Content of cardiolipin of the membrane and sensitivity to cationic surfactants in Pseudomonas putida

Cardiolipin synthesis in Pseudomonas fluorescens UM270 plays a relevant role in stimulating plant growth under salt stress

Membrane cardiolipin (CL) phospholipids play a fundamental role in the adaptation of bacteria to various environmental conditions, including saline stress. Here, we constructed deletion mutants of two CL synthetase genes, clsA (UM270 ∆clsA) and clsB (UM270 ∆clsB), in the rhizobacterium Pseudomonas fluorescens UM270, and evaluated their role in plant growth promotion under salt stress. UM270 ∆clsA and UM270 ∆clsB mutants showed a significant reduction in CL synthesis compared to the P. fluorescens UM270 wild-type (UM270 wt) strain (58% ∆clsA and 53% ∆clsB), and their growth rate was not affected, except when grown at 100 and 200 mM NaCl. Additionally, the root colonization capacity of both mutant strains was impaired compared with that of the wild type. Concomitant with the deletion of clsA and clsB genes, some physiological changes were observed in the UM270 ∆clsA and UM270 ∆clsB mutants, such as a reduction in indole acetic acid and biofilm production. By contrast, an increase in siderophore biosynthesis was observed. Further, inoculation of the UM270 wt strain in tomato plants (Solanum lycopersicum) grown under salt stress conditions (100 and 200 mM NaCl) resulted in an increase in root and shoot length, chlorophyll content, and dry weight. On the contrary, when each of the mutants were inoculated in tomato plants, a reduction in root length was observed when grown at 200 mM NaCl, but the shoot length, chlorophyll content, and total plant dry weight parameters were significantly reduced under normal or saline conditions (100 and 200 mM NaCl), compared to UM270 wt-inoculated plants. In conclusion, these results suggest that CL synthesis in P. fluorescens UM270 plays an important role in the promotion of tomato plant growth under normal conditions, but to a greater extent, under salt-stress conditions.

Identification of Two Variants of Acinetobacter baumannii Strain ATCC 17978 with Distinct Genotypes and Phenotypes

Acinetobacter baumannii is a nosocomial pathogen that exhibits substantial genomic plasticity. Here, the identification of two variants of A. baumannii ATCC 17978 that differ based on the presence of a 44-kb accessory locus, named AbaAL44 (A. baumannii accessory locus 44 kb), is described. Analyses of existing deposited data suggest that both variants are found in published studies of A. baumannii ATCC 17978 and that American Type Culture Collection (ATCC)-derived laboratory stocks comprise a mix of these two variants. Yet, each variant exhibits distinct interactions with the host in vitro and in vivo. Infection with the variant that harbors AbaAL44 (A. baumannii 17978 UN) results in decreased bacterial burdens and increased neutrophilic lung inflammation in a mouse model of pneumonia, and affects the production of interleukin 1 beta (IL-1β) and IL-10 by infected macrophages. AbaAL44 harbors putative pathogenesis genes, including those predicted to encode a type I pilus cluster, a catalase, and a cardiolipin synthase. The accessory catalase increases A. baumannii resistance to oxidative stress and neutrophil-mediated killing in vitro. The accessory cardiolipin synthase plays a dichotomous role by promoting bacterial uptake and increasing IL-1β production by macrophages, but also by enhancing bacterial resistance to cell envelope stress. Collectively, these findings highlight the phenotypic consequences of the genomic dynamism of A. baumannii through the evolution of two variants of a common type strain with distinct infection-related attributes.

High-speed atomic force microscopy highlights new molecular mechanism of daptomycin action

The increase in speed of the high-speed atomic force microscopy (HS-AFM) compared to that of the conventional AFM made possible the first-ever visualisation at the molecular-level of the activity of an antimicrobial peptide on a membrane. We investigated the medically prescribed but poorly understood lipopeptide Daptomycin under infection-like conditions (37 °C, bacterial lipid composition and antibiotic concentrations). We confirmed so far hypothetical models: Dap oligomerization and the existence of half pores. Moreover, we detected unknown molecular mechanisms: new mechanisms to form toroidal pores or to resist Dap action, and to unprecedently quantify the energy profile of interacting oligomers. Finally, the biological and medical relevance of the findings was ensured by a multi-scale multi-nativeness-from the molecule to the cell-correlation of molecular-level information from living bacteria (Bacillus subtilis strains) to liquid-suspended vesicles and supported-membranes using electron and optical microscopies and the lipid tension probe FliptR, where we found that the cells with a healthier state of their cell wall show smaller membrane deformations.

