Abstract
Bce-like systems mediate resistance against antimicrobial peptides in Firmicutes bacteria. Lactobacillus casei BL23 encodes an "orphan" ABC transporter that, based on homology to BceAB-like systems, was proposed to contribute to antimicrobial peptide resistance. A mutant lacking the permease subunit was tested for sensitivity against a collection of peptides derived from bacteria, fungi, insects, and humans. Our results show that the transporter specifically conferred resistance against insect-derived cysteine-stabilized αβ defensins, and it was therefore renamed DerAB for defensin resistance ABC transporter. Surprisingly, cells lacking DerAB showed a marked increase in resistance against the lantibiotic nisin. This could be explained by significantly increased expression of the antimicrobial peptide resistance determinants regulated by the Bce-like systems PsdRSAB (formerly module 09) and ApsRSAB (formerly module 12). Bacterial two-hybrid studies in Escherichia coli showed that DerB could interact with proteins of the sensory complex in the Psd resistance system. We therefore propose that interaction of DerAB with this complex in the cell creates signaling interference and reduces the cell's potential to mount an effective nisin resistance response. In the absence of DerB, this negative interference is relieved, leading to the observed hyperactivation of the Psd module and thus increased resistance to nisin. Our results unravel the function of a previously uncharacterized Bce-like orphan resistance transporter with pleiotropic biological effects on the cell.IMPORTANCE Antimicrobial peptides (AMPs) play an important role in suppressing the growth of microorganisms. They can be produced by bacteria themselves-to inhibit competitors-but are also widely distributed in higher eukaryotes, including insects and mammals, where they form an important component of innate immunity. In low-GC-content Gram-positive bacteria, BceAB-like transporters play a crucial role in AMP resistance but have so far been primarily associated with interbacterial competition. Here, we show that the orphan transporter DerAB from the lactic acid bacterium Lactobacillus casei is crucial for high-level resistance against insect-derived AMPs. It therefore represents an important mechanism for interkingdom defense. Furthermore, our results support a signaling interference from DerAB on the PsdRSAB module that might prevent the activation of a full nisin response. The Bce modules from L. casei BL23 illustrate a biological paradox in which the intrinsic nisin detoxification potential only arises in the absence of a defensin-specific ABC transporter.