Tat transport in Escherichia coli requires zwitterionic phosphatidylethanolamine but no specific negatively charged phospholipid

Coping with stress: How bacteria fine-tune protein synthesis and protein transport

Maintaining a functional proteome under different environmental conditions is challenging for every organism, in particular for unicellular organisms, such as bacteria. In order to cope with changing environments and stress conditions, bacteria depend on strictly coordinated proteostasis networks that control protein production, folding, trafficking, and degradation. Regulation of ribosome biogenesis and protein synthesis are cornerstones of this cellular adaptation in all domains of life, which is rationalized by the high energy demand of both processes and the increased resistance of translationally silent cells against internal or external poisons. Reduced protein synthesis ultimately also reduces the substrate load for protein transport systems, which are required for maintaining the periplasmic, inner, and outer membrane subproteomes. Consequences of impaired protein transport have been analyzed in several studies and generally induce a multifaceted response that includes the upregulation of chaperones and proteases and the simultaneous downregulation of protein synthesis. In contrast, generally less is known on how bacteria adjust the protein targeting and transport machineries to reduced protein synthesis, e.g., when cells encounter stress conditions or face nutrient deprivation. In the current review, which is mainly focused on studies using Escherichia coli as a model organism, we summarize basic concepts on how ribosome biogenesis and activity are regulated under stress conditions. In addition, we highlight some recent developments on how stress conditions directly impair protein targeting to the bacterial membrane. Finally, we describe mechanisms that allow bacteria to maintain the transport of stress-responsive proteins under conditions when the canonical protein targeting pathways are impaired.

Characterization of the Role of Two-Component Systems in Antibiotic Resistance Formation in Salmonella enterica Serovar Enteritidis

The two-component system (TCS) is one of the primary pathways by which bacteria adapt to environmental stresses such as antibiotics. This study aimed to systematically explore the role of TCSs in the development of multidrug resistance (MDR) in Salmonella enterica serovar Enteritidis. Twenty-six in-frame deletion mutants of TCSs were generated from S. Enteritidis SJTUF12367 (the wild type [WT]). Antimicrobial susceptibility tests with these mutants revealed that 10 TCSs were involved in the development of antibiotic resistance in S. Enteritidis. In these 10 pairs of TCSs, functional defects in CpxAR, PhoPQ, and GlnGL in various S. Enteritidis isolates led to a frequent decrease in MIC values against at least three classes of clinically important antibiotics, including cephalosporins and quinolones, which indicated the importance of these TCSs to the formation of MDR. Interaction network analysis via STRING revealed that the genes cpxA, cpxR, phoP, and phoQ played important roles in the direct interaction with global regulatory genes and the relevant genes of efflux pumps and outer membrane porins. Quantitative reverse transcription-PCR analysis further demonstrated that the increased susceptibility to cephalosporins and quinolones in ΔphoP and ΔcpxR mutant cells was accompanied by increased expression of membrane porin genes (ompC, ompD, and ompF) and reduced expression of efflux pump genes (acrA, macB, and mdtK), as well as an adverse transcription of the global regulatory genes (ramA and crp). These results indicated that CpxAR and PhoPQ played an important role in the development of MDR in S. Enteritidis through regulation of cell membrane permeability and efflux pump activity. IMPORTANCE S. Enteritidis is a predominant Salmonella serotype that causes human salmonellosis and frequently exhibits high-level resistance to commonly used antibiotics, including cephalosporins and quinolones. Although TCSs are known as regulators for bacterial adaptation to stressful conditions, which modulates β-lactam resistance in Vibrio parahaemolyticus and colistin resistance in Salmonella enterica serovar Typhimurium, there is little knowledge of their functional mechanisms underlying the development of antibiotic resistance in S. Enteritidis. Here, we systematically identified the TCS elements in S. Enteritidis SJTUF12367, revealed that the three TCSs CpxAR, PhoPQ, and GlnGL were crucial for the MDR formation in S. Enteritidis, and preliminarily illustrated the regulatory functions of CpxAR and PhoPQ for antimicrobial resistance genes. Our work provides the basis to understand the important TCSs that regulate formation of antibiotic resistance in S. Enteritidis.

Octenidine's Efficacy: A Matter of Interpretation or the Influence of Experimental Setups?

