TonB-dependent iron supply in Salmonella by alpha-ketoacids and alpha-hydroxyacids

Iron metabolism in the cell as a target in the development of potential antimicrobial and antiviral agents

The search and creation of innovative antimicrobial drugs, acting against resistant and multiresistant strains of bacteria and fungi, are one of the most important tasks of modern bioorganic chemistry and pharmaceuticals. Since iron is essential for the vital activity of almost all organisms, including mammals and bacteria, the proteins involved in its metabolism can serve as potential targets in the development of new promising antimicrobial agents. Such targets include endogenous mammalian biomolecules, heme oxygenases, siderophores, protein 24p3, as well as bacterial heme oxygenases and siderophores. Other proteins that are responsible for the delivery of iron to cells and its balance between bacteria and the host organism also attract certain particular interest. The review summarizes data on the development of inhibitors and inducers (activators) of heme oxygenases, selective for mammals and bacteria, and considers the characteristic features of their mechanisms of action and structure. Based on the reviewed literature data, it was concluded that the use of hemin, the most powerful hemooxygenase inducer, and its derivatives as potential antimicrobial and antiviral agents, in particular against COVID-19 and other dangerous infections, would be a promising approach. In this case, an important role is attributed to the products of hemin degradation formed by heme oxygenases in vitro and in vivo. Certain attention has been paid to the data on the antimicrobial action of iron-free protoporphyrinates, namely complexes with Co, Ga, Zn, Mn, their advantages and disadvantages compared to hemin. Modification of the well-known antibiotic ceftazidime with a siderophore molecule increased its effectiveness against resistant bacteria.

Differences between Predicted Outer Membrane Proteins of Pasteurella multocida, Histophilus somni, and Genotype 1 and 2 Mannheimia haemolytica Strains Isolated from Cattle

Common bacterial causes of bovine respiratory disease (BRD) include Histophilus somni, Mannheimia haemolytica, and Pasteurella multocida. Within M. haemolytica, two major genotypes are commonly found in cattle (1 and 2), however, genotype 2 strains are isolated from diseased lungs much more frequently than genotype 1 strains. Outer membrane proteins (OMPs) of H. somni, P. multocida, and genotype 2 M. haemolytica may be important factors for acquired host immunity. Predicted OMP differences between genotype 1 and 2 M. haemolytica have been previously identified. In this study, we expanded that focus to include bovine-isolated strain genomes representing all three species and the two M. haemolytica genotypes. Reported here are the core genomes unique to each of them, core genomes shared between some or all combinations of the three species and two M. haemolytica genotypes, and predicted OMPs within these core genomes. The OMPs identified in this study are potential candidates for further study and the development of interventions against BRD.

Proteus mirabilis and Urinary Tract Infections

Proteus mirabilis is a Gram-negative bacterium and is well known for its ability to robustly swarm across surfaces in a striking bulls'-eye pattern. Clinically, this organism is most frequently a pathogen of the urinary tract, particularly in patients undergoing long-term catheterization. This review covers P. mirabilis with a focus on urinary tract infections (UTI), including disease models, vaccine development efforts, and clinical perspectives. Flagella-mediated motility, both swimming and swarming, is a central facet of this organism. The regulation of this complex process and its contribution to virulence is discussed, along with the type VI-secretion system-dependent intra-strain competition, which occurs during swarming. P. mirabilis uses a diverse set of virulence factors to access and colonize the host urinary tract, including urease and stone formation, fimbriae and other adhesins, iron and zinc acquisition, proteases and toxins, biofilm formation, and regulation of pathogenesis. While significant advances in this field have been made, challenges remain to combatting complicated UTI and deciphering P. mirabilis pathogenesis.

Nutritional iron turned inside out: intestinal stress from a gut microbial perspective

Iron is abundantly present on earth, essential for most microorganisms and crucial for human health. Human iron deficiency that is nevertheless highly prevalent in developing regions of the world can be effectively treated by oral iron administration. Accumulating evidence indicates that excess of unabsorbed iron that enters the colonic lumen causes unwanted side effects at the intestinal host-microbiota interface. The chemical properties of iron, the luminal environment and host iron withdrawal mechanisms, especially during inflammation, can turn the intestine in a rather stressful milieu. Certain pathogenic enteric bacteria can, however, deal with this stress at the expense of other members of the gut microbiota, while their virulence also seems to be stimulated in an iron-rich intestinal environment. This review covers the multifaceted aspects of nutritional iron stress with respect to growth, composition, metabolism and pathogenicity of the gut microbiota in relation to human health. We aim to present an unpreceded view on the dynamic effects and impact of oral iron administration on intestinal host-microbiota interactions to provide leads for future research and other applications.

