Functional assessment of microbial superoxide dismutase isozymes suggests a differential role for each isozyme

Unravelling the key steps impairing the metabolic state of Xanthomonas cells undergoing programmed cell death

Programmed cell death (PCD) has been reported in Xanthomonas axonopodis pv. glycines (Xag) wild type earlier and was indirectly shown to be induced by metabolic stress; however, deciphering the key proteins regulating the metabolic stress remained unrevealed. In this study, transcriptomic and proteomic analyses were performed to investigate the prominent pathways, having a role in the induction of metabolic stress in Xag cells undergoing PCD. A comprehensive analysis of transcriptome and proteome data revealed the major involvement of metabolic pathways related to branched chain amino acid degradation, such as acyl-CoA dehydrogenase and energy-yielding, ubiquinol:cytochrome c oxidoreductase complex, in Xag cells undergoing PCD. Consequently, oxidative stress response genes showed major upregulation in Xag cells in PCD-inducing medium; however, no such upregulation was observed at the protein level, indicative of depleted protein levels under excessive stress conditions. Activation of stress response and DNA repair proteins was also observed in Xag cells grown in PCD-inducing medium, which is indicative of excessive cellular damage. Thus, the findings indicate that programmed cell death in Xag is an outcome of metabolic stress in nutrient condition not suitable for a plant pathogen like Xanthomonas, which is more acclimatised with altogether a different nutritional requirement predominantly having an enriched carbohydrate source.

Environmental and genetic regulation of Streptococcus pneumoniae galactose catabolic pathways

Efficient utilization of nutrients is crucial for microbial survival and virulence. The same nutrient may be utilized by multiple catabolic pathways, indicating that the physical and chemical environments for induction as well as their functional roles may differ. Here, we study the tagatose and Leloir pathways for galactose catabolism of the human pathogen Streptococcus pneumoniae. We show that galactose utilization potentiates pneumococcal virulence, the induction of galactose catabolic pathways is influenced differentially by the concentration of galactose and temperature, and sialic acid downregulates galactose catabolism. Furthermore, the genetic regulation and in vivo induction of each pathway differ, and both galactose catabolic pathways can be turned off with a galactose analogue in a substrate-specific manner, indicating that galactose catabolic pathways can be potential drug targets.

Zinc acquisition and its contribution to Klebsiella pneumoniae virulence

Klebsiella pneumoniae is a World Health Organization priority pathogen and a significant clinical concern for infections of the respiratory and urinary tracts due to widespread and increasing resistance to antimicrobials. In the absence of a vaccine, there is an urgent need to identify novel targets for therapeutic development. Bacterial pathogens, including K. pneumoniae, require the d-block metal ion zinc as an essential micronutrient, which serves as a cofactor for ~6% of the proteome. During infection, zinc acquisition necessitates the use of high affinity uptake systems to overcome niche-specific zinc limitation and host-mediated nutritional immunity. Here, we report the identification of ZnuCBA and ZniCBA, two ATP-binding cassette permeases that are highly conserved in Klebsiella species and contribute to K. pneumoniae AJ218 zinc homeostasis, and the high-resolution structure of the zinc-recruiting solute-binding protein ZniA. The Znu and Zni permeases appear functionally redundant with abrogation of both systems required to reduce K. pneumoniae zinc accumulation. Disruption of both systems also exerted pleiotropic effects on the homeostasis of other d-block elements. Zinc limitation perturbed K. pneumoniae cell morphology and compromised resistance to stressors, such as salt and oxidative stress. The mutant strain lacking both systems showed significantly impaired virulence in acute lung infection models, highlighting the necessity of zinc acquisition in the virulence and pathogenicity of K. pneumoniae.

Potential of White Jack Bean (Canavalia ensiformis L. DC) Kefir as a Microencapsulated Antioxidant