With its broad antimicrobial spectrum and non-specific mode of action via membrane disruption, any resistance to octenidine (OCT) seems unlikely and has not been observed in clinical settings so far. In this study, we aimed to investigate the efficacy of OCT against Escherichia coli and mutants lacking specific lipid head groups which, due to altered membrane properties, might be the root cause for resistance development of membrane-active compounds. Furthermore, we aimed to test its efficacy under different experimental conditions including different solvents for OCT, bacterial concentration and methods for analysis. Our primary goal was to estimate how many OCT molecules are needed to kill one bacterium. We performed susceptibility assays by observing bacterial growth behavior, using a Bioscreen in an analogous manner for every condition. The growth curves were recorded for 20 h at 420-580 nm in presence of different OCT concentrations and were used to assess the inhibitory concentrations (IC_100%) for OCT. Bacterial concentrations given in cell numbers were determined, followed by Bioscreen measurement by manual colony counting on agar plates and QUANTOM^TM cell staining. This indicated a significant variance between both methods, which influenced IC_100% of OCT, especially when used at low doses. The binding capacity of OCT to E. coli was investigated by measuring UV-absorbance of OCT exposed to bacteria and a common thermodynamic framework based on Bioscreen measurements. Results showed that OCT's antimicrobial activity in E. coli is not affected by changes at the membrane level but strongly dependent on experimental settings in respect to solvents and applied bacterial counts. More OCT was required when the active was dissolved in phosphate or Hepes buffers instead of water and when higher bacterial concentration was used. Furthermore, binding studies revealed that 10⁷-10⁸ OCT molecules bind to bacteria, which is necessary for the saturation of the bacterial surface to initiate the killing cascade. Our results clearly demonstrate that in vitro data, depending on the applied materials and the methods for determination of IC_100%, can easily be misinterpreted as reduced bacterial susceptibility towards OCT.

Genetic analysis of tellurate reduction reveals the selenate/tellurate reductase genes ynfEF and the transcriptional regulation of moeA by NsrR in Escherichia coli

Several bacteria can reduce tellurate into the less toxic elemental tellurium, but the genes responsible for this process have not yet been identified. In this study, we screened the Keio collection of single-gene knockouts of Escherichia coli responsible for decreased tellurate reduction and found that deletions of 29 genes, including those for molybdenum cofactor (Moco) biosynthesis, iron-sulphur biosynthesis, and the twin-arginine translocation pathway resulted in decreased tellurate reduction. Among the gene knockouts, deletions of nsrR, moeA, yjbB, ynbA, ydaS and yidH affected tellurate reduction more severely than those of other genes. Based on our findings, we determined that the ynfEF genes, which code for the components of the selenate reductase YnfEFGH, are responsible for tellurate reduction. Assays of several molybdoenzymes in the knockouts suggested that nsrR, yjbB, ynbA, ydaS and yidH are essential for the activities of molybdoenzymes in E. coli. Furthermore, we found that the nitric oxide sensor NsrR positively regulated the transcription of the Moco biosynthesis gene moeA. These findings provided new insights into the complexity and regulation of Moco biosynthesis in E. coli.

Phospholipid distribution in the cytoplasmic membrane of Gram-negative bacteria is highly asymmetric, dynamic, and cell shape-dependent

The distribution of phospholipids across the inner membrane (IM) of Gram-negative bacteria is unknown. We demonstrate that the IMs of Escherichia coli and Yersinia pseudotuberculosis are asymmetric, with a 75%/25% (cytoplasmic/periplasmic leaflet) distribution of phosphatidylethanolamine (PE) in rod-shaped cells and an opposite distribution in E. coli filamentous cells. In initially filamentous PE-lacking E. coli cells, nascent PE appears first in the periplasmic leaflet. As the total PE content increases from nearly zero to 75%, cells progressively adopt a rod shape and PE appears in the cytoplasmic leaflet of the IM. The redistribution of PE influences the distribution of the other lipids between the leaflets. This correlates with the tendency of PE and cardiolipin to regulate antagonistically lipid order of the bilayer. The results suggest that PE asymmetry is metabolically controlled to balance temporally the net rates of synthesis and translocation, satisfy envelope growth capacity, and adjust bilayer chemical and physical properties.

The Dynamic ATP-Driven Mechanism of Bacterial Protein Translocation and the Critical Role of Phospholipids

Protein secretion from the cell cytoplasm to the outside is essential for life. Bacteria do so for a range of membrane associated and extracellular activities, including envelope biogenesis, surface adherence, pathogenicity, and degradation of noxious chemicals such as antibiotics. The major route for this process is via the ubiquitous Sec system, residing in the plasma membrane. Translocation across (secretion) or into (insertion) the membrane is driven through the translocon by the action of associated energy-transducing factors or translating ribosomes. This review seeks to summarize the recent advances in the dynamic mechanisms of protein transport and the critical role played by lipids in this process. The article will include an exploration of how lipids are actively involved in protein translocation and the consequences of these interactions for energy transduction from ATP hydrolysis and the trans-membrane proton-motive-force (PMF).

Precursor-Receptor Interactions in the Twin Arginine Protein Transport Pathway Probed with a New Receptor Complex Preparation

The twin arginine translocation (Tat) system moves folded proteins across the cytoplasmic membrane of bacteria and the thylakoid membrane of plant chloroplasts. Signal peptide-bearing substrates of the Tat pathway (precursor proteins) are recognized at the membrane by the TatBC receptor complex. The only established preparation of the TatBC complex uses the detergent digitonin, rendering it unsuitable for biophysical analysis. Here we show that the detergent glyco-diosgenin (GDN) can be used in place of digitonin to isolate homogeneous TatBC complexes that bind precursor proteins with physiological specificity. We use this new preparation to quantitatively characterize TatBC-precursor interactions in a fully defined system. Additionally, we show that the GDN-solubilized TatBC complex co-purifies with substantial quantities of phospholipids.