The microcins

Non-classical roles of bacterial siderophores in pathogenesis

Within host environments, iron availability is limited, which instigates competition for this essential trace element. In response, bacteria produce siderophores, secondary metabolites that scavenge iron and deliver it to bacterial cells via specific receptors. This role in iron acquisition contributes significantly to bacterial pathogenesis, thereby designating siderophores as virulence factors. While prior research has primarily focused on unravelling the molecular mechanisms underlying siderophore biosynthesis, uptake, and iron sequestration, recent investigations have unveiled additional non-iron chelating functions of siderophores. These emerging roles are being consistently shown to support bacterial pathogenesis. In this review, we present the current understanding of siderophores in various roles: acquiring non-iron metal ions, supporting tolerance to metal-induced and reactive oxygen species (ROS)-induced stresses, mediating siderophore signalling, inducing ROS formation, and functioning in class IIb microcins. By integrating recent findings, this review aims to provide an overview of the diverse roles of siderophores in bacterial pathogenesis.

Opportunities and challenges of RiPP-based therapeutics

Covering: up to 2024Ribosomally synthesised and post-translationally modified peptides (RiPPs) comprise a substantial group of peptide natural products exhibiting noteworthy bioactivities ranging from antiinfective to anticancer and analgesic effects. Furthermore, RiPP biosynthetic pathways represent promising production routes for complex peptide drugs, and the RiPP technology is well-suited for peptide engineering to produce derivatives with specific functions. Thus, RiPP natural products possess features that render them potentially ideal candidates for drug discovery and development. Nonetheless, only a small number of RiPP-derived compounds have successfully reached the market thus far. This review initially outlines the therapeutic opportunities that RiPP-based compounds can offer, whilst subsequently discussing the limitations that require resolution in order to fully exploit the potential of RiPPs towards the development of innovative drugs.

Influence of PepF peptidase and sporulation on microcin J25 production in Bacillus subtilis

The lasso peptide microcin J25 (MccJ25) possesses strong antibacterial properties and is considered a potential effective component of bacterial disease treatment drugs and safe food preservatives. Although MccJ25 can be heterologously expressed in Bacillus subtilis as we have previously reported, its regulation and accumulation are yet to be understood. Here, we investigated the expression level and stability of MccJ25 in B. subtilis strains with disruption in peptidase genes pepA, pepF, and pepT. Oligoendopeptidase F (PepF) was found to be involved in reduction of the production of MccJ25 by degradation of its precursor peptide. In the pepF mutant, the MccJ25 reached a concentration of 1.68 µM after a cultivation time exceeding 60 hours, while the wild-type strain exhibited a concentration of only 0.14 µM. Moreover, the production of MccJ25 in B. subtilis downregulated the genes associated with sporulation, and this may contribute to its accumulation. Finally, this study provides a strategy to improve the stability and production of MccJ25 in B. subtilis.

IMPORTANCE

MccJ25 displays significant antibacterial activity, a well-defined mode of action, exceptional safety, and remarkable stability. Hence, it presents itself as a compelling candidate for an optimal antibacterial or anti-endotoxin medication. The successful establishment of exogenous production of MccJ25 in Bacillus subtilis provides a strategy for reducing its production cost and diversifying its utilization. In this study, we have provided evidence indicating that both peptidase PepF and sporulation are significant factors that limit the expression of MccJ25 in B. subtilis. The ΔpepF and ΔsigF mutants of B. subtilis express MccJ25 with higher production yield and enhanced stability. To sum up, this study developed several better engineered strains of B. subtilis, which greatly reduced the consumption of MccJ25 during the nutrient depletion stage of the host strain, improved its production, and elucidated factors that may be involved in reducing MccJ25 accumulation in B. subtilis.

The Prevalence of Antibiotic Tolerance in Neisseria gonorrhoeae Varies by Anatomical Site

BACKGROUND

Tolerance enables bacteria to survive intermittent antibiotic exposure without an increase in antimicrobial susceptibility. In this study, we investigated the presence of tolerance to three antimicrobials, ceftriaxone, azithromycin and ciprofloxacin, in clinical isolates and the WHO (World Health Organization) reference panel of Neisseria gonorrhoeae.

METHODS

We used the modified tolerance disk (TD test) to assess for tolerance to ceftriaxone, azithromycin and ciprofloxacin in 14 WHO reference strains and 62 N. gonorrhoeae clinical isolates-evenly divided between anorectal and urogenital infections. The isolates underwent a three-step incubation process wherein the isolates were exposed to an antibiotic disk for 20 h of incubation (Step I), followed by the replacement of the antibiotic disk with a nutrient disk for overnight incubation (Step II) and additional overnight incubation with extra nutrients (Step III).

RESULTS

A total of 4 of the 62 clinical anorectal isolates and none of the urogenital isolates exhibited tolerance to azithromycin (p = 0.033). Tolerance to ceftriaxone and ciprofloxacin was observed in eight and four isolates, respectively, with no difference between infection sites. Tolerance was also detected in 8 (K, M, N, O, P, U, V, W) out of the 14 WHO reference strains, with varying patterns of tolerance to ceftriaxone (n = 8), ciprofloxacin (n = 2) and azithromycin (n = 1).

CONCLUSIONS

This study identified ceftriaxone, azithromycin and ciprofloxacin tolerance in clinical and WHO reference N. gonorrhoeae isolates. Azithromycin tolerance was more common in anorectal than urogenital infections.

Peptide Dendrimer-Based Antibacterial Agents: Synthesis and Applications

Pathogenic bacteria cause the deaths of millions of people every year. With the development of antibiotics, hundreds and thousands of people's lives have been saved. Nevertheless, bacteria can develop resistance to antibiotics, rendering them insensitive to antibiotics over time. Peptides containing specific amino acids can be used as antibacterial agents; however, they can be easily degraded by proteases in vivo. To address these issues, branched peptide dendrimers are now being considered as good antibacterial agents due to their high efficacy, resistance to protease degradation, and low cytotoxicity. The ease with which peptide dendrimers can be synthesized and modified makes them accessible for use in various biological and nonbiological fields. That is, peptide dendrimers hold a promising future as antibacterial agents with prolonged efficacy without bacterial resistance development. Their in vivo stability and multivalence allow them to effectively target multi-drug-resistant strains and prevent biofilm formation. Thus, it is interesting to have an overview of the development and applications of peptide dendrimers in antibacterial research, including the possibility of employing machine learning approaches for the design of AMPs and dendrimers. This review summarizes the synthesis and applications of peptide dendrimers as antibacterial agents. The challenges and perspectives of using peptide dendrimers as the antibacterial agents are also discussed.

