Comparative proteomic analysis of Salmonella Typhimurium wild type and its isogenic fnr null mutant during anaerobiosis reveals new insight into bacterial metabolism and virulence

Critical role of msgA in invasive capacity and intracellular survivability of Salmonella

Salmonella enterica serovar Typhimurium, which is a common foodborne pathogen, causes both intestinal and systemic infections in hosts. Salmonella has a complex pathogenic mechanism that involves invasive capacity and intracellular survivability, which hampers research on virulence of Salmonella. The virulence of Salmonella is primarily studied through Salmonella pathogenicity islands (SPIs). However, there are also genes outside these SPIs that significantly impact virulence. Macrophage survival gene msgA is positioned at a region independent of the SPIs and conserved in Salmonella. However, there has been limited research on msgA to date. This study aims to investigate the virulent function of msgA to deepen our understanding of Salmonella virulence. Proteomic and RT-qPCR analyses reveal that MsgA influences multiple metabolic pathways and the expression of SPIs. The depletion of msgA led to the significantly reduced invasive capacity and intracellular survivability, and thus the decreased virulence of Salmonella. In conclusion, our study suggests that MsgA is an important regulator that mainly regulates virulence. Further research into the function of MsgA will enhance the understanding of Salmonella pathogenesis and promote the application of Salmonella for medical treatment.

IMPORTANCE

Salmonella enterica serovar Typhimurium is a common foodborne pathogen, it has a complex pathogenic mechanism that involves invasive capacity and intracellular survivability. The virulence of Salmonella is primarily studied through its pathogenicity islands. In contrast, virulence genes located outside the Salmonella pathogenicity islands (SPIs) have received less attention. Macrophage survival gene (MsgA) is positioned at a region independent of the SPIs and conserved in Salmonella. Our research indicates that MsgA is a novel global regulator influencing the metabolic pathways and SPIs. Further research into the function of MsgA will enhance the understanding of Salmonella pathogenesis and promote the application of Salmonella for medical treatment.

Role of narL gene in the pathogenesis of Salmonella Typhimurium

Salmonella Typhimurium (STM) is a facultative anaerobe and one of the causative agents of nontyphoidal salmonellosis (NTS). Its anaerobic metabolism is enabled under the hypoxic environment that is encountered inside macrophages and the gut lumen of the host. In both of these niches, free radicals and oxidative intermediates are released by neutrophils as an inflammatory response. These chemical species further undergo reactions to produce nitrate, which is preferably taken up by STM as an electron acceptor in the absence of oxygen. NarL, the response regulator of the two-component regulatory system NarX/L, and a transcription factor, gets activated under anaerobic nitrate-rich conditions and upregulates the nitrate reduction during anaerobic respiration of STM. To understand the role of NarL in the pathogenesis of STM, we generated a narL-knockout (STM:ΔnarL) as well as a narL-complemented strain of STM. Anaerobically, the mutant displayed no growth defect but a significant attenuation in the swimming (26%) and swarming (61%) motility, and biofilm-forming ability (73%) in vitro, while these morphotypes got rescued upon genetic complementation. We also observed a downregulation in the expression of genes associated with nitrate reduction (narG) and biofilm formation (csgA and csgD) in anaerobically grown STM:ΔnarL. As compared with wild STM, narL mutant exhibited a threefold reduction in the intracellular replication in both intestinal epithelial cells (INT- 407) and monocyte-derived macrophages of poultry origin. Further, in vivo competitive assay in the liver and spleen of the murine model showed a competitive index of 0.48 ± 0.58 and 0.403668 ± 0.32, respectively, for STM:ΔnarL.

Proteomics approaches: A review regarding an importance of proteome analyses in understanding the pathogens and diseases

Proteomics is playing an increasingly important role in identifying pathogens, emerging and re-emerging infectious agents, understanding pathogenesis, and diagnosis of diseases. Recently, more advanced and sophisticated proteomics technologies have transformed disease diagnostics and vaccines development. The detection of pathogens is made possible by more accurate and time-constrained technologies, resulting in an early diagnosis. More detailed and comprehensive information regarding the proteome of any noxious agent is made possible by combining mass spectrometry with various gel-based or short-gun proteomics approaches recently. MALDI-ToF has been proved quite useful in identifying and distinguishing bacterial pathogens. Other quantitative approaches are doing their best to investigate bacterial virulent factors, diagnostic markers and vaccine candidates. Proteomics is also helping in the identification of secreted proteins and their virulence-related functions. This review aims to highlight the role of cutting-edge proteomics approaches in better understanding the functional genomics of pathogens. This also underlines the limitations of proteomics in bacterial secretome research.

Effect of ethanolamine utilization on the pathogenicity and metabolic profile of enterotoxigenic Escherichia coli

Bacterial pathogenicity is greatly affected by nutrient recognition and utilization in the host microenvironment. The characterization of enteral nutrients that promote intestinal pathogen virulence is helpful for developing new adjuvant therapies and inhibiting host damage. Ethanolamine (EA), as a major component of intestinal epithelial cells and bacterial membranes, is abundant in the intestine. Here, we provide the first demonstration that the critical human and porcine pathogen enterotoxigenic Escherichia coli (ETEC) can utilize EA as a nitrogen source, which affects its virulence phenotype. We found that compared with that in M9 medium (containing NH₄Cl), EA inhibited ETEC growth to a certain extent; however, the relative expression levels of virulence-related genes, such as ltA (3.0-fold), fimH (2.9-fold), CfaD (2.6-fold), gspD (3.6-fold), and qesE (1.3-fold), increased significantly with 15 mM EA as a nitrogen source (P < 0.05), and the adhesion efficiency of ETEC to Caco-2 cells increased approximately 4.2-fold. In Caco-2 cells, the relative cell viability decreased from 74.8 to 63.4%, and the transepithelial electrical resistance (TEER) cells decreased to 74.8% with intestinal EA (4 mM). In addition, the relative expression levels of proinflammatory factors, such as TNF-α (3.2-fold), INF-γ (2.9-fold), and IL-1β (1.98-fold), in ETEC-infected Caco-2 cells were significantly upregulated (P < 0.05) under EA exposure; however, the above virulence changes were not found in ΔeutR and ΔeutB ETEC. A gas chromatography-mass spectrometry (GC-MS)-based untargeted metabolomics approach was then employed to reveal EA-induced metabolic reprogramming related to ETEC virulence. The data showed that most metabolites related to carbohydrate, aspartate and glutamate metabolism, shikimic acid metabolism, and serine metabolism in ETEC exhibited a decreasing trend with increases in the EA concentration from 0 to 15 mM, but the branched-chain amino acid (BCAA) levels in ETEC increased in a dose-dependent manner under EA exposure. Our data suggest that the intestinal EA concentration can significantly affect the virulence phenotype, metabolic profile, and pathogenicity of ETEC. KEY POINTS: • ETEC growth and virulence gene expression could be regulated by ethanolamine. • The intestinal concentration of EA promoted the damaging effect of ETEC on the host epithelial barrier. • The promoting effect of EA on ETEC toxicity may be related to BCAA metabolism.