Amphiphilic Aminoglycosides as Medicinal Agents

The conjugation of hydrophobic group(s) to the polycationic hydrophilic core of the antibiotic drugs aminoglycosides (AGs), targeting ribosomal RNA, has led to the development of amphiphilic aminoglycosides (AAGs). These drugs exhibit numerous biological effects, including good antibacterial effects against susceptible and multidrug-resistant bacteria due to the targeting of bacterial membranes. In the first part of this review, we summarize our work in identifying and developing broad-spectrum antibacterial AAGs that constitute a new class of antibiotic agents acting on bacterial membranes. The target-shift strongly improves antibiotic activity against bacterial strains that are resistant to the parent AG drugs and to antibiotic drugs of other classes, and renders the emergence of resistant Pseudomonas aeruginosa strains highly difficult. Structure-activity and structure-eukaryotic cytotoxicity relationships, specificity and barriers that need to be crossed in their development as antibacterial agents are delineated, with a focus on their targets in membranes, lipopolysaccharides (LPS) and cardiolipin (CL), and the corresponding mode of action against Gram-negative bacteria. At the end of the first part, we summarize the other recent advances in the field of antibacterial AAGs, mainly published since 2016, with an emphasis on the emerging AAGs which are made of an AG core conjugated to an adjuvant or an antibiotic drug of another class (antibiotic hybrids). In the second part, we briefly illustrate other biological and biochemical effects of AAGs, i.e., their antifungal activity, their use as delivery vehicles of nucleic acids, of short peptide (polyamide) nucleic acids (PNAs) and of drugs, as well as their ability to cleave DNA at abasic sites and to inhibit the functioning of connexin hemichannels. Finally, we discuss some aspects of structure-activity relationships in order to explain and improve the target selectivity of AAGs.

Membrane Rigidity and Phosphatidic Acid (PtdOH) Signal: Two Important Events in Acinetobacter guillouiae SFC 500-1A Exposed to Chromium(VI) and Phenol

The remodeling of membrane lipids is a mechanism that allows microorganisms to survive in unfavorable environments such as industrial effluents, which often contain inorganic and organic pollutants, like chromium and phenol. In the present work, we evaluated the effect of Cr(VI) and phenol on the membrane of Acinetobacter guillouiae SFC 500-1A, a bacterial strain isolated from tannery sediments where such pollutants can be found. The presence of lipid kinases and phospholipases and the changes in their activities under exposure to these pollutants were determined. Cr(VI) and Cr(VI) + phenol caused the membrane to become more rigid for up to 16 h after exposure. This could be due to an increase in cardiolipin (Ptd₂ Gro) and a decrease in phosphatidylethanolamine (PtdEtn), which are indicative of more order and rigidity in the membrane. Increased phospholipase A activity (PLA, EC 3.1.1.4) could be responsible for the decrease in PtdEtn levels. Moreover, our results indicate that Cr(VI) and Cr(VI) + phenol trigger the phosphatidic acid (PtdOH) signal. The finding of significantly increased phosphatidylinositol-4-phosphate (PtdIns-4-P) levels means this is likely achieved via PtdIns-PLC/DGK. This report provides the first evidence that A. guillouiae SFC 500-1A is able to sense Cr(VI) and phenol, transduce this signal through changes in the physical state of the membrane, and trigger lipid-signaling events.

Phospholipid Content of Pseudomonas aeruginosa PAO1 Is Modulated by the Growth Phase Rather Than the Immobilization State

Biofilms have significance in medical, industrial, and environmental settings, and can cause important damage. As biofilms are tolerant to various stresses, including antibiotics, it is necessary to better understand their formation. For this reason, we characterized the phospholipidome of Pseudomonas aeruginosa, an opportunistic pathogen involved in numerous infections, during the first steps of the biofilm development. By a liquid chromatography-tandem mass spectrometry time-course analysis over a 24-h period, we compared the phospholipid (PL) composition of immobilized (attached) and planktonic (unattached) P. aeruginosa PAO1 cells. Our results showed that the PL content of P. aeruginosa PAO1 was mainly modulated by the incubation time, thus related to bacterial growth but also, more modestly, by the immobilization state. We observed that relative amounts of PL varied over time with two main profiles and that these profiles are correlated to its fatty acid composition, including the degree of unsaturation. A statistical analysis revealed that the PL contents of both attached and unattached PAO1 cells were significantly different mainly after 3 and 6 h of incubation and that the amounts of two PL presented a statistical difference between attached and unattached cells all along the 24-h period: PtdEtn 16:0_18:1 and PtdEtn 18:1_18:1.

