Expression of the translocator protein (TSPO) from Pseudomonas fluorescens Pf0-1 requires the stress regulatory sigma factors AlgU and RpoH

The Human Gut and Dietary Salt: The Bacteroides/Prevotella Ratio as a Potential Marker of Sodium Intake and Beyond

The gut microbiota is a dynamic ecosystem that plays a pivotal role in maintaining host health. The perturbation of these microbes has been linked to several health conditions. Hence, they have emerged as promising targets for understanding and promoting good health. Despite the growing body of research on the role of sodium in health, its effects on the human gut microbiome remain under-explored. Here, using nutrition and metagenomics methods, we investigate the influence of dietary sodium intake and alterations of the human gut microbiota. We found that a high-sodium diet (HSD) altered the gut microbiota composition with a significant reduction in Bacteroides and inverse increase in Prevotella compared to a low-sodium diet (LSD). However, there is no clear distinction in the Firmicutes/Bacteroidetes (F/B) ratio between the two diet types. Metabolic pathway reconstruction revealed the presence of sodium reabsorption genes in the HSD, but not LSD. Since it is currently difficult in microbiome studies to confidently associate the F/B ratio with what is considered healthy (e.g., low sodium) or unhealthy (e.g., high sodium), we suggest that the use of a genus-based ratio such as the Bacteroides/Prevotella (B/P) ratio may be more beneficial for the application of microbiome studies in health.

Elucidating the pivotal role of TSPO in porphyrin-related cellular processes, in Bacillus cereus

A structural homolog of the mammalian TSPO has been identified in the human pathogen Bacillus cereus. BcTSPO, in its recombinant form, has previously been shown to bind and degrade porphyrins. In this study, we generated a ΔtspO mutant strain in B. cereus ATCC 14579 and assessed the impact of the absence of BcTSPO on cellular proteomics and physiological characteristics. The proteomic analysis revealed correlations between the lack of BcTSPO and the observed growth defects, increased oxygen consumption, ATP deficiency, heightened tryptophan catabolism, reduced motility, and impaired biofilm formation in the ΔtspO mutant strain. Our results also suggested that BcTSPO plays a crucial role in regulating intracellular levels of metabolites from the coproporphyrin-dependent branch of the heme biosynthetic pathway. This regulation potentially underlies alterations in the metabolic landscape, emphasizing the pivotal role of BcTSPO in B. cereus aerobic metabolism. Notably, our study unveils, for the first time, the involvement of TSPO in tryptophan metabolism. These findings underscore the multifaceted role of TSPO, not only in metabolic pathways but also potentially in the microorganism's virulence mechanisms.

Combined Transcriptome and Proteome Analysis of RpoS Regulon Reveals Its Role in Spoilage Potential of Pseudomonas fluorescens

Microbial contamination is considered the main cause of food spoilage. Pseudomonas fluorescens is a typical spoilage bacterium contributing to a large extent to the spoilage process of proteinaceous foods. RpoS is known as an alternative sigma factor controlling stress resistance and virulence in many pathogens. Our previous work revealed that RpoS contributes to the spoilage activities of P. fluorescens by regulating resistance to different stress conditions, extracellular acylated homoserine lactone (AHL) levels, extracellular protease and total volatile basic nitrogen (TVB-N) production. However, RpoS-dependent genes in P. fluorescens remained undefined. RNA-seq transcriptomics analysis combined with quantitative proteomics analysis based on multiplexed isobaric tandem mass tag (TMT) labeling was performed in the P. fluorescens wild-type strain UK4 and its derivative carrying an rpoS mutation. A total of 375 differentially expressed coding sequences (DECs) and 212 differentially expressed proteins (DEPs) were identified. The DECs were further verified by qRT-PCR. The combined transcriptome and proteome analyses revealed the involvement of this regulator in several cellular processes, mainly including polysaccharide metabolism, intracellular secretion, extracellular structures, cell wall biogenesis, stress responses, and amino acid and biogenic amine metabolism, which may contribute to the biofilm formation, stress resistance, and spoilage activities of P. fluorescens. Moreover, we indeed observed that RpoS contributed to the production of the macrocolony biofilm's matrix. Our results provide insights into the regulatory network of RpoS and expand the knowledge about the role of RpoS in the functioning of P. fluorescens in food spoilage.

