In recent years, multidrug resistance of Shigella strains associated with genetic elements like pathogenicity islands, have become a public health problem. The Shigella resistance locus pathogenicity island (SRL PAI) of S. flexneri 2a harbors a 16Kbp region that contributes to the multidrug resistance phenotype. However, there is not much information about other functions such as metabolic, physiologic or ecological ones. For that, wild type S. flexneri YSH6000 strain, and its spontaneous SRL PAI mutant, 1363, were used to study the contribution of the island in different growth conditions. Interestingly, when both strains were compared by the Phenotype Microarrays, the ability to metabolize D-aspartic acid as a carbon source was detected in the wild type strain but not in the mutant. When D-aspartate was added to minimal medium with other carbon sources such as mannose or mannitol, the SRL PAI-positive strain was able to metabolize it, while the SRL PAI-negative strain did not. In order to identify the genetic elements responsible for this phenotype, a bioinformatic analysis was performed and two genes belonging to SRL PAI were found: orf8, coding for a putative aspartate racemase, and orf9, coding for a transporter. Thus, it was possible to measure, by an indirect analysis of racemization activity in minimal medium supplemented only with D-aspartate, that YSH6000 strain was able to transform the D-form into L-, while the mutant was impaired to do it. When the orf8-orf9 region from SRL island was transformed into S. flexneri and S. sonnei SRL PAI-negative strains, the phenotype was restored. Although, when single genes were cloned into plasmids, no complementation was observed. Our results strongly suggest that the aspartate racemase and the transporter encoded in the SRL pathogenicity island are important for bacterial survival in environments rich in D-aspartate.

中文翻译：

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SRL致病岛通过福氏志贺氏菌YSH6000中的天冬氨酸消旋酶促进D-天冬氨酸的代谢。

近年来，与致病岛等遗传因素相关的志贺氏菌菌株的多药耐药性已成为公共卫生问题。弗氏链球菌2a的志贺氏菌抗性基因座病原性岛（SRL PAI）具有一个16Kbp的区域，该区域有助于产生多药耐药性表型。但是，关于其他功能（例如代谢功能，生理功能或生态功能）的信息并不多。为此，野生型弗氏链球菌YSH6000菌株及其自发的SRL PAI突变体1363被用于研究该岛在不同生长条件下的贡献。有趣的是，当通过表型微阵列比较两个菌株时，在野生型菌株中检测到了代谢D-天冬氨酸作为碳源的能力，但在突变体中却未检测到。当将D-天门冬氨酸添加到含有其他碳源（如甘露糖或甘露醇）的基本培养基中时，SRL PAI阳性菌株能够代谢它，而SRL PAI阴性菌株则不能代谢。为了鉴定造成该表型的遗传元件，进行了生物信息学分析，发现了两个属于SRL PAI的基因：orf8，编码假定的天冬氨酸消旋酶，orf9，编码转运蛋白。因此，通过在仅添加D-天冬氨酸的基本培养基中对消旋活性的间接分析，有可能测定YSH6000菌株能够将D-形式转化为L-，而突变体却不能做到这一点。当将SRL岛的orf8-orf9区域转化为弗氏链球菌和S. sonnei SRL PAI阴性菌株时，表型得以恢复。虽然，当将单个基因克隆到质粒中时，没有观察到互补。我们的结果有力地表明，SRL致病岛中编码的天冬氨酸消旋酶和转运蛋白对于富含D-天冬氨酸的环境中的细菌存活至关重要。