Salmonella enterica relies on carbon metabolism to adapt to agricultural environments.

Salmonella enterica, a foodborne and human pathogen, is a constant threat to human health. Agricultural environments, for example, soil and plants, can be ecological niches and vectors for Salmonella transmission. Salmonella persistence in such environments increases the risk for consumers. Therefore, it is necessary to investigate the mechanisms used by Salmonella to adapt to agricultural environments. We assessed the adaptation strategy of S. enterica serovar Typhimurium strain 14028s to agricultural-relevant situations by analyzing the abundance of intermediates in glycolysis and the tricarboxylic acid pathway in tested environments (diluvial sand soil suspension and leaf-based media from tomato and lettuce), as well as in bacterial cells grown in such conditions. By reanalyzing the transcriptome data of Salmonella grown in those environments and using an independent RT-qPCR approach for verification, several genes were identified as important for persistence in root or leaf tissues, including the pyruvate dehydrogenase subunit E1 encoding gene aceE. In vivo persistence assay in tomato leaves confirmed the crucial role of aceE. A mutant in another tomato leaf persistence-related gene, aceB, encoding malate synthase A, displayed opposite persistence features. By comparing the metabolites and gene expression of the wild-type strain and its aceB mutant, fumarate accumulation was discovered as a potential way to replenish the effects of the aceB mutation. Our research interprets the mechanism of S. enterica adaptation to agriculture by adapting its carbon metabolism to the carbon sources available in the environment. These insights may assist in the development of strategies aimed at diminishing Salmonella persistence in food production systems.

Minimal SPI1-T3SS effector requirement for Salmonella enterocyte invasion and intracellular proliferation in vivo.

Effector molecules translocated by the Salmonella pathogenicity island (SPI)1-encoded type 3 secretion system (T3SS) critically contribute to the pathogenesis of human Salmonella infection. They facilitate internalization by non-phagocytic enterocytes rendering the intestinal epithelium an entry site for infection. Their function in vivo has remained ill-defined due to the lack of a suitable animal model that allows visualization of intraepithelial Salmonella. Here, we took advantage of our novel neonatal mouse model and analyzed various bacterial mutants and reporter strains as well as gene deficient mice. Our results demonstrate the critical but redundant role of SopE2 and SipA for enterocyte invasion, prerequisite for transcriptional stimulation and mucosal translocation in vivo. In contrast, the generation of a replicative intraepithelial endosomal compartment required the cooperative action of SipA and SopE2 or SipA and SopB but was independent of SopA or host MyD88 signaling. Intraepithelial growth had no critical influence on systemic spread. Our results define the role of SPI1-T3SS effector molecules during enterocyte invasion and intraepithelial proliferation in vivo providing novel insight in the early course of Salmonella infection.

Self-labelling enzymes as universal tags for fluorescence microscopy, super-resolution microscopy and electron microscopy.

Research in cell biology demands advanced microscopy techniques such as confocal fluorescence microscopy (FM), super-resolution microscopy (SRM) and transmission electron microscopy (TEM). Correlative light and electron microscopy (CLEM) is an approach to combine data on the dynamics of proteins or protein complexes in living cells with the ultrastructural details in the low nanometre scale. To correlate both data sets, markers functional in FM, SRM and TEM are required. Genetically encoded markers such as fluorescent proteins or self-labelling enzyme tags allow observations in living cells. Various genetically encoded tags are available for FM and SRM, but only few tags are suitable for CLEM. Here, we describe the red fluorescent dye tetramethylrhodamine (TMR) as a multimodal marker for CLEM. TMR is used as fluorochrome coupled to ligands of genetically encoded self-labelling enzyme tags HaloTag, SNAP-tag and CLIP-tag in FM and SRM. We demonstrate that TMR can additionally photooxidize diaminobenzidine (DAB) to an osmiophilic polymer visible on TEM sections, thus being a marker suitable for FM, SRM and TEM. We evaluated various organelle markers with enzymatic tags in mammalian cells labelled with TMR-coupled ligands and demonstrate the use as efficient and versatile DAB photooxidizer for CLEM approaches.

Characterization of the type III export signal of the flagellar hook scaffolding protein FlgD of Escherichia coli.