The pearl jubilee of microcin J25: thirty years of research on an exceptional lasso peptide

Covering: 1992 up to 2023Since their discovery, lasso peptides went from peculiarities to be recognized as a major family of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products that were shown to be spread throughout the bacterial kingdom. Microcin J25 was first described in 1992, making it one of the earliest known lasso peptides. No other lasso peptide has since then been studied to such an extent as microcin J25, yet, previous review articles merely skimmed over all the research done on this exceptional lasso peptide. Therefore, to commemorate the 30th anniversary of its first report, we give a comprehensive overview of all literature related to microcin J25. This review article spans the early work towards the discovery of microcin J25, its biosynthetic gene cluster, and the elucidation of its three-dimensional, threaded lasso structure. Furthermore, the current knowledge about the biosynthesis of microcin J25 and lasso peptides in general is summarized and a detailed overview is given on the biological activities associated with microcin J25, including means of self-immunity, uptake into target bacteria, inhibition of the Gram-negative RNA polymerase, and the effects of microcin J25 on mitochondria. The in vitro and in vivo models used to study the potential utility of microcin J25 in a (veterinary) medicine context are discussed and the efforts that went into employing the microcin J25 scaffold in bioengineering contexts are summed up.

TonB-Dependent Transport Across the Bacterial Outer Membrane

TonB-dependent transporters (TBDTs) are present in all gram-negative bacteria and mediate energy-dependent uptake of molecules that are too scarce or large to be taken up efficiently by outer membrane (OM) diffusion channels. This process requires energy that is derived from the proton motive force and delivered to TBDTs by the TonB-ExbBD motor complex in the inner membrane. Together with the need to preserve the OM permeability barrier, this has led to an extremely complex and fascinating transport mechanism for which the fundamentals, despite decades of research, are still unclear. In this review, we describe our current understanding of the transport mechanism of TBDTs, their potential role in the delivery of novel antibiotics, and the important contributions made by TBDT-associated (lipo)proteins.

Evidence for Widespread Class II Microcins in Enterobacterales Genomes

Microcins are a class of antimicrobial peptides produced by certain Gram-negative bacterial species to kill or inhibit the growth of competing bacteria. Only 10 unique, experimentally validated class II microcins have been identified, and the majority of these come from Escherichia coli. Although the current representation of microcins is sparse, they exhibit a diverse array of molecular functionalities, uptake mechanisms, and target specificities. This broad diversity from such a small representation suggests that microcins may have untapped potential for bioprospecting peptide antibiotics from genomic data sets. We used a systematic bioinformatics approach to search for verified and novel class II microcins in E. coli and other species within its family, Enterobacteriaceae. Nearly one-quarter of the E. coli genome assemblies contained one or more microcins, where the prevalence of hits to specific microcins varied by isolate phylogroup. E. coli isolates from human extraintestinal and poultry meat sources were enriched for microcins, while those from freshwater were depleted. Putative microcins were found in various abundances across all five distinct phylogenetic lineages of Enterobacteriaceae, with a particularly high prevalence in the "Klebsiella" clade. Representative genome assemblies from species across the Enterobacterales order, as well as a few outgroup species, also contained putative microcin sequences. This study suggests that microcins have a complicated evolutionary history, spanning far beyond our limited knowledge of the currently validated microcins. Efforts to functionally characterize these newly identified microcins have great potential to open a new field of peptide antibiotics and microbiome modulators and elucidate the ways in which bacteria compete with each other. IMPORTANCE Class II microcins are small bacteriocins produced by strains of Gram-negative bacteria in the Enterobacteriaceae. They are generally understood to play a role in interbacterial competition, although direct evidence of this is limited, and they could prove informative in developing new peptide antibiotics. However, few examples of verified class II microcins exist, and novel microcins are difficult to identify due to their sequence diversity, making it complicated to study them as a group. Here, we overcome this limitation by developing a bioinformatics pipeline to detect microcins in silico. Using this pipeline, we demonstrate that both verified and novel class II microcins are widespread within and outside the Enterobacteriaceae, which has not been systematically shown previously. The observed prevalence of class II microcins suggests that they are ecologically important, and the elucidation of novel microcins provides a resource that can be used to expand our knowledge of the structure and function of microcins as antibacterials.

Colicins and Microcins Produced by Enterobacteriaceae: Characterization, Mode of Action, and Putative Applications

Enterobacteriaceae are widely present in many environments related to humans, including the human body and the food that they consume, from both plant or animal origin. Hence, they are considered relevant members of the gastrointestinal tract microbiota. On the other hand, these bacteria are also recognized as putative pathogens, able to impair human health and, in food, they are considered indicators for the microbiological quality and hygiene status of a production process. Nevertheless, beneficial properties have also been associated with Enterobacteriaceae, such as the ability to synthesize peptides and proteins, which can have a role in the structure of microbial communities. Among these antimicrobial molecules, those with higher molecular mass are called colicins, while those with lower molecular mass are named microcins. In recent years, some studies show an emphasis on molecules that can help control the development of pathogens. However, not enough data are available on this subject, especially related to microcins. Hence, this review gathers and summarizes current knowledge on colicins and microcins, potential usage in the treatment of pathogen-associated diseases and cancer, as well as putative applications in food biotechnology.

Interventions in Nicotinamide Adenine Dinucleotide Metabolism, the Intestinal Microbiota and Microcin Peptide Antimicrobials

A proper NADH/NAD + balance allows for the flow of metabolic and catabolic activities determining cellular growth. In Escherichia coli, more than 80 NAD + dependent enzymes are involved in all major metabolic pathways, including the post-transcriptional build-up of thiazole and oxazole rings from small linear peptides, which is a critical step for the antibiotic activity of some microcins. In recent years, NAD metabolism boosting drugs have been explored, mostly precursors of NAD + synthesis in human cells, with beneficial effects on the aging process and in preventing oncological and neurological diseases. These compounds also enhance NAD + metabolism in the human microbiota, which contributes to these beneficial effects. On the other hand, inhibition of NAD + metabolism has been proposed as a therapeutic approach to reduce the growth and propagation of tumor cells and mitigating inflammatory bowel diseases; in this case, the activity of the microbiota might mitigate therapeutic efficacy. Antibiotics, which reduce the effect of microbiota, should synergize with NAD + metabolism inhibitors, but these drugs might increase the proportion of antibiotic persistent populations. Conversely, antibiotics might have a stronger killing effect on bacteria with active NAD + production and reduce the cooperation of NAD + producing bacteria with tumoral cells. The use of NADH/NAD + modulators should take into consideration the use of antibiotics and the population structure of the microbiota.