Oxidative stress plays a major role in the pathogenesis and progression of noncommunicable diseases. Kefir is a fermented food that has been reported to repress oxidative stress. This study aimed to assess the antioxidant activity, bioactive composition, and encapsulation efficiency of white jack bean (WJB) kefir. The following procedures were conducted: WJB was prepared and converted into juice using water solvent. The sterilized WJB juice was then fermented with kefir grain (10%) for 24∼72 h. Every 24 h, the kefir was evaluated for antioxidant activity, and the dominant bioactive component suspected to be the source of the antioxidant activity was identified. The final stage was the encapsulation process. WJB kefir showed high antioxidant activity, inhibiting DPPH radicals by 90.51±4.73% and ABTS radicals by 86.63±2.34% after 72 h of fermentation. WJB kefir contained 0.35±0.01 mg GAE/g total phenolics and 0.08 mg/g total flavonoids. The LC/MS identification suggested that the bioactive antioxidant components of the WJB kefir were from the alkaloid, saponin, phenolic, and flavonoid groups. The encapsulation with maltodextrin using freeze drying resulted in microencapsulation of WJB kefir with a particle size of 6.42±0.13 μm. The encapsulation efficiency was 79.61%, and the IC50 value was 32.62 ppm. The encapsulation method was able to maintain the antioxidant stability of the kefir and extend its shelf life. WJB kefir, a nondairy, lactose-free kefir, can be used as an antioxidant functional food.

Metalloenzymes play major roles to achieve high-rate nitrogen removal in N-damo communities: Lessons from metaproteomics

Nitrite-driven anaerobic methane oxidation (N-damo) is a promising biological process to achieve carbon-neutral wastewater treatment solutions, aligned with the sustainable development goals. Here, the enzymatic activities in a membrane bioreactor highly enriched in N-damo bacteria operated at high nitrogen removal rates were investigated. Metaproteomic analyses, with a special focus on metalloenzymes, revealed the complete enzymatic route of N-damo including their unique nitric oxide dismutases. The relative protein abundance evidenced that "Ca. Methylomirabilis lanthanidiphila" was the predominant N-damo species, attributed to the induction of its lanthanide-binding methanol dehydrogenase in the presence of cerium. Metaproteomics also disclosed the activity of the accompanying taxa in denitrification, methylotrophy and methanotrophy. The most abundant functional metalloenzymes from this community require copper, iron, and cerium as cofactors which was correlated with the metal consumptions in the bioreactor. This study highlights the usefulness of metaproteomics for evaluating the enzymatic activities in engineering systems to optimize microbial management.

Metal manipulators and regulators in human pathogens: A comprehensive review on microbial redox copper metalloenzymes "multicopper oxidases and superoxide dismutases"

The chemistry of metal ions with human pathogens is essential for their survival, energy generation, redox signaling, and niche dominance. To regulate and manipulate the metal ions, various enzymes and metal chelators are present in pathogenic bacteria. Metalloenzymes incorporate transition metal such as iron, zinc, cobalt, and copper in their reaction centers to perform essential metabolic functions; however, iron and copper have gained more importance. Multicopper oxidases have the ability to perform redox reaction on phenolic substrates with the help of copper ions. They have been reported from Enterobacteriaceae, namely Salmonella enterica, Escherichia coli, and Yersinia enterocolitica, but their role in virulence is still poorly understood. Similarly, superoxide dismutases participate in reducing oxidative stress and allow the survival of pathogens. Their role in virulence and survival is well established in Salmonella typhimurium and Mycobacterium tuberculosis. Further, to ensure survival against stress, like metal starvation or metal toxicity, redox metalloenzymes and metal transportation systems of pathogens actively participate in metal homeostasis. Recently, the omics and protein structure biology studies have helped to predict new targets for regulation the colonization potential of the pathogenic strains. The current review is focused on the major roles of redox metalloenzymes, especially MCOs and SODs of human pathogenic bacteria.

The Klebsiella pneumoniae ter Operon Enhances Stress Tolerance

Healthcare-acquired infections are a leading cause of disease in patients that are hospitalized or in long-term-care facilities. Klebsiella pneumoniae (Kp) is a leading cause of bacteremia, pneumonia, and urinary tract infections in these settings. Previous studies have established that the ter operon, a genetic locus that confers tellurite oxide (K2TeO3) resistance, is associated with infection in colonized patients. Rather than enhancing fitness during infection, the ter operon increases Kp fitness during gut colonization; however, the biologically relevant function of this operon is unknown. First, using a murine model of urinary tract infection, we demonstrate a novel role for the ter operon protein TerC as a bladder fitness factor. To further characterize TerC, we explored a variety of functions, including resistance to metal-induced stress, resistance to radical oxygen species-induced stress, and growth on specific sugars, all of which were independent of TerC. Then, using well-defined experimental guidelines, we determined that TerC is necessary for tolerance to ofloxacin, polymyxin B, and cetylpyridinium chloride. We used an ordered transposon library constructed in a Kp strain lacking the ter operon to identify the genes that are required to resist K2TeO3-induced and polymyxin B-induced stress, which suggested that K2TeO3-induced stress is experienced at the bacterial cell envelope. Finally, we confirmed that K2TeO3 disrupts the Kp cell envelope, though these effects are independent of ter. Collectively, the results from these studies indicate a novel role for the ter operon as a stress tolerance factor, thereby explaining its role in enhancing fitness in the gut and bladder.