Transport of flagellar structural proteins beyond the cytoplasmic membrane is accomplished by a type III secretory pathway [flagellar type III secretion system (fTTSS)]. The mechanism of substrate recognition by the fTTSS is still enigmatic. Using the hook scaffolding protein FlgD of Escherichia coli as a model substrate, it is demonstrated that the export signal is contained within the N-terminal 71 amino acids of FlgD. Analysis of frame-shift mutations and alterations of the nucleotide sequence suggest a proteinaceous nature of the signal. Furthermore, the physicochemical properties of the first about eight amino acids are crucial for export.

Interresidual distance determination by four-pulse double electron-electron resonance in an integral membrane protein: the Na+/proline transporter PutP of Escherichia coli.

Proximity relationships within three doubly spin-labeled variants of the Na+/proline transporter PutP of Escherichia coli were studied by means of four-pulse double electron-electron resonance spectroscopy. The large value of 4.8 nm for the interspin distance determined between positions 107 in loop 4 and 223 in loop 7 strongly supports the idea of these positions being located on opposite sides of the membrane. Significant smaller values of between 1.8 and 2.5 nm were found for the average interspin distances between spin labels attached to the cytoplasmic loops 2 and 4 (position 37 and 107) and loops 2 and 6 (position 37 and 187). The large distance distribution widths visible in the pair correlation functions reveal a high flexibility of the studied loop regions. An increase of the distance between positions 37 and 187 upon Na+ binding suggests ligand-induced structural alterations of PutP. The results demonstrate that four-pulse double electron-electron resonance spectroscopy is a powerful means to investigate the structure and conformational changes of integral membrane proteins reconstituted in proteoliposomes.

CaiT of Escherichia coli, a new transporter catalyzing L-carnitine/gamma -butyrobetaine exchange.

l-Carnitine is essential for beta-oxidation of fatty acids in mitochondria. Bacterial metabolic pathways are used for the production of this medically important compound. Here, we report the first detailed functional characterization of the caiT gene product, a putative transport protein whose function is required for l-carnitine conversion in Escherichia coli. The caiT gene was overexpressed in E. coli, and the gene product was purified by affinity chromatography and reconstituted into proteoliposomes. Functional analyses with intact cells and proteoliposomes demonstrated that CaiT is able to catalyze the exchange of l-carnitine for gamma-butyrobetaine, the excreted end product of l-carnitine conversion in E. coli, and related betaines. Electrochemical ion gradients did not significantly stimulate l-carnitine uptake. Analysis of l-carnitine counterflow yielded an apparent external K(m) of 105 microm and a turnover number of 5.5 s(-1). Contrary to related proteins, CaiT activity was not modulated by osmotic stress. l-Carnitine binding to CaiT increased the protein fluorescence and caused a red shift in the emission maximum, an observation explained by ligand-induced conformational alterations. The fluorescence effect was specific for betaine structures, for which the distance between trimethylammonium and carboxyl groups proved to be crucial for affinity. Taken together, the results suggest that CaiT functions as an exchanger (antiporter) for l-carnitine and gamma-butyrobetaine according to the substrate/product antiport principle.

Sites important for Na+ and substrate binding in the Na+/proline transporter of Escherichia coli, a member of the Na+/solute symporter family.

To elucidate the functional importance of transmembrane domain II in the Na(+)/proline transporter (PutP) of Escherichia coli we analyzed the effect of replacing Ser-54 through Gly-58. Substitution of Asp-55 or Met-56 dramatically reduces the apparent affinity for Na(+) and Li(+) in a cation-dependent manner. Conversely, Cys in place of Gly-58 significantly reduces only the apparent proline affinity while substitution of Ser-57 results in a dramatic reduction of the apparent proline and cation affinities. Interestingly, upon increasing the proline concentration the apparent Na(+) affinity of Ser-57 replacement mutants converges toward the wild-type value, indicating a close cooperativity between cation and substrate site(s). This notion is supported by the fact that Na(+)-stimulated site-specific fluorescence labeling of a single Cys at position 57 is completely reversed by the addition of proline. Similar results are obtained upon labeling of a Cys at position 54 or 58. Taken together, these results indicate that Asp-55 and Met-56 are located at or close to the ion-binding site while Ser-54, Ser-57, and Gly-58 may be close to the proline translocation pathway. In addition, the data prod at an involvement of the latter residues in ligand-induced conformational dynamics that are crucial for cation-coupled transport.