Biosynthetic Microcin J25 Exerts Strong Antibacterial, Anti-Inflammatory Activities, Low Cytotoxicity Without Increasing Drug-Resistance to Bacteria Target

Multidrug resistant (MDR) bacterial infection has emerged, raising concerns about untreatable infections, and posing the highest health risks. Antimicrobial peptides (AMPs) are thought to be the best remedy for this problem. Here, we showed biosynthetic microcin J25 (MccJ25) exhibited excellent bactericidal activity against standard and clinically relevant veterinary MDR strains with high stability, no cytotoxicity, and no increase in drug resistance. Analysis of antimicrobial mechanism possessed by sensitive enterotoxigenic Escherichia coli (ETEC) based on electron microscopy and Sytox Green methods was carried out. Results showed excellent activity against ETEC was due to permeabilizing bacterial membranes and strong affinity. MccJ25 exhibited high endotoxin-neutralizing activity in both in vivo and in vitro environments, and mice exposed to lipopolysaccharide (LPS) showed decreased plasma LPS levels and improved survival after administration of MccJ25. In an LPS-treated mouse septicemia model, MccJ25 treatment significantly alleviated inflammatory responses by inhibiting proinflammatory factor secretion and expression. In a mouse E. coli infection model, administration of MccJ25 effectively improved host defense against clinically source cocktail of multidrug-resistant E. coli strains induced intestinal inflammation and bacteria dissemination. Results of studies on anti-inflammatory mechanisms showed that MccJ25 downregulated nuclear factor kappa B kinase and mitogen-activated protein kinase, thereby reducing the production of toll-like receptor 4, myeloid differentiation factor 88 and decreasing the key proinflammatory cytokines. These findings clarify MccJ25 may be an ideal antibacterial/antiendotoxic drug candidate that has the potential to further guide the development of anti-inflammatory and/or antimicrobial agents in the war against MDR bacterial infection.

A Novel Polymeric Nanohybrid Antimicrobial Engineered by Antimicrobial Peptide MccJ25 and Chitosan Nanoparticles Exerts Strong Antibacterial and Anti-Inflammatory Activities

Infection caused by antibiotic-resistant microorganisms (ARMs) has been declared a global threat to public health. Polymeric nanoparticles (PNPs) formed by antimicrobial peptides (AMPs) and synthetic PNPs against ARM infections are emerging. PNPs are also considered to be a promising natural biological preservative that prevents microbial spoilage through food processing and preservation. We engineered CNMs, a novel nanocomposite antibacterial agent based on chitosan nanoparticles and AMP microcin J25. In this study, we aimed to evaluate the comprehensive antimicrobial activity, potential antimicrobial mechanism, and anti-inflammatory activity of CNMs. We demonstrated that CNMs harbor excellent bactericidal activity against clinical foodborne pathogens and ARMs. CNMs caused fast mortality against different growth phases of tetracycline (Tet)-resistant enterotoxigenic E. coli (ETEC) and significantly killed Tet-resistant ETEC in food biological environments. Mechanistically, CNMs have the ability to bind lipopolysaccharides (LPS), neutralize endotoxin, and promote diaphragm permeability by damaging the cell membrane. CNMs did not cause mouse RAW264.7 cell cytotoxicity. Notably, CNMs significantly reduced the cytotoxicity of RAW264.7 macrophages induced by LPS. The LPS-induced inflammatory response was significantly ameliorated by CNMs by reducing the levels of nitric oxide and proinflammatory cytokines, including tumor necrosis factor α, interleukin (IL)-6, IL-8, IL-1β, Toll-like receptor 4, and nuclear factor κB (NF-κB), in LPS-challenged RAW264.7 macrophages. CNMs downregulated the NF-κB and mitogen-activated protein kinase signaling pathways, thereby inhibiting inflammatory responses upon LPS stimulation. Taken together, CNMs could be applied as effective antimicrobial/anti-inflammatory agents with lower cytotoxicity in food, medicine, and agriculture to prevent bacterial contamination and infection, respectively.

A Novel Nano-Antimicrobial Polymer Engineered with Chitosan Nanoparticles and Bioactive Peptides as Promising Food Biopreservative Effective against Foodborne Pathogen E. coli O157-Caused Epithelial Barrier Dysfunction and Inflammatory Responses

For food quality and safety issues, the emergence of foodborne pathogenic bacteria has further accelerated the spread of antibiotic residues and drug resistance genes. To alleviate the harm caused by bacterial infections, it is necessary to seek novel antimicrobial agents as biopreservatives to prevent microbial spoilage. Nanoantimicrobials have been widely used in the direct treatment of bacterial infections. CNMs, formed by chitosan nanoparticles and peptides, are promising antibiotic alternatives for use as excellent new antibacterial drugs against pathogenic bacteria. Herein, the current study evaluated the function of CNMs in the protection of foodborne pathogen Escherichia coli (E. coli) O157 infection using an intestinal epithelial cell model. Antibacterial activity assays indicated that CNMs exerted excellent bactericidal activity against E. coli O157. Assessment of the cytotoxicity risks toward cells demonstrated that 0.0125–0.02% of CNMs did not cause toxicity, but 0.4% of CNMs caused cytotoxicity. Additionally, CNMs did not induced genotoxicity either. CNMs protected against E. coli O157-induced barrier dysfunction by increasing transepithelial electrical resistance, decreasing lactate dehydrogenase and promoting the protein expression of occludin. CNMs were further found to ameliorate inflammation via modulation of tumor factor α, toll-like receptor 4 and nuclear factor κB (NF-κB) expression via inhibition of mitogen-activated protein kinase and NF-κB activation and improved antioxidant activity. Taken together, CNMs could protect the host against E. coli O157-induced intestinal barrier damage and inflammation, showing that CNMs have great advantages and potential application as novel antimicrobial polymers in the food industry as food biopreservatives, bringing new hope for the treatment of bacterial infections.

Putative Antimicrobial Peptides Within Bacterial Proteomes Affect Bacterial Predominance: A Network Analysis Perspective

The predominance of bacterial taxa in the gut, was examined in view of the putative antimicrobial peptide sequences (AMPs) within their proteomes. The working assumption was that compatible bacteria would share homology and thus immunity to their putative AMPs, while competing taxa would have dissimilarities in their proteome-hidden AMPs. A network-based method ("Bacterial Wars") was developed to handle sequence similarities of predicted AMPs among UniProt-derived protein sequences from different bacterial taxa, while a resulting parameter ("Die" score) suggested which taxa would prevail in a defined microbiome. T he working hypothesis was examined by correlating the calculated Die scores, to the abundance of bacterial taxa from gut microbiomes from different states of health and disease. Eleven publicly available 16S rRNA datasets and a dataset from a full shotgun metagenomics served for the analysis. The overall conclusion was that AMPs encrypted within bacterial proteomes affected the predominance of bacterial taxa in chemospheres.