Biosynthesis and IroC-dependent export of the siderophore salmochelin are essential for virulence of Salmonella enterica serovar Typhimurium

In response to iron deprivation, Salmonella enterica serovar Typhimurium secretes two catecholate-type siderophores, enterobactin and its glucosylated derivative salmochelin. Although the systems responsible for enterobactin synthesis and acquisition are well characterized, the mechanisms of salmochelin secretion and acquisition, as well as its role in Salmonella virulence, are incompletely understood. Herein we show by liquid chromatography-mass spectrometry analysis of culture supernatants from wild type and isogenic mutant bacterial strains that the Major Facilitator Superfamily pump EntS is the major exporter of enterobactin and the ABC transporter IroC exports both salmochelin and enterobactin. Growth promotion experiments demonstrate that IroC is not required for utilization of Fe-enterobactin or Fe-salmochelin, as had been previously suggested, but the ABC transporter protein FepD is required for utilization of both siderophores. Salmonella mutants deficient in salmochelin synthesis or secretion exhibit reduced virulence during systemic infection of mice.

Role of receptor proteins for enterobactin and 2,3-dihydroxybenzoylserine in virulence of Salmonella enterica

Single, double, and triple mutants of an enterobactin-deficient mutant strain of Salmonella enterica serovar Typhimurium were constructed that were defective in the expression of the iron-regulated outer membrane proteins (IROMPs) FepA, IroN, and Cir, which are proposed to function as catecholate receptors. Uptake of naturally occurring and chemically synthesized catecholate molecules by these mutants was assessed in standard growth promotion assays. Unique patterns of uptake were identified for each IROMP; specifically, FepA and IroN were confirmed to be required for transport of enterobactin, and all three proteins were shown to function as receptors for the enterobactin breakdown product 2,3-dihydroxybenzoylserine. The fepA, iroN, and cir alleles were transduced to enterobactin-proficient strains of S. enterica serovar Typhimurium and S. enterica serovar Enteritidis, and the resulting phenotypes were confirmed by analysis of outer membrane protein profiles, by sensitivity to KP-736, a catecholate-cephalosporin conjugate, and by growth promotion tests on egg white agar. Intragastric infections of mice with the S. enterica serovar Typhimurium strains indicated that the parental strain and the fepA iroN double mutant were similarly virulent but that the fepA iroN cir triple mutant was significantly attenuated. Moreover, in mixed infections, the fepA iroN mutant showed similar cecal colonization and invasion of the liver to the parental strain, while the triple mutant showed significantly reduced cecal colonization and no measurable spread to the liver. Infections of 4-day-old chicks with S. enterica serovar Enteritidis strains also indicated that mutation of the fepA iroN genes did not significantly reduce cecal colonization and systemic spread compared with those of the parental strain. The results indicate that, while enterobactin uptake is not essential for the virulence of S. enterica serovars in mouse and chicken infection models, the ability to take up 2,3-dihydroxybenzoylserine via any of the three catecholate siderophore receptors appears to play an important role, since the S. enterica serovar Typhimurium triple mutant was significantly attenuated in the mouse model. Salmochelins appear not to be involved in the virulence of S. enterica.

Cloning and inactivation of a branched-chain-amino-acid aminotransferase gene from Staphylococcus carnosus and characterization of the enzyme

Staphylococcus carnosus and Staphylococcus xylosus are widely used as aroma producers in the manufacture of dried fermented sausages. Catabolism of branched-chain amino acids (BCAAs) by these strains contributes to aroma formation by production of methyl-branched aldehydes and carboxy acids. The first step in the catabolism is most likely a transamination reaction catalyzed by BCAA aminotransferases (IlvE proteins). In this study, we cloned the ilvE gene from S. carnosus by using degenerate oligonucleotides and PCR. We found that the deduced amino acid sequence was 80% identical to that of the corresponding enzyme in Staphylococcus aureus and that the ilvE gene was constitutively expressed as a monocistronic transcript. To study the influence of ilvE on BCAA catabolism, we constructed an ilvE deletion mutant by gene replacement. The IlvE protein from S. carnosus was shown mainly to catalyze the transamination of isoleucine, valine, leucine, and, to some extent, methionine using pyridoxal 5'-phosphate as a coenzyme. The ilvE mutant degraded less than 5% of the BCAAs, while the wild-type strain degraded 75 to 95%. Furthermore, the mutant strain produced approximately 100-fold less of the methyl-branched carboxy acids, 2-methylpropanoic acid, 2-methylbutanoic acid, and 3-methylbutanoic acid, which derived from the BCAA catabolism, clearly emphasizing the role of IlvE in aroma formation. In contrast to previous reports, we found that IlvE was the only enzyme that catalyzed the deamination of BCAAs in S. carnosus. The ilvE mutant strain showed remarkably lower growth rate and biomass yield compared to those of the wild-type strain when grown in rich medium. Normal growth rate and biomass yield were restored by addition of the three BCAA-derived alpha-keto acids, showing that degradation products of BCAAs were essential for optimal cell growth.