Pseudomonas putida Δ9-fatty acid desaturase: Gene cloning, expression, and function in the cationic surfactants stress

Pseudomonas putida counteract the fluidizing effect of cationic surfactants decreasing the content of membrane unsaturated fatty acid (UFA). A Δ9-fatty acid desaturase gene (desA) from P. putida was isolated, cloned, and successfully expressed in Escherichia coli, a Δ9 desaturase deficient organism. desA consists of 1185 bp and codes for 394 amino acids. The deduced amino acid sequence reveals three histidine clusters and a hydropathy profile, typical of membrane-bound desaturases. Validating desA expression in E. coli cells, the amount of palmitoleic acid increased from 2.05 to 7.36%, with the concomitant increase in membrane fluidity (fluorescence polarization value decrease from 0.13 ± 0.03 to 0.09 ± 0.02). Also, when DesA activity was assayed in vivo, the percentage of UFA obtained from exogenous palmitic acid [1-¹⁴ C] increased 10-fold. In contrast, when cells expressing desA were exposed 15 min at sublethal concentration of cationic surfactants, the amount of UFA was 82% lower than that detected in cells non-exposed. Thus, the decrease in UFA content to counteract the fluidizing effect of cationic surfactants can be correlated with reduction of DesA activity.

Effect of cardiolipin on the antimicrobial activity of a new amphiphilic aminoglycoside derivative on Pseudomonas aeruginosa

Amphiphilic aminoglycoside derivatives are promising new antibacterials active against Gram-negative bacteria such as Pseudomonas aeruginosa, including colistin resistant strains. In this study, we demonstrated that addition of cardiolipin to the culture medium delayed growth of P. aeruginosa, favored asymmetrical growth and enhanced the efficiency of a new amphiphilic aminoglycoside derivative, the 3’,6-dinonylneamine. By using membrane models mimicking P. aeruginosa plasma membrane composition (POPE:POPG:CL), we demonstrated the ability of 3’6-dinonylneamine to induce changes in the biophysical properties of membrane model lipid systems in a cardiolipin dependent manner. These changes include an increased membrane permeability associated with a reduced hydration and a decreased ability of membrane to mix and fuse as shown by monitoring calcein release, Generalized Polarization of Laurdan and fluorescence dequenching of octadecyl rhodamine B, respectively. Altogether, results shed light on how cardiolipin may be critical for improving antibacterial action of new amphiphilic aminoglycoside derivatives.

Targeting Bacterial Cardiolipin Enriched Microdomains: An Antimicrobial Strategy Used by Amphiphilic Aminoglycoside Antibiotics

Some bacterial proteins involved in cell division and oxidative phosphorylation are tightly bound to cardiolipin. Cardiolipin is a non-bilayer anionic phospholipid found in bacterial inner membrane. It forms lipid microdomains located at the cell poles and division plane. Mechanisms by which microdomains are affected by membrane-acting antibiotics and the impact of these alterations on membrane properties and protein functions remain unclear. In this study, we demonstrated cardiolipin relocation and clustering as a result of exposure to a cardiolipin-acting amphiphilic aminoglycoside antibiotic, the 3′,6-dinonyl neamine. Changes in the biophysical properties of the bacterial membrane of P. aeruginosa, including decreased fluidity and increased permeability, were observed. Cardiolipin-interacting proteins and functions regulated by cardiolipin were impacted by the amphiphilic aminoglycoside as we demonstrated an inhibition of respiratory chain and changes in bacterial shape. The latter effect was characterized by the loss of bacterial rod shape through a decrease in length and increase in curvature. It resulted from the effect on MreB, a cardiolipin dependent cytoskeleton protein as well as a direct effect of 3′,6-dinonyl neamine on cardiolipin. These results shed light on how targeting cardiolipin microdomains may be of great interest for developing new antibacterial therapies.