Mcc1229, an Stx2a-amplifying microcin, is produced in vivo and requires CirA for activity

Enterohemorrhagic Escherichia coli (EHEC) strains, including the foodborne pathogen E. coli O157:H7, are responsible for thousands of hospitalizations each year. Various environmental triggers can modulate pathogenicity in EHEC by inducing the expression of Shiga toxin (Stx), which is encoded on a lambdoid prophage and transcribed together with phage late genes. Cell-free supernatants of the sequence type 73 (ST73) E. coli strain 0.1229 are potent inducers of Stx2a production in EHEC, suggesting that 0.1229 secretes a factor that activates the SOS response and leads to phage lysis. We previously demonstrated that this factor, designated microcin 1229 (Mcc1229), was proteinaceous and plasmid-encoded. To further characterize Mcc1229 and support its classification as a microcin, we investigated its regulation, determined its receptor, and identified loci providing immunity. The production of Mcc1229 was increased upon iron limitation, as determined by an enzyme-linked immunosorbent assay (ELISA), lacZ fusions, and quantitative real-time PCR (qRT-PCR). Spontaneous Mcc1229-resistant mutants and targeted gene deletion revealed that CirA was the Mcc1229 receptor. TonB, which interacts with CirA in the periplasm, was also essential for Mcc1229 import. Subcloning of the Mcc1229 plasmid indicated that Mcc activity was neutralized by two open reading frames (ORFs), each predicted to encode a domain of unknown function (DUF)-containing protein. In a germfree mouse model of infection, colonization with 0.1229 suppressed subsequent colonization by EHEC. Although Mcc1229 was produced in vivo, it was dispensable for colonization suppression. The regulation, import, and immunity determinants identified here are consistent with features of other Mccs, suggesting that Mcc1229 should be included in this class of small molecules.

The impact of the gut microbiome on toxigenic bacteria

Millions of symbiotic and pathogenic microorganisms known as microbiota colonize the host body. The microbiome plays an important role in human health and colonizes hundreds of different species of multicellular organisms so that they are introduced as the metaorganisms. Changes in the microbial population of the gut microbiome may cause resistance to pathogenic bacteria-induced infection. Understanding the principles of Host-Microbiota Interactions (HMIs) is important because it clarifies our insight towards the mechanisms of infections established in the host. Interactions between the host and the microbiota help answer the question of how a microorganism can contribute to the health or disease of the host. Microbiota can increase host resistance to colonization of pathogenic species. Studying the HMIs network can in several ways delineate the pathogenic mechanisms of pathogens and thereby help to increase useful and novel therapeutic pathways. For example, the potentially unique microbial effects that target the distinct host or interfere with the endogenous host interactions can be identified. In addition, the way mutations in essential proteins in the host and/or in the microbes can influence the interactions between them may be determined. Furthermore, HMIs help in identifying host cell regulatory modules.

Engineering and characterization of human β-defensin-3 and its analogues and microcin J25 peptides against Mannheimia haemolytica and bovine neutrophils

Mannheimia haemolytica-induced bovine respiratory disease causes loss of millions of dollars to Canadian cattle industry. Current antimicrobials are proving to be ineffective and leave residues in meat. Antimicrobial peptides (AMPs) may be effective against M. haemolytica while minimizing the risk of drug residues. Cationic AMPs can kill bacteria through interactions with the anionic bacterial membrane. Human β-Defensin 3 (HBD3) and microcin J25 (MccJ25) are AMPs with potent activity against many Gram-negative bacteria. We tested the microbicidal activity of wild-type HBD3, three HBD3 peptide analogues (28 amino acid, 20AA, and 10AA) derived from the sequence of natural HBD3, and MccJ25 in vitro against M. haemolytica. Three C-terminal analogues of HBD3 with all cysteines replaced with valines were manually synthesized using solid phase peptide synthesis. Since AMPs can act as chemoattractant we tested the chemotactic effect of HBD3, 28AA, 20AA, and 10AA peptides on bovine neutrophils in Boyden chamber. Minimum bactericidal concentration (MBC) assay showed that M. haemolytica was intermediately sensitive to HBD3, 28AA and 20AA analogues with an MBC of 50 µg/mL. The 10AA analogue had MBC 6.3 µg/mL which is likely a result of lower final inoculum size. MccJ25 didn't have significant bactericidal effect below an MBC < 100 µg/mL. Bovine neutrophils showed chemotaxis towards HBD3 and 20AA peptides (P < 0.05) but not towards 28AA analogue. Co-incubation of neutrophils with any of the peptides did not affect their chemotaxis towards N-formyl-L-methionyl-L-leucyl-phenylalanine (fMLP). The data show that these peptides are effective against M. haemolytica and are chemotactic for neutrophils in vitro.

Bacteriocins as a new generation of antimicrobials: toxicity aspects and regulations

In recent decades, bacteriocins have received substantial attention as antimicrobial compounds. Although bacteriocins have been predominantly exploited as food preservatives, they are now receiving increased attention as potential clinical antimicrobials and as possible immune-modulating agents. Infections caused by antibiotic-resistant bacteria have been declared as a global threat to public health. Bacteriocins represent a potential solution to this worldwide threat due to their broad- or narrow-spectrum activity against antibiotic-resistant bacteria. Notably, despite their role in food safety as natural alternatives to chemical preservatives, nisin remains the only bacteriocin legally approved by regulatory agencies as a food preservative. Moreover, insufficient data on the safety and toxicity of bacteriocins represent a barrier against the more widespread use of bacteriocins by the food and medical industry. Here, we focus on the most recent trends relating to the application of bacteriocins, their toxicity and impacts.

Living Therapeutics: The Next Frontier of Precision Medicine

Modern medicine has long studied the mechanism and impact of pathogenic microbes on human hosts, but has only recently shifted attention toward the complex and vital roles that commensal and probiotic microbes play in both health and dysbiosis. Fueled by an enhanced appreciation of the human-microbe holobiont, the past decade has yielded countless insights and established many new avenues of investigation in this area. In this review, we discuss advances, limitations, and emerging frontiers for microbes as agents of health maintenance, disease prevention, and cure. We highlight the flexibility of microbial therapeutics across disease states, with special consideration for the rational engineering of microbes toward precision medicine outcomes. As the field advances, we anticipate that tools of synthetic biology will be increasingly employed to engineer functional living therapeutics with the potential to address longstanding limitations of traditional drugs.