The mammalian neuroendocrine hormone norepinephrine supplies iron for bacterial growth in the presence of transferrin or lactoferrin

Norepinephrine stimulates the growth of a range of bacterial species in nutritionally poor SAPI minimal salts medium containing 30% serum. Addition of size-fractionated serum components to SAPI medium indicated that transferrin was required for norepinephrine stimulation of growth of Escherichia coli. Since bacteriostasis by serum is primarily due to the iron-withholding capacity of transferrin, we considered the possibility that norepinephrine can overcome this effect by supplying transferrin-bound iron for growth. Incubation with concentrations of norepinephrine that stimulated bacterial growth in serum-SAPI medium resulted in loss of bound iron from iron-saturated transferrin, as indicated by the appearance of monoferric and apo- isoforms upon electrophoresis in denaturing gels. Norepinephrine also caused the loss of iron from lactoferrin. The pharmacologically inactive metabolite norepinephrine 3-O-sulfate, by contrast, did not result in iron loss from transferrin or lactoferrin and did not stimulate bacterial growth in serum-SAPI medium. Norepinephrine formed stable complexes with transferrin, lactoferrin, and serum albumin. Norepinephrine-transferrin and norepinephrine-lactoferrin complexes, but not norepinephrine-apotransferrin or norepinephrine-albumin complexes, stimulated bacterial growth in serum-SAPI medium in the absence of additional norepinephrine. Norepinephrine-stimulated growth in medium containing (55)Fe complexed with transferrin or lactoferrin resulted in uptake of radioactivity by bacterial cells. Moreover, norepinephrine-stimulated growth in medium containing [(3)H]norepinephrine indicated concomitant uptake of norepinephrine. In each case, addition of excess iron did not affect growth but significantly reduced levels of radioactivity ((55)Fe or (3)H) associated with bacterial cells. A role for catecholamine-mediated iron supply in the pathophysiology of infectious diseases is proposed.

Metabolic defects caused by mutations in the isc gene cluster in Salmonella enterica serovar typhimurium: implications for thiamine synthesis

The metabolic consequences of two insertions, iscR1::MudJ and iscA2::MudJ, in the isc gene cluster of Salmonella enterica serovar Typhimurium were studied. Each of these insertions had polar effects and caused a nutritional requirement for the thiazole moiety of thiamine. Data showed that IscS was required for the synthesis of nicotinic acid and the thiazole moiety of thiamine and that one or more additional isc gene products were required for a distinct step in the thiazole biosynthetic pathway. Strains with isc lesions had reduced succinate dehydrogenase and aconitase activities. Furthermore, isc mutants accumulated increased levels of pyruvate in the growth medium in response to exogenously added iron (FeCl(3)), and this response required a functional ferric uptake regulator, Fur.

Gene cloning, sequencing, and inactivation of the branched-chain aminotransferase of Lactococcus lactis LM0230

A branched-chain aminotransferase gene (ilvE) from Lactococcus lactis LM0230 was identified on a 9-kb chromosomal insert by complementation in Escherichia coli DL39. Sequencing of a 2.0-kbp fragment resulted in the identification of a 1,023-bp open reading frame that could encode a 340-amino-acid protein. Sequence analysis of the deduced amino acid sequence revealed 62% identity to IlvE of Haemophilus influenzae and high similarity to IlvEs from a variety of organisms found in GenBank classified as class IV aminotransferases. Under logarithmic growth in complex medium, ilvE is transcribed monocistronically as a 1.1-kb transcript. Hydrophobicity plot analysis of the deduced amino acid sequence and the lack of a signal peptide sequence suggest IlvE is a cytosolic protein. A derivative of LM0230 lacking IlvE activity was constructed by gene replacement. Comparison of the IlvE-deficient strain's ability to grow in defined media lacking an amino acid but containing its alpha-keto acid biosynthetic precursor to that of the wild-type strain indicated that IlvE is the only enzyme capable of synthesis of Ile and Val from their biosynthetic precursors. Comparison of the aminotransferase activity of the IlvE mutant to LM0230 revealed that the mutant retained <2, 4.5, 43, 40, and 76% of its aminotransferase activity with Ile, Val, Leu, Met, and Phe, respectively. No difference in growth or acidification rate between LM0230 and the IlvE-deficient strain was observed in milk.

