct lineages. The backbone sequence of each plasmid was determined by removing translocatable genetic elements and reversing their effects in silico. The sequence identities of replication regions and complete backbones were then considered for typing. This supported the distinction of L and M plasmids and revealed that there are five L and eight M types, where each type is comprised of further sub-lineages that are distinguished by variation in their backbone and translocatable element content. Regions containing antibiotic resistance genes in L and M sub-lineages have often formed by initial rare insertion events, followed by insertion of other translocatable elements within the inceptive element. As such, islands evolve in situ to contain genes conferring resistance to multiple antibiotics. In some cases, different plasmid sub-lineages have acquired the same or related resistance genes independently. This highlights the importance of these plasmids in acting as vehicles for the dissemination of emerging resistance genes. Materials are provided here for typing plasmids of the L/M complex from complete sequences or draft genomes. This should enable rapid identification of novel types and facilitate tracking the evolution of existing lineages.

自 1970 年代以来，L/M 复合体的接合性、广泛宿主范围质粒一直与抗生素抗性有关。它们存在于引起人类感染并在医院环境中持续存在的革兰氏阴性细菌属中。对这些质粒进行准确分型至关重要，这样才能在流行病学研究中追踪它们的临床和全球传播。L/M 复合体以前被分为 L、M1 和 M2 亚型。但是，这些类型并不包括该组中的所有多样性。在这里，我们检查了 148 个完整的 L/M 质粒序列，以了解复合物的多样性并追踪不同谱系的进化。每个质粒的骨架序列是通过去除易位遗传元件并在计算机中逆转它们的作用来确定的. 然后考虑复制区域和完整骨架的序列身份进行分型。这支持了 L 和 M 质粒的区别，并揭示了有五种 L 和八种 M 类型，其中每种类型由进一步的子谱系组成，这些子谱系通过其主链和易位元件含量的变化来区分。L 和 M 亚谱系中含有抗生素抗性基因的区域通常由最初的罕见插入事件形成，随后在起始元件中插入其他易位元件。因此，岛屿就地进化包含赋予多种抗生素抗性的基因。在某些情况下，不同的质粒亚系独立地获得了相同或相关的抗性基因。这突出了这些质粒作为传播新出现的抗性基因的载体的重要性。此处提供了用于从完整序列或基因组草图对 L/M 复合体的质粒进行分型的材料。这应该能够快速识别新类型并促进跟踪现有谱系的演变。