Harnessing Iron Acquisition Machinery to Target Enterobacteriaceae

Infections caused by Gram-negative bacteria can be challenging to treat due to the outer membrane permeability barrier and the increasing emergence of antibiotic resistance. During infection, Gram-negative pathogens must acquire iron, an essential nutrient, in the host. Many Gram-negative bacteria utilize sophisticated iron acquisition machineries based on siderophores, small molecules that bind iron with high affinity. In this review, we provide an overview of siderophore-mediated iron acquisition in Enterobacteriaceae and show how these systems provide a foundation for the conceptualization and development of approaches to prevent and/or treat bacterial infections. Differences between the siderophore-based iron uptake machineries of pathogenic Enterobacteriaceae and commensal microbes may lead to the development of selective "Trojan-horse" antimicrobials and immunization strategies that will not harm the host microbiota.

A Toxic Environment: a Growing Understanding of How Microbial Communities Affect Escherichia coli O157:H7 Shiga Toxin Expression

Enterohemorrhagic Escherichia coli (EHEC) strains, including E. coli O157:H7, cause severe illness in humans due to the production of Shiga toxin (Stx) and other virulence factors. Because Stx is coregulated with lambdoid prophage induction, its expression is especially susceptible to environmental cues. Infections with Stx-producing E. coli can be difficult to model due to the wide range of disease outcomes: some infections are relatively mild, while others have serious complications. Probiotic organisms, members of the gut microbiome, and organic acids can depress Stx production, in many cases by inhibiting the growth of EHEC strains. On the other hand, the factors currently known to amplify Stx act via their effect on the stx-converting phage. Here, we characterize two interactive mechanisms that increase Stx production by O157:H7 strains: first, direct interactions with phage-susceptible E. coli, and second, indirect amplification by secreted factors. Infection of susceptible strains by the stx-converting phage can expand the Stx-producing population in a human or animal host, and phage infection has been shown to modulate virulence in vitro and in vivo Acellular factors, particularly colicins and microcins, can kill O157:H7 cells but may also trigger Stx expression in the process. Colicins, microcins, and other bacteriocins have diverse cellular targets, and many such molecules remain uncharacterized. The identification of additional Stx-amplifying microbial interactions will improve our understanding of E. coli O157:H7 infections and help elucidate the intricate regulation of pathogenicity in EHEC strains.

Non-antibiotic antibacterial peptides and proteins of Escherichia coli: efficacy and potency of bacteriocins

INTRODUCTION

The emergence and spread of antibiotic resistance among pathogenic bacteria drives the search for alternative antimicrobial therapies. Bacteriocins represent a potential alternative to antibiotic treatment. In contrast to antibiotics, bacteriocins are peptides or proteins that have relatively narrow spectra of antibacterial activities and are produced by a wide range of bacterial species. Bacteriocins of Escherichia coli are historically classified as microcins and colicins, and, until now, more than 30 different bacteriocin types have been identified and characterized.

AREAS COVERED

We performed bibliographical searches of online databases to review the literature regarding bacteriocins produced by E. coli with respect to their occurrence, bacteriocin role in bacterial colonization and pathogenicity, and application of their antimicrobial effect.

EXPERT OPINION

The potential use of bacteriocins for applications in human and animal medicine and the food industry includes (i) the use of bacteriocin-producing probiotic strains, (ii) recombinant production in plants and application in food, and (iii) application of purified bacteriocins.

How Food Affects Colonization Resistance Against Enteropathogenic Bacteria

Food has a major impact on all aspects of health. Recent data suggest that food composition can also affect susceptibility to infections by enteropathogenic bacteria. Here, we discuss how food may alter the microbiota as well as mucosal defenses and how this can affect infection. Salmonella Typhimurium diarrhea serves as a paradigm, and complementary evidence comes from other pathogens. We discuss the effects of food composition on colonization resistance, host defenses, and the infection process as well as the merits and limitations of mouse models and experimental foods, which are available to decipher the underlying mechanisms.

Bacteriocins, Antimicrobial Peptides from Bacterial Origin: Overview of Their Biology and Their Impact against Multidrug-Resistant Bacteria

Currently, the emergence and ongoing dissemination of antimicrobial resistance among bacteria are critical health and economic issue, leading to increased rates of morbidity and mortality related to bacterial infections. Research and development for new antimicrobial agents is currently needed to overcome this problem. Among the different approaches studied, bacteriocins seem to be a promising possibility. These molecules are peptides naturally synthesized by ribosomes, produced by both Gram-positive bacteria (GPB) and Gram-negative bacteria (GNB), which will allow these bacteriocin producers to survive in highly competitive polymicrobial environment. Bacteriocins exhibit antimicrobial activity with variable spectrum depending on the peptide, which may target several bacteria. Already used in some areas such as agro-food, bacteriocins may be considered as interesting candidates for further development as antimicrobial agents used in health contexts, particularly considering the issue of antimicrobial resistance. The aim of this review is to present an updated global report on the biology of bacteriocins produced by GPB and GNB, as well as their antibacterial activity against relevant bacterial pathogens, and especially against multidrug-resistant bacteria.

The manifold roles of microbial ribosomal peptide-based natural products in physiology and ecology

The ribosomally synthesized and posttranslationally modified peptides (RiPPs), also called ribosomal peptide natural products (RPNPs), form a growing superfamily of natural products that are produced by many different organisms and particularly by bacteria. They are derived from precursor polypeptides whose modification by various dedicated enzymes helps to establish a vast array of chemical motifs. RiPPs have attracted much interest as a source of potential therapeutic agents, and in particular as alternatives to conventional antibiotics to address the bacterial resistance crisis. However, their ecological roles in nature are poorly understood and explored. The present review describes major RiPP actors in competition within microbial communities, the main ecological and physiological functions currently evidenced for RiPPs, and the microbial ecosystems that are the sites for these functions. We envision that the study of RiPPs may lead to discoveries of new biological functions and highlight that a better knowledge of how bacterial RiPPs mediate inter-/intraspecies and interkingdom interactions will hold promise for devising alternative strategies in antibiotic development.

Construction of sulfur-containing moieties in the total synthesis of natural products

This review surveys the total syntheses of sulfur-containing natural products where sulfur atoms are introduced with different sulfurization agents to construct related sulfur-containing moieties.

