Peptidoglycan types of bacterial cell walls and their taxonomic implications.

Metals have been used as antimicrobial agents since antiquity, but throughout most of history their modes of action have remained unclear. Recent studies indicate that different metals cause discrete and distinct types of injuries to microbial cells as a result of oxidative stress, protein dysfunction or membrane damage. Here, we describe the chemical and toxicological principles that underlie the antimicrobial activity of metals and discuss the preferences of metal atoms for specific microbial targets. Interdisciplinary research is advancing not only our understanding of metal toxicity but also the design of metal-based compounds for use as antimicrobial agents and alternatives to antibiotics.

Antimicrobial activity of metals: mechanisms, molecular targets and applications

▪ Abstract The ability of pathogens to obtain iron from transferrins, ferritin, hemoglobin, and other iron-containing proteins of their host is central to whether they live or die. To combat invading bacteria, animals go into an iron-withholding mode and also use a protein (Nramp1) to generate reactive oxygen species in an attempt to kill the pathogens. Some invading bacteria respond by producing specific iron chelators—siderophores—that remove the iron from the host sources. Other bacteria rely on direct contact with host iron proteins, either abstracting the iron at their surface or, as with heme, taking it up into the cytoplasm. The expression of a large number of genes (>40 in some cases) is directly controlled by the prevailing intracellular concentration of Fe(II) via its complexing to a regulatory protein (the Fur protein or equivalent). In this way, the biochemistry of the bacterial cell can accommodate the challenges from the host. Agents that interfere with bacterial iron metabolism may prove ex...

Iron metabolism in pathogenic bacteria.

Summary: High-affinity iron acquisition is mediated by siderophore-dependent pathways in the majority of pathogenic and nonpathogenic bacteria and fungi. Considerable progress has been made in characterizing and understanding mechanisms of siderophore synthesis, secretion, iron scavenging, and siderophore-delivered iron uptake and its release. The regulation of siderophore pathways reveals multilayer networks at the transcriptional and posttranscriptional levels. Due to the key role of many siderophores during virulence, coevolution led to sophisticated strategies of siderophore neutralization by mammals and (re)utilization by bacterial pathogens. Surprisingly, hosts also developed essential siderophore-based iron delivery and cell conversion pathways, which are of interest for diagnostic and therapeutic studies. In the last decades, natural and synthetic compounds have gained attention as potential therapeutics for iron-dependent treatment of infections and further diseases. Promising results for pathogen inhibition were obtained with various siderophore-antibiotic conjugates acting as “Trojan horse” toxins and siderophore pathway inhibitors. In this article, general aspects of siderophore-mediated iron acquisition, recent findings regarding iron-related pathogen-host interactions, and current strategies for iron-dependent pathogen control will be reviewed. Further concepts including the inhibition of novel siderophore pathway targets are discussed.

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Siderophore-Based Iron Acquisition and Pathogen Control

Salmonella typhimurium is a facultative intracellular pathogen capable of surviving within phagocytic cells of the reticuloendothelial system. To identify the genes important for intracellular survival, 9516 independent Tn10 insertional mutations were isolated in a virulent strain of S. typhimurium. By using an in vitro assay for survival within macrophages, 83 Tn10 mutants have been identified that have a diminished capacity for intracellular survival (designated MS or macrophage survival mutants). All of the MS mutants are less virulent than the parent strain in vivo, demonstrating that, for Salmonella, survival within the macrophage is essential for virulence. Thirty-seven of the MS mutants have been characterized as to their phenotype, including several mutations that confer sensitivity to specific microbiocidal mechanisms of the macrophage.

https://www.pnas.org/content/pnas/83/14/5189.full.pdf

Mutants of Salmonella typhimurium that cannot survive within the macrophage are avirulent

Previous work suggested that virulent bacteria, which can grow rapidly in serum, must possess a specific mechanism for removing iron from its transferrin complex. Two strains of Escherichia coli were examined with this in mind. Strain O141, which showed inoculum-dependent growth in serum and multiplied in the mouse peritoneum, secreted iron-binding catechols into both synthetic medium and serum. One of these compounds has an association constant for iron similar to that of transferrin. Both transferrin and ethylenediamine-di-o-hydroxyphenyl acetic acid (EDDA), which have very high affinities for ferric iron, induced catechol synthesis in growing cultures of strain O111. This organism was inhibited by normal horse serum. Further work showed that traces of specific antibody inhibited catechol synthesis by O111 exposed to EDDA; therefore, the existence of this inhibitory process means that the organism can no longer obtain Fe3+, which all remains bound to transferrin in serum. In vivo, the inhibition of O111 is similar to that produced by serum in vitro. Neither phagocytosis nor killing by complement appeared to be of any significance during the first 4 h of the infections. Significantly, the purified catechol was capable of abolishing bacteriostasis in vivo. Since these results show that the production of iron-binding catechols is essential for rapid bacterial growth both in vitro and in vivo, these compounds should therefore be considered as true virulence factors. Conversely, any interference by the host with the production or activity of these compounds would constitute an important aspect of antibacterial defense. Images

Iron-Binding Catechols and Virulence in Escherichia coli.

