Arginine glycosylation regulates UDP-GlcNAc biosynthesis in Salmonella enterica

Role of glycosylation in bacterial resistance to carbapenems

Carbapenems are a class of β-lactam antibacterial drugs with a broad antibacterial spectrum and strong activity, commonly used to treat serious bacterial infections. However, improper or excessive use of carbapenems can lead to increased bacterial resistance, which is a significant concern as they are often used as last resort for treating multidrug-resistant (MDR) gram-negative bacteria. Confronted with this challenge, it is crucial to comprehensively understand the mechanism of carbapenem resistance to develop effective therapeutic strategies and innovative drugs. In recent years, emerging research on the glycosylation of bacterial proteins has highlighted the crucial role of glycans in various bacterial processes, including carbapenem resistance. Given the limited understanding of bacterial glycosylation, its role in in carbapenem resistance may be more pivotal than currently acknowledged. In this review, we summarize the direct and multifunctional role of glycosylation in bacterial resistance as well as the classical and recently reported mechanisms of bacterial carbapenem resistance, focusing on illuminating the potential role of glycosylation in carbapenem resistance. We also discuss the potential of leveraging this knowledge to develop more effective strategies for combating clinically resistant bacteria.

Metabolomic profiling reveals biomarkers for diverse flesh colors in jelly fungi (Auricularia cornea)

Auricularia cornea has garnered attention due to its nutrition, culinary applications, and promising commercial prospects. However, there is little information available regarding the metabolic profiling of various colors strains. In this study, 642 metabolites across 64 classes were identified by LC-MS/MS to understand the metabolic variations between white, pink and dark brown strains. Notably, prenol lipids, carboxylic acids and fatty acyls accounted for 46.8 % of the total. Comparative analysis revealed 17 shared differential metabolites (DMs) among them. ACP vs ACW exhibited 17 unique metabolites, including d-arginine and maleic acid, etc. ACP vs ACB showed 5 unique metabolites, with only PS(18:1(9Z)/0:0) demonstrating up-regulation. ACB vs ACW showed 8 unique metabolites, including 4-hydroxymandelic acid and 5'-methylthioadenosine, etc. KEGG enrichment analysis highlighted pathway variations, and MetPA analysis identified key-pathways influencing DMs accumulation in A. cornea. This pioneering metabolomics study offers insights into A. cornea metabolic profiling, potential applications, and guides further research.

Salmonella T3SS effector SseK1 arginine-glycosylates the two-component response regulator OmpR to alter bile salt resistance

Type III secretion system (T3SS) effector proteins are primarily recognized for binding host proteins to subvert host immune response during infection. Besides their known host target proteins, several T3SS effectors also interact with endogenous bacterial proteins. Here we demonstrate that the Salmonella T3SS effector glycosyltransferase SseK1 glycosylates the bacterial two-component response regulator OmpR on two arginine residues, R15 and R122. Arg-glycosylation of OmpR results in reduced expression of ompF, a major outer membrane porin gene. Glycosylated OmpR has reduced affinity to the ompF promoter region, as compared to the unglycosylated form of OmpR. Additionally, the Salmonella ΔsseK1 mutant strain had higher bile salt resistance and increased capacity to form biofilms, as compared to WT Salmonella, thus linking OmpR glycosylation to several important aspects of bacterial physiology.

Intrabacterial Regulation of a Cytotoxic Effector by Its Cognate Metaeffector Promotes Legionella pneumophila Virulence