Properties of an Antimicrobial Molecule Produced by an Escherichia coli Champion

Over recent decades, the number and frequency of severe pathogen infections have been increasing. Pathogen mitigation strategies in human medicine or in livestock operations are vital to combat emerging arsenals of bacterial virulence and defense mechanisms. Since the emergence of antimicrobial resistance, the competitive nature of bacteria has been considered for the potential treatment or mitigation of pathogens. Previously, we identified a strong E. coli competitor with probiotic properties producing a diffusible antimicrobial molecule(s) that inhibited the growth of Shiga toxin-producing E. coli (STEC). Our current objective was to isolate and examine the properties of this antimicrobial molecule(s). Molecules were isolated by filter sterilization after 12 h incubation, and bacterial inhibition was compared to relevant controls. Isolated antimicrobial molecule(s) and controls were subjected to temperature, pH, or protease digestion treatments. Changes in inhibition properties were evaluated by comparing the incremental cell growth in the presence of treated and untreated antimicrobial molecule(s). No treatment affected the antimicrobial molecule(s) properties of STEC inhibition, suggesting that at least one molecule produced is an efficacious microcin. The molecule persistence to physiochemical and enzymatic treatments could open a wide window to technical industry-scale applications.

Microcins in Enterobacteriaceae: Peptide Antimicrobials in the Eco-Active Intestinal Chemosphere

Microcins are low-molecular-weight, ribosomally produced, highly stable, bacterial-inhibitory molecules involved in competitive, and amensalistic interactions between Enterobacteriaceae in the intestine. These interactions take place in a highly complex chemical landscape, the intestinal eco-active chemosphere, composed of chemical substances that positively or negatively influence bacterial growth, including those originated from nutrient uptake, and those produced by the action of the human or animal host and the intestinal microbiome. The contribution of bacteria results from their effect on the host generated molecules, on food and digested food, and organic substances from microbial origin, including from bacterial degradation. Here, we comprehensively review the main chemical substances present in the human intestinal chemosphere, particularly of those having inhibitory effects on microorganisms. With this background, and focusing on Enterobacteriaceae, the most relevant human pathogens from the intestinal microbiota, the microcin’s history and classification, mechanisms of action, and mechanisms involved in microcin’s immunity (in microcin producers) and resistance (non-producers) are reviewed. Products from the chemosphere likely modulate the ecological effects of microcin activity. Several cross-resistance mechanisms are shared by microcins, colicins, bacteriophages, and some conventional antibiotics, which are expected to produce cross-effects. Double-microcin-producing strains (such as microcins MccM and MccH47) have been successfully used for decades in the control of pathogenic gut organisms. Microcins are associated with successful gut colonization, facilitating translocation and invasion, leading to bacteremia, and urinary tract infections. In fact, Escherichia coli strains from the more invasive phylogroups (e.g., B2) are frequently microcinogenic. A publicly accessible APD3 database http://aps.unmc.edu/AP/ shows particular genes encoding microcins in 34.1% of E. coli strains (mostly MccV, MccM, MccH47, and MccI47), and much less in Shigella and Salmonella (<2%). Some 4.65% of Klebsiella pneumoniae are microcinogenic (mostly with MccE492), and even less in Enterobacter or Citrobacter (mostly MccS). The high frequency and variety of microcins in some Enterobacteriaceae indicate key ecological functions, a notion supported by their dominance in the intestinal microbiota of biosynthetic gene clusters involved in the synthesis of post-translationally modified peptide microcins.

Evolutionary Stability of Salmonella Competition with the Gut Microbiota: How the Environment Fosters Heterogeneity in Exploitative and Interference Competition

Following ingestion, gastrointestinal pathogens compete against the gastrointestinal microbiota and overcome host immune defenses in order to cause infections. Besides employing direct killing mechanisms, the commensal microbiota occupies metabolic niches to outcompete invading pathogens. Salmonella enterica serovar Typhimurium (S. Typhimurium) uses several strategies to successfully colonize the gut and establish infection, of which an increasing number is based on phenotypic heterogeneity within the S. Typhimurium population. The utilization of myo-inositol (MI) and the production of colicin confer a selective advantage over the microbiota in terms of exploitative and interference competition, respectively. In this review, we summarize the genetic basis underlying bistability of MI catabolism and colicin production. As demonstrated by single-cell analyses, a stochastic switch in the expression of the genes responsible for colicin production and MI degradation constitutes the heterogeneity of the two phenotypes. Both genetic systems are tightly regulated to avoid their expression under non-appropriate conditions and possible detrimental effects on bacterial fitness. Moreover, evolutionary mechanisms underlying formation and stability of these phenotypes in S. Typhimurium are discussed. We propose that both MI catabolism and colicin production create a bet-hedging strategy, which provides an adaptive benefit for S. Typhimurium in the fluctuating environment of the mammalian gut.

Risks Related to High-Dosage Recombinant Antimicrobial Peptide Microcin J25 in Mice Model: Intestinal Microbiota, Intestinal Barrier Function, and Immune Regulation

Antimicrobial peptide (AMP) can be a promising alternative in various domains. However, further risk information is required. In this study, mice were orally administrated different dosages of recombinant AMP microcin J25 (4.55, 9.1, and 18.2 mg/kg; MccJ25) for 1 week, and the toxicity risk impacts were examined. We evidenced that middle-dosage administration mice had a lower inflammation, better body weight, and ameliorated mucosal morphology, accompanied by reduced intestinal permeability and tighter intestinal barrier. Fecal microbiota composition analysis in middle- or low-dosage mice revealed the Bifidobacterium count was increased and the coliform bacteria count was decreased, and increased in short-chain fatty acid levels. Unexpectedly, there was a risk that high-dosage mice increased intestinal permeability and imbalance of intestinal bacteria. Taken together, these data indicated a safe threshold for usage of MccJ25 in clinical practice. Such studies can effectively enhance the safety of various aspects such as food preservative and drug.

Effects of Metabolites Derived From Gut Microbiota and Hosts on Pathogens

Intestinal metabolites participate in various physiological processes, including energy metabolism, cell-to-cell communication, and host immunity. These metabolites mainly originate from gut microbiota and hosts. Although many host metabolites are dominant in intestines, such as free fatty acids, amino acids and vitamins, the metabolites derived from gut microbiota are also essential for intestinal homeostasis. In addition, some metabolites are only generated and released by gut microbiota, such as bacteriocins, short-chain fatty acids, and quorum-sensing autoinducers. In this review, we summarize recent studies regarding the crosstalk between pathogens and metabolites from different sources, including the influence on bacterial development and the activation/inhibition of immune responses of hosts. All of these functions would affect the colonization of and infection by pathogens. This review provides clear ideas and directions for further exploring the regulatory mechanisms and effects of metabolites on pathogens.