Salmonella typhimurium encodes a putative iron transport system within the centisome 63 pathogenicity island

Upon entry into the host, Salmonella enterica strains are presumed to encounter an iron-restricted environment. Consequently, these bacteria have evolved a variety of often-redundant high-affinity acquisition systems to obtain iron in this restricted environment. We have identified an iron transport system that is encoded within the centisome 63 pathogenicity island of Salmonella typhimurium. The nucleotide composition of this locus is significantly different from that of the rest of this pathogenicity island, suggesting a different ancestry and a mosaic structure for this region of the S. typhimurium chromosome. This locus, designated sit, consists of four open reading frames which encode polypeptides with extensive homology to the yfe ABC iron transport system of Yersinia pestis, as well as other ABC transporters. The sitA gene encodes a putative periplasmic binding protein, sitB encodes an ATP-binding protein, and sitC and sitD encode two putative permeases (integral membrane proteins). This operon is capable of complementing the growth defect of the enterobactin-deficient Escherichia coli strain SAB11 in iron-restricted minimal medium. Transcription of the sit operon is repressed under iron-rich growth conditions in a fur-dependent manner. Introduction of a sitBCD deletion into wild-type S. typhimurium resulted in no apparent growth defect in either nutrient-rich or minimal medium and no measurable virulence phenotype. These results further support the existence of redundant iron uptake systems in S. enterica.

Ferrioxamine-mediated Iron(III) utilization by Salmonella enterica

Utilization of ferrioxamines as sole sources of iron distinguishes Salmonella enterica serotypes Typhimurium and Enteritidis from a number of related species, including Escherichia coli. Ferrioxamine supplements have therefore been used in preenrichment and selection media to increase the bacterial growth rate while selectivity is maintained. We characterized the determinants involved in utilization of ferrioxamines B, E, and G by S. enterica serotype Typhimurium by performing siderophore cross-feeding bioassays. Transport of all three ferric siderophores across the outer membrane was dependent on the FoxA receptor encoded by the Fur-repressible foxA gene. However, only the transport of ferrioxamine G was dependent on the energy-transducing protein TonB, since growth stimulation of a tonB strain by ferrioxamines B and E was observed, albeit at lower efficiencies than in the parental strain. Transport across the inner membrane was dependent on the periplasmic binding protein-dependent ABC transporter complex comprising FhuBCD, as has been reported for other hydroxamate siderophores of enteric bacteria. The distribution of the foxA gene in the genus Salmonella, as indicated by DNA hybridization studies and correlated with the ability to utilize ferrioxamine E, was restricted to subspecies I, II, and IIIb, and this gene was absent from subspecies IIIa, IV, VI, and VII (formerly subspecies IV) and Salmonella bongori (formerly subspecies V). S. enterica serotype Typhimurium mutants with either a transposon insertion or a defined nonpolar frameshift (+2) mutation in the foxA gene were not able to utilize any of the three ferrioxamines tested. A strain carrying the nonpolar foxA mutation exhibited a significantly reduced ability to colonize rabbit ileal loops compared to the foxA+ parent. In addition, a foxA mutant was markedly attenuated in mice inoculated by either the intragastric or intravenous route. Mice inoculated with the foxA mutant were protected against subsequent challenge by the foxA+ parent strain.

Iron-regulated excretion of alpha-keto acids by Salmonella typhimurium

Excretion of alpha-keto acids by clinical isolates and laboratory strains of Salmonella typhimurium was determined by high-performance liquid chromatography analysis of culture supernatants. The levels of excretion increased markedly with increasing iron stress imposed by the presence of alpha,alpha'-dipyridyl or conalbumin in the medium. The major product was pyruvic acid, but significant concentrations of alpha-ketoglutaric acid, alpha-ketoisovaleric acid, and alpha-ketoisocaproic acid were also observed. Maximal excretion occurred at iron stress levels that initially inhibited bacterial growth; the concentration of alpha,alpha'-dipyridyl at which this was observed differed between strains depending on their ability to secrete and utilize siderophores, suggesting that the intracellular iron status was important in determining alpha-keto acid excretion. However, prolonged incubation of the siderophore-deficient S. typhimurium strain enb-7 under conditions of high iron stress resulted in significant delayed bacterial growth, promoted by tonB-dependent uptake of iron complexed with the high accumulated levels of pyruvic acid and other alpha-keto acids. Strain RB181, a fur derivative of enb-7, excreted massive amounts of alpha-keto acids into the culture medium even in the absence of any iron chelators (the concentration of pyruvic acid, for example, was >25 mM). Moreover, RB181 was able to grow and excrete alpha-keto acids in the presence of alpha,alpha'-dipyridyl at concentrations threefold greater than that which inhibited the growth of enb-7.