Legionella pneumophila is a natural pathogen of unicellular protozoa that can opportunistically infect macrophages and cause Legionnaires' Disease. Intracellular replication is driven by hundreds of bacterial effector proteins that are translocated into infected host cells by a Dot/Icm type IV secretion system. L. pneumophila effectors are temporally regulated in part by a unique family of translocated regulatory effectors, termed metaeffectors, which bind and modulate the function of a cognate effector in host cells. Regulation of the cytotoxic effector SidI by its cognate metaeffector, MesI, is critical for L. pneumophila virulence in natural and opportunistic hosts. MesI binds and negatively regulates SidI activity in vitro, but how impaired regulation of SidI impairs L. pneumophila intracellular replication is unclear. Using a chromosomally encoded inducible expression system, we found that SidI was toxic to L. pneumophila when uncoupled from MesI. SidI enzymatic activity was required for intrabacterial toxicity since L. pneumophila growth was unaffected by induced expression of a catalytically inactive sidI allele. We also found that MesI translocation into host cells was dispensable for intracellular replication and that MesI-deficient bacteria were rapidly degraded within host cells. These data suggest that MesI promotes L. pneumophila intracellular replication by regulating SidI within the bacterium and reveal a unique role for intrabacterial effector regulation by a translocated metaeffector in L. pneumophila virulence. IMPORTANCE Legionella pneumophila replicates within phagocytic host cells using hundreds of effector protein virulence factors, which canonically subvert the function of host proteins and pathways. L. pneumophila encodes a unique family of translocated effectors called metaeffectors, which bind and regulate the function of a cognate effector in host cells. The metaeffector MesI promotes L. pneumophila virulence by regulating the cytotoxic effector SidI; however, the MesI regulatory mechanism is poorly understood. We discovered a unique intrabacterial role for MesI in L. pneumophila virulence. When uncoupled from MesI, SidI was toxic to L. pneumophila in vitro and triggered robust bacterial degradation in host cells. Furthermore, translocation of MesI was dispensable for intracellular replication, demonstrating that intrabacterial regulation of SidI contributes to L. pneumophila virulence. These data show a novel and important role for translocated effector activity within the bacterium, which challenges the dogma that L. pneumophila effectors function exclusively within host cells.

L-arginine metabolism as pivotal interface of mutual host-microbe interactions in the gut

L-arginine (L-arg) is a versatile amino acid and a central intestinal metabolite in mammalian and microbial organisms. Thus, L-arg participates as precursor of multiple metabolic pathways in the regulation of cell division and growth. It also serves as a source of carbon, nitrogen, and energy or as a substrate for protein synthesis. Consequently, L-arg can simultaneously modify mammalian immune functions, intraluminal metabolism, intestinal microbiota, and microbial pathogenesis. While dietary intake, protein turnover or de novo synthesis usually supply L-arg in sufficient amounts, the expression of several key enzymes of L-arg metabolism can change rapidly and dramatically following inflammation, sepsis, or injury. Consequently, the availability of L-arg can be restricted due to increased catabolism, transforming L-arg into an essential amino acid. Here, we review the enzymatic pathways of L-arg metabolism in microbial and mammalian cells and their role in immune function, intraluminal metabolism, colonization resistance, and microbial pathogenesis in the gut.

FXR/ASS1 axis attenuates the TAA-induced liver injury through arginine metabolism

Previous studies demonstrated that arginine biosynthesis was frequently impaired in acute liver injury. However, the underlying mechanisms remain elusive. In this study, we found that Argininosuccinate synthetase 1 (ASS1), a rate-limiting enzyme in arginine metabolism, was downregulated in the TAA-induced liver injury model. Single-cell RNA-seq data found that ASS1 was highly enriched in the hepatocytes. The reduction of ASS1 was attributed to the decreased expression of Farnesoid X receptor (FXR), which is a bile acid-activated nuclear hormone receptor with high expression in the liver. Subsequent studies demonstrated that activation of FXR by its agonist obeticholic acid (OCA) directly promoted ASS1 transcription and enhanced arginine synthesis, leading to the alleviation of TAA-mediated liver injury. Further experiments found that OCA, ASS1, and arginine supplement can rescue TAA-mediated hepatocytes apoptosis by decreasing the protein levels of Cyto C, PARP, and Caspase 3. Taken together, our study illustrated a protective role of the FXR/ASS1 axis in TAA-induced liver injury by targeting arginine metabolism, which might shed light on the development of novel therapeutic approaches for acute liver injury.