Bacteriocinogenic properties of Escherichia coli P2C isolated from pig gastrointestinal tract: purification and characterization of microcin V

The aim of this study was to isolate and investigate the bacteriocinogenic and probiotic potential of new Gram-negative isolates. Of 22 bacterial isolates from pig intestine and chicken crops, ten isolates had demonstrated a good activity, and the most potent five strains were identified as four E. coli and one as Proteus sp. No virulence factors were detected for E. coli strains isolated from pig intestine. The semi-purified microcins proved to be resistant to temperature and pH variation, but sensitive to proteolytic enzymes. Of particular interest, strain E. coli P2C was the most potent, free of virulence genes and sensitive to tested antibiotics. Purification procedure revealed the presence of a single pure peak having a molecular mass of 8733.94 Da and matching microcin V (MccV). The sequence obtained by LC-MS/MS confirmed the presence of MccV. Purified MccV showed a good activity against pathogenic coliforms, especially E. coli O₁K₁H₇ involved in avian colibacillosis. The present study provides evidence that E. coli strains isolated from pig intestine produce microcin-like substances. E. coli P2C is a safe MccV producer that could be a good candidate for its application as novel probiotic strain to protect livestock and enhance growth performance.

Intracellular Targeting Mechanisms by Antimicrobial Peptides

Antimicrobial peptides (AMPs) are expressed in various living organisms as first-line host defenses against potential harmful encounters in their surroundings. AMPs are short polycationic peptides exhibiting various antimicrobial activities. The principal antibacterial activity is attributed to the membrane-lytic mechanism which directly interferes with the integrity of the bacterial cell membrane and cell wall. In addition, a number of AMPs form a transmembrane channel in the membrane by self-aggregation or polymerization, leading to cytoplasm leakage and cell death. However, an increasing body of evidence has demonstrated that AMPs are able to exert intracellular inhibitory activities as the primary or supportive mechanisms to achieve efficient killing. In this review, we focus on the major intracellular targeting activities reported in AMPs, which include nucleic acids and protein biosynthesis and protein-folding, protease, cell division, cell wall biosynthesis, and lipopolysaccharide inhibition. These multifunctional AMPs could serve as the potential lead peptides for the future development of novel antibacterial agents with improved therapeutic profiles.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

Microcin PDI regulation and proteolytic cleavage are unique among known microcins

Microcin PDI inhibits a diversity of pathogenic Escherichia coli through the action of an effector protein, McpM. In this study we demonstrated that expression of the inhibitory phenotype is induced under low osmolarity conditions and expression is primarily controlled by the EnvZ/OmpR two-component regulatory system. Functional, mutagenesis and complementation experiments were used to empirically demonstrate that EnvZ is required for the inhibitory phenotype and that regulation of mcpM is dependent on binding of the phosphorylated OmpR to the mcpM promoter region. The phosphorylated OmpR may recognize three different binding sites within this promoter region. Site-directed mutagenesis revealed that the McpM precursor peptide includes two leader peptides that undergo sequential cleavage at positions G17/G18 and G35/A36 during export through the type I secretion system. Competition assays showed that both cleaved products are required for the PDI phenotype although we could not distinguish loss of function from loss of secretion in these assays. McpM has four cysteines within the mature peptide and site-directed mutagenesis experiments demonstrated that the first two cysteines are necessary for McpM to inhibit susceptible cells. Together these data combined with previous work indicate that MccPDI is unique amongst the microcins that have been described to date.

Microcins mediate competition among Enterobacteriaceae in the inflamed gut

The Enterobacteriaceae are a family of Gram-negative bacteria that include commensal organisms as well as primary and opportunistic pathogens that are among the leading causes of morbidity and mortality worldwide. Although Enterobacteriaceae often comprise less than 1% of a healthy intestine's microbiota, some of these organisms can bloom in the inflamed gut; expansion of enterobacteria is a hallmark of microbial imbalance known as dysbiosis. Microcins are small secreted proteins that possess antimicrobial activity in vitro, but whose role in vivo has been unclear. Here we demonstrate that microcins enable the probiotic bacterium Escherichia coli Nissle 1917 (EcN) to limit the expansion of competing Enterobacteriaceae (including pathogens and pathobionts) during intestinal inflammation. Microcin-producing EcN limits the growth of competitors in the inflamed intestine, including commensal E. coli, adherent-invasive E. coli and the related pathogen Salmonella enterica. Moreover, only therapeutic administration of the wild-type, microcin-producing EcN to mice previously infected with S. enterica substantially reduced intestinal colonization by the pathogen. Our work provides the first evidence that microcins mediate inter- and intraspecies competition among the Enterobacteriaceae in the inflamed gut. Moreover, we show that microcins can act as narrow-spectrum therapeutics to inhibit enteric pathogens and reduce enterobacterial blooms.

Escherichia coli strain Nissle 1917-from bench to bedside and back: history of a special Escherichia coli strain with probiotic properties

Among the gram-negative microorganisms with probiotic properties, Escherichia coli strain Nissle 1917 (briefly EcN) is probably the most intensively investigated bacterial strain today. Since nearly 100 years, the EcN strain is used as the active pharmaceutical ingredient in a licensed medicinal product that is distributed in Germany and several other countries. Over the last few decades, novel probiotic activities have been detected, which taken together are specific of this versatile E. coli strain. This review gives a short overview on the discovery and history of the EcN strain.

Beyond iron: non-classical biological functions of bacterial siderophores

Bacteria secrete small molecules known as siderophores to acquire iron from their surroundings. For over 60 years, investigations into the bioinorganic chemistry of these molecules, including fundamental coordination chemistry studies, have provided insight into the crucial role that siderophores play in bacterial iron homeostasis. The importance of understanding the fundamental chemistry underlying bacterial life has been highlighted evermore in recent years because of the emergence of antibiotic-resistant bacteria and the need to prevent the global rise of these superbugs. Increasing reports of siderophores functioning in capacities other than iron transport have appeared recently, but reports of "non-classical" siderophore functions have long paralleled those of iron transport. One particular non-classical function of these iron chelators, namely antibiotic activity, was documented before the role of siderophores in iron transport was established. In this Perspective, we present an exposition of past and current work into non-classical functions of siderophores and highlight the directions in which we anticipate that this research is headed. Examples include the ability of siderophores to function as zincophores, chalkophores, and metallophores for a variety of other metals, sequester heavy metal toxins, transport boron, act as signalling molecules, regulate oxidative stress, and provide antibacterial activity.

