Department of Microbiology, University of Barcelona, Barcelona, Spain.
PLoS One. 2012;7(2):e32393. doi: 10.1371/journal.pone.0032393. Epub 2012 Feb 29.
The bacteriophage life cycle has an important role in Shiga toxin (Stx) expression. The induction of Shiga toxin-encoding phages (Stx phages) increases toxin production as a result of replication of the phage genome, and phage lysis of the host cell also provides a means of Stx toxin to exit the cell. Previous studies suggested that prophage induction might also occur in the absence of SOS response, independently of RecA.
METHODOLOGY/PRINCIPAL FINDINGS: The influence of EDTA on RecA-independent Stx2 phage induction was assessed, in laboratory lysogens and in EHEC strains carrying Stx2 phages in their genome, by Real-Time PCR. RecA-independent mechanisms described for phage λ induction (RcsA and DsrA) were not involved in Stx2 phage induction. In addition, mutations in the pathway for the stress response of the bacterial envelope to EDTA did not contribute to Stx2 phage induction. The effect of EDTA on Stx phage induction is due to its chelating properties, which was also confirmed by the use of citrate, another chelating agent. Our results indicate that EDTA affects Stx2 phage induction by disruption of the bacterial outer membrane due to chelation of Mg(2+). In all the conditions evaluated, the pH value had a decisive role in Stx2 phage induction.
CONCLUSIONS/SIGNIFICANCE: Chelating agents, such as EDTA and citrate, induce Stx phages, which raises concerns due to their frequent use in food and pharmaceutical products. This study contributes to our understanding of the phenomenon of induction and release of Stx phages as an important factor in the pathogenicity of Shiga toxin-producing Escherichia coli (STEC) and in the emergence of new pathogenic strains.
噬菌体生命周期在志贺毒素（Stx）表达中起着重要作用。诱导编码志贺毒素的噬菌体（Stx 噬菌体）会导致噬菌体基因组复制，从而增加毒素的产生，而噬菌体裂解宿主细胞也为 Stx 毒素离开细胞提供了一种途径。先前的研究表明，前噬菌体的诱导也可能在没有 SOS 反应的情况下发生，而不依赖于 RecA。
方法/主要发现：通过实时 PCR 评估 EDTA 对 RecA 独立的 Stx2 噬菌体诱导的影响，在实验室溶原菌和基因组中携带 Stx2 噬菌体的 EHEC 菌株中进行评估。不涉及 λ噬菌体诱导的 RecA 独立机制（RcsA 和 DsrA）。此外，细菌包膜对 EDTA 的应激反应途径中的突变也不能促进 Stx2 噬菌体的诱导。EDTA 对 Stx 噬菌体诱导的影响是由于其螯合特性，这也通过使用另一种螯合剂柠檬酸盐得到了证实。我们的研究结果表明，EDTA 通过螯合 Mg2+破坏细菌外膜来影响 Stx2 噬菌体的诱导。在所有评估的条件下，pH 值对 Stx2 噬菌体的诱导起着决定性的作用。
结论/意义：螯合剂，如 EDTA 和柠檬酸盐，可诱导 Stx 噬菌体，这引起了人们的关注，因为它们在食品和制药产品中经常使用。本研究有助于我们理解 Stx 噬菌体的诱导和释放现象，这是产志贺毒素大肠杆菌（STEC）致病性的一个重要因素，也是新的致病性菌株出现的一个重要因素。