Why is manganese so valuable to bacterial pathogens?

Apart from oxygenic photosynthesis, the extent of manganese utilization in bacteria varies from species to species and also appears to depend on external conditions. This observation is in striking contrast to iron, which is similar to manganese but essential for the vast majority of bacteria. To adequately explain the role of manganese in pathogens, we first present in this review that the accumulation of molecular oxygen in the Earth's atmosphere was a key event that linked manganese utilization to iron utilization and put pressure on the use of manganese in general. We devote a large part of our contribution to explanation of how molecular oxygen interferes with iron so that it enhances oxidative stress in cells, and how bacteria have learned to control the concentration of free iron in the cytosol. The functioning of iron in the presence of molecular oxygen serves as a springboard for a fundamental understanding of why manganese is so valued by bacterial pathogens. The bulk of this review addresses how manganese can replace iron in enzymes. Redox-active enzymes must cope with the higher redox potential of manganese compared to iron. Therefore, specific manganese-dependent isoenzymes have evolved that either lower the redox potential of the bound metal or use a stronger oxidant. In contrast, redox-inactive enzymes can exchange the metal directly within the individual active site, so no isoenzymes are required. It appears that in the physiological context, only redox-inactive mononuclear or dinuclear enzymes are capable of replacing iron with manganese within the same active site. In both cases, cytosolic conditions play an important role in the selection of the metal used. In conclusion, we summarize both well-characterized and less-studied mechanisms of the tug-of-war for manganese between host and pathogen.

Genomic Landscape Highlights Molecular Mechanisms Involved in Silicate Solubilization, Stress Tolerance, and Potential Growth-Promoting Activity of Bacterium Enterobacter sp. LR6

Silicon (Si) is gaining widespread attention due to its prophylactic activity to protect plants under stress conditions. Despite Si's abundance in the earth's crust, most soils do not have enough soluble Si for plants to absorb. In the present study, a silicate-solubilizing bacterium, Enterobacter sp. LR6, was isolated from the rhizospheric soil of rice and subsequently characterized through whole-genome sequencing. The size of the LR6 genome is 5.2 Mb with a GC content of 54.9% and 5182 protein-coding genes. In taxogenomic terms, it is similar to E. hormaechei subsp. xiangfangensis based on average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH). LR6 genomic data provided insight into potential genes involved in stress response, secondary metabolite production, and growth promotion. The LR6 genome contains two aquaporins, of which the aquaglyceroporin (GlpF) is responsible for the uptake of metalloids including arsenic (As) and antimony (Sb). The yeast survivability assay confirmed the metalloid transport activity of GlpF. As a biofertilizer, LR6 isolate has a great deal of tolerance to high temperatures (45 °C), salinity (7%), and acidic environments (pH 9). Most importantly, the present study provides an understanding of plant-growth-promoting activity of the silicate-solubilizing bacterium, its adaptation to various stresses, and its uptake of different metalloids including As, Ge, and Si.

Quantitative proteomic analysis of marine biofilms formed by filamentous cyanobacterium

Cyanobacterial molecular biology can identify pathways that affect the adhesion and settlement of biofouling organisms and, consequently, obtain novel antifouling strategies for marine applications. Proteomic analyses can provide an essential understanding of how cyanobacteria adapt to different environmental settings. However, only a few qualitative studies have been performed in some cyanobacterial strains. Considering the limited knowledge about protein expression in cyanobacteria in different growing conditions, a quantitative proteomic analysis by LC-MS/MS of biofilm cells from a filamentous strain was performed. Biofilms were also analysed through standard methodologies for following cyanobacterial biofilm development. Biofilms were formed on glass and perspex at two relevant hydrodynamic conditions for marine environments (average shear rates of 4 s^-1 and 40 s^-1). Biofilm development was higher at 4 s^-1 and no significant differences were found between surfaces. Proteomic analysis identified 546 proteins and 41 were differentially expressed. Differences in protein expression were more noticeable between biofilms formed on glass and perspex at 4 s^-1. When comparing biofilms formed on different surfaces, results suggest that biofilm development may be related to the expression of several proteins like a beta-propeller domain-containing protein, chaperone DnaK, SLH domain-containing proteins, an OMF family outer membrane protein, and/or additional uncharacterized proteins. Regarding the hydrodynamic effect, biofilm development can be related to SOD enzyme expression, to proteins related to photosynthetic processes and to a set of uncharacterized proteins with calcium binding domains, disordered proteins, and others involved in electron transfer activity. Studies that combine distinct approaches are essential for finding new targets for antibiofilm agents. The characterisation performed in this work provides new insights into how shear rate and surface affect cyanobacterial biofilm development and how cyanobacteria adapt to these different environmental settings from a macroscopic standpoint to a proteomics context.