Antagonistic activity expressed by Shigella sonnei: identification of a putative new bacteriocin

Bacteriocins are antibacterial, proteinaceous substances that mediate microbial dynamics. Bacteriocin production is a highly disseminated property among all major lineages of bacteria, including Shigella. In this paper, we addressed the purification and characterisation of a bacteriocin produced by a Shigella sonnei strain (SS9) isolated from a child with acute diarrhoea. The substance was purified through ammonium-sulphate precipitation and sequential steps of chromatography. The intracellular fraction obtained at 75% ammonium sulphate maintained activity following exposure to pH values from 1-11 and storage at -80ºC for more than two years and was inactivated by high temperatures and proteases. The molecular mass of the purified bacteriocin was determined by mass spectrometry to be 18.56 kDa. The N-terminal sequence of the bacteriocin did not match any other antibacterial proteins described. A putative new bacteriocin produced by S. sonnei has been detected. This bacteriocin may represent a newly described protein or a previously described protein with a newly detected function. Considering that SS9 expresses antagonism against other diarrhoeagenic bacteria, the bacteriocin may contribute to S. sonnei virulence and is potentially applicable to either preventing or controlling diarrhoeal disease.

Synthesis of full length and truncated microcin B17 analogues as DNA gyrase poisons

Using a combination of solid-phase peptide synthesis and fragment assembly strategies a library of full-length and truncated analogues of the antibacterial post-translationally modified peptide microcin B17 have been synthesised. Both antibacterial and DNA gyrase poisoning activities are also described for the synthetic analogues.

Role of a microcin-C-like biosynthetic gene cluster in allelopathic interactions in marine Synechococcus

Competition between phytoplankton species for nutrients and light has been studied for many years, but allelopathic interactions between them have been more difficult to characterize. We used liquid and plate assays to determine whether these interactions occur between marine unicellular cyanobacteria of the genus Synechococcus. We have found a clear growth impairment of Synechococcus sp. CC9311 and Synechococcus sp. WH8102 when they are cultured in the presence of Synechococcus sp. CC9605. The genome of CC9605 contains a region showing homology to genes of the Escherichia coli Microcin C (McC) biosynthetic pathway. McC is a ribosome-synthesized peptide that inhibits translation in susceptible strains. We show that the CC9605 McC gene cluster is expressed and that three genes (mccD, mccA, and mccB) are further induced by coculture with CC9311. CC9605 was resistant to McC purified from E. coli, whereas strains CC9311 and WH8102 were sensitive. Cloning the CC9605 McC biosynthetic gene cluster into sensitive CC9311 led this strain to become resistant to both purified E. coli McC and Synechococcus sp. CC9605. A CC9605 mutant lacking mccA1, mccA2, and the N-terminal domain of mccB did not inhibit CC9311 growth, whereas the inhibition of WH8102 was reduced. Our results suggest that an McC-like molecule is involved in the allelopathic interactions with CC9605.

Microcins in action: amazing defence strategies of Enterobacteria

Probably the oldest and most widespread antimicrobial strategy in living organisms is the use of antimicrobial peptides. Bacteria secrete such defence peptides, termed bacteriocins, that they use for microbial competitions. Microcins are bacteriocins of less than 10 kDa produced by Escherichia coli and related enterobacteria through the ribosomal pathway. They are synthesized as linear precursors, which can further undergo complex post-translational modifications resulting from dedicated maturation enzymes encoded in the microcin gene clusters, and are processed by proteolytic cleavage. Microcins exert potent bactericidal activities that use subtle and clever mechanisms to cross outer and inner membranes of Gram-negative bacteria. To cross the outer membrane, siderophore-microcins hijack receptors involved in iron acquisition. The lasso-peptide microcin J25, which is characterized by a knotted arrangement where the C-terminal tail is threaded through an N-terminal macrolactam ring, uses a hydroxamate siderophore receptor and the inner-membrane protein SbmA for import in sensitive bacteria, where it inhibits bacterial transcription through binding to RNAP (RNA polymerase). Microcin C produced as a heptapeptide adenylate, requires an outer-membrane porin and an inner-membrane ABC (ATP-binding-cassette) transporter to reach the cytoplasm of target bacteria, where it is processed by proteases into a non-hydrolysable aspartyl-adenylate analogue. Therefore, despite showing different killing mechanisms and the absence of any structural homology, microcins have the common characteristic to use Trojan horse strategies to destroy their competitors. They offer new and promising tracks for further design and engineering of novel efficient antibiotics.

Ribosomally synthesized and post-translationally modified peptide natural products: overview and recommendations for a universal nomenclature

This review presents recommended nomenclature for the biosynthesis of ribosomally synthesized and post-translationally modified peptides (RiPPs), a rapidly growing class of natural products. The current knowledge regarding the biosynthesis of the >20 distinct compound classes is also reviewed, and commonalities are discussed.

Bacteriocins - a viable alternative to antibiotics?

Solutions are urgently required for the growing number of infections caused by antibiotic-resistant bacteria. Bacteriocins, which are antimicrobial peptides produced by certain bacteria, might warrant serious consideration as alternatives to traditional antibiotics. These molecules exhibit significant potency against other bacteria (including antibiotic-resistant strains), are stable and can have narrow or broad activity spectra. Bacteriocins can even be produced in situ in the gut by probiotic bacteria to combat intestinal infections. Although the application of specific bacteriocins might be curtailed by the development of resistance, an understanding of the mechanisms by which such resistance could emerge will enable researchers to develop strategies to minimize this potential problem.

Microcin amyloid fibrils A are reservoir of toxic oligomeric species

Microcin E492 (Mcc), a low molecular weight bacteriocin produced by Klebsiella pneumoniae RYC492, has been shown to exist in two forms: soluble forms that are believed to be toxic to the bacterial cell by forming pores and non-toxic fibrillar forms that share similar biochemical and biophysical properties with amyloids associated with several human diseases. Here we report that fibrils polymerized in vitro from soluble forms sequester toxic species that can be released upon changing environmental conditions such as pH, ionic strength, and upon dilution. Our results indicate that basic pH (≥8.5), low NaCl concentrations (≤50 mm), and dilution (>10-fold) destabilize Mcc fibrils into more soluble species that are found to be toxic to the target cells. Additionally, we also found a similar conversion of non-toxic fibrils into highly toxic oligomers using Mcc aggregates produced in vivo. Moreover, the soluble protein released from fibrils is able to rapidly polymerize into amyloid fibrils under fibril-forming conditions and to efficiently seed aggregation of monomeric Mcc. Our findings indicate that fibrillar forms of Mcc constitute a reservoir of toxic oligomeric species that is released into the medium upon changing the environmental conditions. These findings may have substantial implications to understand the dynamic process of interconversion between toxic and non-toxic aggregated species implicated in protein misfolding diseases.