Tryptophan-Rich Sensory Protein/Translocator Protein (TSPO) from Cyanobacterium Fremyella diplosiphon Binds a Broad Range of Functionally Relevant Tetrapyrroles

Tryptophan-rich sensory protein/translocator protein (TSPO) is a membrane protein involved in stress adaptation in the cyanobacterium Fremyella diplosiphon. Characterized mammalian and proteobacterial TSPO homologues bind tetrapyrroles and cholesterol ligands. We investigated the ligand binding properties of TSPO from F. diplosiphon (FdTSPO1), which was functionally characterized in prior genetic studies. Two additional TSPO proteins (FdTSPO2 and FdTSPO3) are present in F. diplosiphon; they are similar in size to reported bacterial TSPOs and smaller than FdTSPO1. The longer cyanobacterial TSPO1 is found almost exclusively in filamentous cyanobacteria and has a relatively low degree of homology to bacterial and mammalian TSPO homologues with confirmed tetrapyrrole binding. To probe distinctions of long-form TSPOs, we tested the binding of porphyrin and bilin to FdTSPO1 and measured binding affinities in the low micromolar range, with the highest binding affinity detected for heme. Although tetrapyrrole ligands bound FdTSPO1 with affinities similar to those previously reported for proteobacterial TSPO, binding of cholesterol to FdTSPO1 was particularly poor and was not improved by introducing an amino acid motif known to enhance cholesterol binding in other bacterial TSPO homologues. Additionally, we detected limited binding of bacterial hopanoids to FdTSPO1. Cyanobacterial TSPO1 from the oxygenic photosynthetic F. diplosiphon, thus, binds a range of tetrapyrroles of functional relevance with efficiencies similar to those of mammalian and proteobacterial homologues, but the level of cholesterol binding is greatly reduced compared to that of mammalian TSPO. Furthermore, the ΔFdTSPO1 mutant exhibits altered growth in the presence of biliverdin compared to that of wild-type cells under green light. Together, these results suggest that TSPO molecules may play roles in bilin homeostasis or trafficking in cyanobacteria.

The Tryptophan-Rich Sensory Protein (TSPO) is Involved in Stress-Related and Light-Dependent Processes in the Cyanobacterium Fremyella diplosiphon

The tryptophan-rich sensory protein (TSPO) is a membrane protein, which is a member of the 18 kDa translocator protein/peripheral-type benzodiazepine receptor (MBR) family of proteins that is present in most organisms and is also referred to as Translocator protein 18 kDa. Although TSPO is associated with stress- and disease-related processes in organisms from bacteria to mammals, full elucidation of the functional role of the TSPO protein is lacking for most organisms in which it is found. In this study, we describe the regulation and function of a TSPO homolog in the cyanobacterium Fremyella diplosiphon, designated FdTSPO. Accumulation of the FdTSPO transcript is upregulated by green light and in response to nutrient deficiency and stress. A F. diplosiphon TSPO deletion mutant (i.e., ΔFdTSPO) showed altered responses compared to the wild type (WT) strain under stress conditions, including salt treatment, osmotic stress, and induced oxidative stress. Under salt stress, the FdTSPO transcript is upregulated and a ΔFdTSPO mutant accumulates lower levels of reactive oxygen species (ROS) and displays increased growth compared to WT. In response to osmotic stress, FdTSPO transcript levels are upregulated and ΔFdTSPO mutant cells exhibit impaired growth compared to the WT. By comparison, methyl viologen-induced oxidative stress results in higher ROS levels in the ΔFdTSPO mutant compared to the WT strain. Taken together, our results provide support for the involvement of membrane-localized FdTSPO in mediating cellular responses to stress in F. diplosiphon and represent detailed functional analysis of a cyanobacterial TSPO. This study advances our understanding of the functional roles of TSPO homologs in vivo.