Bacterial resistance to antibiotics is a major threat to clinical medicine. However, natural resistance to bacterial infection, which does not depend on antibiotics, is a powerful protective mechanism common to all mankind. The availability of iron is the heart of the matter and the successful functioning of these antibacterial systems depends entirely upon an extremely low level of free ionic iron (10−18 M) in normal tissue fluids. This in turn depends on well-oxygenated tissues where the oxidation–reduction potential (Eh) and pH control the binding of iron by unsaturated transferrin and lactoferrin.

Bacterial virulence is greatly enhanced by freely available iron, such as that in fully-saturated transferrin or free haemoglobin. Following trauma a fall in tissue Eh and pH due to ischaemia, plus the reducing powers of bacteria, can make iron in transferrin freely available and abolish the bactericidal properties of tissue fluids with disastrous results for the host. Hyperbaric oxygen is a possible therapeutic measure that could restore normal bactericidal systems in infected tissues by raising the Eh and pH.

Iron and infection: the heart of the matter

Natural resistance to infection, which does not depend on antibiotics, is a powerful protective mechanism common to all mankind that has been responsible for the survival of our species during countless millennia in the past The normal functioning of this complex system of phagocytic cells and tissue fluids is entirely dependent on an extremely low level of free ionic iron (10(-18) M) in tissue fluids This low-iron environment is maintained by the unsaturated iron-binding proteins transferrin and lactoferrin, which depend on well-oxygenated tissues, where a relatively high oxidation-reduction potential (Eh) and pH are essential for the binding of ferric iron Freely available iron is derived from iron overload, free haem compounds, or hypoxia in injured tissue leading to a fall in Eh and pH This can severely damage or abolish normal bactericidal mechanisms in tissue fluids leading to overwhelming growth of bacteria or fungi The challenge for clinical medicine is to reduce or eliminate the presence of freely available iron in clinical disease In injured or hypoxic tissue, treatment with hyperbaric oxygen might prove very useful by increasing tissue oxygenation and restoring normal bactericidal mechanisms in tissue fluids, which would be of huge benefit to the patient

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Natural resistance, iron and infection: a challenge for clinical medicine.

MUCH recent work1–4 has shown that a considerable proportion of the chondroitin sulphate in the ground substance of cartilage is combined in some way or other with protein. It is possible that the relative ease with which the mucopolysaccharide can be obtained in a protein-free form5 by extraction with mild alkali is a reflexion of the alkali lability of some of the protein-mucopolysaccharide bonds4. It is also known that papain and trypsin2,4 lower the viscosity of solutions of chondroitin sulphate–protein complex and partially remove the proteins. Plasmin is a protease resulting from the activation of serum plasminogen by diverse means, including the action of the so-called kinases of streptococci6 and staphylococci7,8. In view of the chondrolytic effects of joint infections caused by staphylococci and streptococci which produce kinases, in contrast to infection by other organisms, such as the tubercle, which do not produce kinases, it seemed important to see whether plasmin preparations would liberate chondroitin sulphate from cartilage in vitro: such a liberation might be a step in destruction of cartilage in vivo.

Action of plasmin on cartilage.

Unsaturated transferrin in plasma ensures that the amount of free ferric iron available to bacteria is about 10 -18 mol/L. This low iron environment is essential for the bacteriostatic and bactericidal systems in blood, lymph, and exudates. Antibacterial systems are abolished when iron becomes freely available. This results in rapid extracellular bacterial growth and greatly increased bacterial virulence. In human plasma, a fall in E h (oxidation-reduction potential) or pH results in the abolition or marked reduction of its bactericidal properties. This is highly relevant to infection after trauma, where a fall in E h and pH frequently accompanies tissue damage. Bacterial resistance to antibiotics has put the treatment of serious infections in jeopardy. Reinforcement of natural means of resistance needs to be explored, as well as examining new antibacterials that interfere with bacterial iron metabolism.

H. J. Rogers

Papers

Iron-Binding Catechols and Virulence in Escherichia coli.

Iron and infection: the heart of the matter

Natural resistance, iron and infection: a challenge for clinical medicine.

Action of plasmin on cartilage.

Iron and infection: new developments and their implications.