The resistance-nodulation-division (RND)-type efflux pumps AcrAB and MdtABC contribute to multidrug-resistance (MDR) in Gram-negative bacteria. Photorhabdus is a symbiotic bacterium of soil nematodes that also produces virulence factors killing insects by septicaemia. We previously showed that mdtA deletion in Photorhabdus laumondii TT01 resulted in no detrimental phenotypes. Here, we investigated the roles of the last two putative RND transporters in TT01 genome, AcrAB and AcrAB-like (Plu0759-Plu0758). Only ΔacrA and ΔmdtAΔacrA mutants were multidrug sensitive, even to triphenyltetrazolium chloride and bromothymol blue used for Photorhabdus isolation from nematodes on the nutrient bromothymol blue-triphenyltetrazolium chloride agar (NBTA) medium. Both mutants also displayed slightly attenuated virulence after injection into Spodoptera littoralis. Transcriptional analysis revealed intermediate levels of acrAB expression in vitro, in vivo and post-mortem, whereas its putative transcriptional repressor acrR was weakly expressed. Yet, plasmid-mediated acrR overexpression did not decrease acrAB transcript levels neither MDR in TT01 WT. While no pertinent mutations were detected in acrR of the same P. laumondii strain grown either on NBTA or nutrient agar, we suggest that AcrR-mediated repression of acrAB is not physiologically required under conditions tested. Finally, we propose that AcrAB is the primary RND-efflux pump, which is essential for MDR in Photorhabdus and may confer adaptive advantages during insect infection.

中文翻译：

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AcrAB 是劳氏光棒菌的主要 RND 型外排泵，具有内在的多药抗性并有助于昆虫的毒力

耐药结瘤分裂 (RND) 型外排泵 AcrAB 和 MdtABC 有助于革兰氏阴性菌的多药耐药 (MDR)。Photorhabdus是土壤线虫的共生细菌，它还产生通过败血症杀死昆虫的毒力因子。我们之前表明，Photorhabdus laumondii TT01中的 mdtA 缺失不会导致有害表型。在这里，我们研究了 TT01 基因组中最后两个推定的 RND 转运蛋白 AcrAB 和 AcrAB 样 (Plu0759-Plu0758) 的作用。只有 Δ acrA和 Δ mdtA Δ acrA突变体对多药敏感，甚至对用于Photorhabdus的三苯基四唑氯化物和溴百里酚蓝也是如此在营养溴百里酚蓝-氯化三苯基四唑琼脂琼脂 (NBTA) 培养基上从线虫中分离。两种突变体在注射到Spodoptera littoralis后也表现出略微减弱的毒力。转录分析显示在体外、体内和死后的acrAB表达处于中等水平，而其推定的转录阻遏物acrR表达较弱。然而，质粒介导的acrR过表达并没有降低acrAB转录水平，也没有降低 TT01 WT 中的 MDR。虽然在同一劳氏假单胞菌的 acrR中未检测到相关突变在 NBTA 或营养琼脂上生长的菌株，我们建议在测试条件下生理上不需要AcrR 介导的acrAB抑制。最后，我们提出 AcrAB 是主要的 RND 外排泵，这对于Photorhabdus中的 MDR 至关重要，并且可能在昆虫感染期间赋予适应性优势。