Phenazine-1-carboxylic Acid Produced by Pseudomonas chlororaphis YL-1 Is Effective against Acidovorax citrulli

The bacterial pathogen Acidovorax citrulli causes the destructive fruit blotch (BFB) on cucurbit plants. Pseudomonas chlororaphis YL-1 is a bacterial strain isolated from Mississippi soil and its genome harbors some antimicrobial-related gene clusters, such as phenazine, pyrrolnitrin, and pyoverdine. Here, we evaluated the antimicrobial activity of strain YL-1 as compared with its deficient mutants of antimicrobial-related genes, which were obtained using a sacB-based site-specific mutagenesis strategy. We found that only phenazine-deficient mutants ΔphzE and ΔphzF almost lost the inhibitory effects against A. citrulli in LB plates compared with the wild-type strain YL-1, and that the main antibacterial compound produced by strain YL-1 in LB medium was phenazine-1-carboxylic acid (PCA) based on the liquid chromatography-mass spectrometry (LC-MS) analysis. Gene expression analyses revealed that PCA enhanced the accumulation of reactive oxygen species (ROS) and increased the activity of catalase (CAT) in A. citrulli. The inhibition effect of PCA against A. citrulli was lowered by adding exogenous CAT. PCA significantly upregulated the transcript level of katB from 6 to 10 h, which encodes CAT that helps to protect the bacteria against oxidative stress. Collectively, the findings of this research suggest PCA is one of the key antimicrobial metabolites of bacterial strain YL-1, a promising biocontrol agent for disease management of BFB of cucurbit plants.

The Rgg1518 transcriptional regulator is a necessary facet of sugar metabolism and virulence in Streptococcus pneumoniae

Rggs are a group of transcriptional regulators with diverse roles in metabolism and virulence. Here, we present work on the Rgg1518/SHP1518 quorum sensing system of Streptococcus pneumoniae. The activity of Rgg1518 is induced by its cognate peptide, SHP1518. In vitro analysis showed that the Rgg1518 system is active in conditions rich in galactose and mannose, key nutrients during nasopharyngeal colonization. Rgg1518 expression is highly induced in the presence of these sugars and its isogenic mutant is attenuated in growth on galactose and mannose. When compared with other Rgg systems, Rgg1518 has the largest regulon on galactose. On galactose it controls up- or downregulation of a functionally diverse set of genes involved in galactose metabolism, capsule biosynthesis, iron metabolism, protein translation, as well as other metabolic functions, acting mainly as a repressor of gene expression. Rgg1518 is a repressor of capsule biosynthesis, and binds directly to the capsule regulatory region. Comparison with other Rggs revealed inter-regulatory interactions among Rggs. Finally, the rgg1518 mutant is attenuated in colonization and virulence in a mouse model of colonization and pneumonia. We conclude that Rgg1518 is a virulence determinant that contributes to a regulatory network composed of multiple Rgg systems.

Redox Active Antimicrobial Peptides in Controlling Growth of Microorganisms at Body Barriers

Epithelia in the skin, gut and other environmentally exposed organs display a variety of mechanisms to control microbial communities and limit potential pathogenic microbial invasion. Naturally occurring antimicrobial proteins/peptides and their synthetic derivatives (here collectively referred to as AMPs) reinforce the antimicrobial barrier function of epithelial cells. Understanding how these AMPs are functionally regulated may be important for new therapeutic approaches to combat microbial infections. Some AMPs are subject to redox-dependent regulation. This review aims to: (i) explore cysteine-based redox active AMPs in skin and intestine; (ii) discuss casual links between various redox environments of these barrier tissues and the ability of AMPs to control cutaneous and intestinal microbes; (iii) highlight how bacteria, through intrinsic mechanisms, can influence the bactericidal potential of redox-sensitive AMPs.