LuxS-dependent AI-2 production is not involved in global regulation of natural product biosynthesis inPhotorhabdusandXenorhabdus

Abstract

The Gram-negative bacteria Photorhabdus and Xenorhabdus are known to produce a variety of different natural products (NP). These compounds play different roles since the bacteria live in symbiosis with nematodes and are pathogenic to insect larvae in the soil. Thus, a fine tuned regulatory system controlling NP biosynthesis is indispensable. Global regulators such as Hfq, Lrp, LeuO and HexA have been shown to influence NP production of Photorhabdus and Xenorhabdus. Additionally, photopyrones as quorum sensing (QS) signals were demonstrated to be involved in the regulation of NP production in Photorhabdus. In this study, we investigated the role of another possible QS signal, autoinducer-2 (AI-2), in regulation of NP production. The AI-2 synthase (LuxS) is widely distributed within the bacterial kingdom and has a dual role as a part of the activated methyl cycle pathway, as well as being responsible for AI-2 precursor production. We deleted luxS in three different entomopathogenic bacteria and compared NP levels in the mutant strains to the wild type (WT) but observed no difference to the WT strains. Furthermore, the absence of the small regulatory RNA micA, which is encoded directly upstream of luxS, did not influence NP levels. Phenotypic differences between the P. luminescens luxS deletion mutant and an earlier described luxS deficient strain of P. luminescens suggested that two phenotypically different strains have evolved in different laboratories.

Pyrones Identified as LuxR Signal Molecules in Photorhabdus and Their Synthetic Analogues Can Alter Multicellular Phenotypic Behavior of Bacillus atropheaus

“…The quorum in this case activates a signaling cascade that transcribes the operon responsible for the Photorhabdus clumping factor (Pcf)a biofilm-like phenotype that plays a vital role in the pathogenicity of these bacteria. 14,16,17 These findings encompass the first demonstration of 2-pyrones, specifically 3-alkyl-4-hydroxy-6-isobutyl-2H-pyran-2-ones (Figure 1), acting as signaling molecules in any bacterial strain. Although the findings specified the detection of endogenously produced PPY signals, it is also known that orphan receptors can be used to respond to exogenous signals, produced by coinhabiting microorganisms, for example.…”

“…Although not fully understood, systems lacking any LuxI-like AHL synthase can still encompass proteins with homology to LuxR-type receptors, namely LuxR orphans or solos. , Seminal reports by Heermann and co-workers explicitly target the concept that non-AHL producing bacteria can employ these receptors in the detection of other endogenously synthesized compounds and operate a QS signaling pathway. , Critically, this group unearthed “photopyrones” (PPYs) participating in the QS network of the Gram-negative pathogen Photorhabdus luminescens through an orphan LuxR receptor. The quorum in this case activates a signaling cascade that transcribes the operon responsible for the Photorhabdus clumping factor (Pcf)a biofilm-like phenotype that plays a vital role in the pathogenicity of these bacteria. ,, These findings encompass the first demonstration of 2-pyrones, specifically 3-alkyl-4-hydroxy-6-isobutyl-2 H -pyran-2-ones (Figure ), acting as signaling molecules in any bacterial strain …”

AcrAB, the major RND‐type efflux pump of Photorhabdus laumondii, confers intrinsic multidrug‐resistance and contributes to virulence in insects

“…Recently, a metabolomic‐based study in E. coli confirmed that TolC‐dependent pumps play an important role in cell metabolism by demonstrating that AcrAB pump deletion or overproduction was associated with changes in a large number of central metabolism intermediates (Cauilan et al ., 2019). Therefore, the accumulation of such metabolites and compounds in AcrAB mutants could be toxic or at least disadvantageous at a given moment in Photorhabdus life cycle, producing a negative feedback on the production of attributes directly or indirectly required to infect and colonize the host as shown in Salmonella Typhimurium (Webber et al ., 2009; Heinrich et al ., 2017). In harmony with this, the slight virulence exaltation noted upon acrAB overexpression in mutant strains is suggestive of an enhanced production/export of yet unknown bacterial effectors/toxic compounds via the AcrAB pump, contributing to a better adaptation and survival to the complex stresses within the dying insect.…”