Endotoxin, capsule, and bacterial attachment contribute to Neisseria meningitidis resistance to the human antimicrobial peptide LL-37

The Potential of Fish Oil Components and Manuka Honey in Tackling Chronic Wound Treatment

Chronic wounds are becoming an increasing burden on healthcare services, as they have extended healing times and are susceptible to infection, with many failing to heal, which can lead ultimately to amputation. Due to the additional rise in antimicrobial resistance and emergence of difficult-to-treat Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. (ESKAPE pathogens), novel treatments will soon be required asides from traditional antibiotics. Many natural substances have been identified as having the potential to aid in both preventing infection and increasing the speed of wound closure processes. Manuka honey is already in some cases used as a topical treatment in the form of ointments, which in conjunction with dressings and fish skin grafts are an existing US Food and Drug Administration-approved treatment option. These existing treatment options indicate that fatty acids from fish oil and manuka honey are well tolerated by the body, and if the active components of the treatments were better understood, they could make valuable additions to topical treatment options. This review considers two prominent natural substances with established manufacturing and global distribution-marine based fatty acids (including their metabolites) and manuka honey-their function as antimicrobials and how they can aid in wound repair, two important aspects leading to resolution of chronic wounds.

Molecular Mechanisms of Bacterial Resistance to Antimicrobial Peptides in the Modern Era: An Updated Review

Antimicrobial resistance (AMR) poses a serious global health concern, resulting in a significant number of deaths annually due to infections that are resistant to treatment. Amidst this crisis, antimicrobial peptides (AMPs) have emerged as promising alternatives to conventional antibiotics (ATBs). These cationic peptides, naturally produced by all kingdoms of life, play a crucial role in the innate immune system of multicellular organisms and in bacterial interspecies competition by exhibiting broad-spectrum activity against bacteria, fungi, viruses, and parasites. AMPs target bacterial pathogens through multiple mechanisms, most importantly by disrupting their membranes, leading to cell lysis. However, bacterial resistance to host AMPs has emerged due to a slow co-evolutionary process between microorganisms and their hosts. Alarmingly, the development of resistance to last-resort AMPs in the treatment of MDR infections, such as colistin, is attributed to the misuse of this peptide and the high rate of horizontal genetic transfer of the corresponding resistance genes. AMP-resistant bacteria employ diverse mechanisms, including but not limited to proteolytic degradation, extracellular trapping and inactivation, active efflux, as well as complex modifications in bacterial cell wall and membrane structures. This review comprehensively examines all constitutive and inducible molecular resistance mechanisms to AMPs supported by experimental evidence described to date in bacterial pathogens. We also explore the specificity of these mechanisms toward structurally diverse AMPs to broaden and enhance their potential in developing and applying them as therapeutics for MDR bacteria. Additionally, we provide insights into the significance of AMP resistance within the context of host-pathogen interactions.

A secreted bacterial protein protects bacteria from cationic antimicrobial peptides by entrapment in phase-separated droplets

Mammalian hosts combat bacterial infections through the production of defensive cationic antimicrobial peptides (CAPs). These immune factors are capable of directly killing bacterial invaders; however, many pathogens have evolved resistance evasion mechanisms such as cell surface modification, CAP sequestration, degradation, or efflux. We have discovered that several pathogenic and commensal proteobacteria, including the urgent human threat Neisseria gonorrhoeae, secrete a protein (lactoferrin-binding protein B, LbpB) that contains a low-complexity anionic domain capable of inhibiting the antimicrobial activity of host CAPs. This study focuses on a cattle pathogen, Moraxella bovis, that expresses the largest anionic domain of the LbpB homologs. We used an exhaustive biophysical approach employing circular dichroism, biolayer interferometry, cross-linking mass spectrometry, microscopy, size-exclusion chromatography with multi-angle light scattering coupled to small-angle X-ray scattering (SEC-MALS-SAXS), and NMR to understand the mechanisms of LbpB-mediated protection against CAPs. We found that the anionic domain of this LbpB displays an α-helical secondary structure but lacks a rigid tertiary fold. The addition of antimicrobial peptides derived from lactoferrin (i.e. lactoferricin) to the anionic domain of LbpB or full-length LbpB results in the formation of phase-separated droplets of LbpB together with the antimicrobial peptides. The droplets displayed a low rate of diffusion, suggesting that CAPs become trapped inside and are no longer able to kill bacteria. Our data suggest that pathogens, like M. bovis, leverage anionic intrinsically disordered domains for the broad recognition and neutralization of antimicrobials via the formation of biomolecular condensates.

Susceptible bacteria can survive antibiotic treatment in the mammalian gastrointestinal tract without evolving resistance

Antibiotic resistance and evasion are incompletely understood and complicated by the fact that murine interval dosing models do not fully recapitulate antibiotic pharmacokinetics in humans. To better understand how gastrointestinal bacteria respond to antibiotics, we colonized germ-free mice with a pan-susceptible genetically barcoded Escherichia coli clinical isolate and administered the antibiotic cefepime via programmable subcutaneous pumps, allowing closer emulation of human parenteral antibiotic dynamics. E. coli was only recovered from intestinal tissue, where cefepime concentrations were still inhibitory. Strikingly, "some" E. coli isolates were not cefepime resistant but acquired mutations in genes involved in polysaccharide capsular synthesis increasing their invasion and survival within human intestinal cells. Deleting wbaP involved in capsular polysaccharide synthesis mimicked this phenotype, allowing increased invasion of colonocytes where cefepime concentrations were reduced. Additionally, "some" mutant strains exhibited a persister phenotype upon further cefepime exposure. This work uncovers a mechanism allowing "select" gastrointestinal bacteria to evade antibiotic treatment.

Exploring the arsenal of antimicrobial peptides: Mechanisms, diversity, and applications

Antimicrobial peptides (AMPs) are essential for defence against pathogens in all living organisms and possessed activities against bacteria, fungi, viruses, parasites and even cancer cells. AMPs are short peptides containing 12-100 amino acids conferring a net positive charge and an amphiphilic property in most cases. Although, anionic AMPs also exist. AMPs can be classified based on the types of secondary structures, charge, hydrophobicity, amino acid composition, length, etc. Their mechanism of action usually includes a membrane disruption process through pore formation (three different models have been described, barrel-stave, toroidal or carpet model) but AMPs can also penetrate and impair intracellular functions. Besides their activity against pathogens, they have also shown immunomodulatory properties in complex scenarios through many different interactions. The aim of this review to summarize knowledge about AMP's and discuss the potential application of AMPs as therapeutics, the challenges due to their limitations, including their susceptibility to degradation, the potential generation of AMP resistance, cost, etc. We also discuss the current FDA-approved drugs based on AMPs and strategies to circumvent natural AMPs' limitations.

Antibiotics modulates the virulence of N. meningitidis by regulating capsule synthesis

The opportunistic pathogens residing are frequently exposed to range of antimicrobials which affects virulence attributes. Neisseria meningitidis, is a host-restricted commensal of human upper respiratory tract which is subjected to a variety of stresses within the host, including antibiotic exposure. One of the most important virulence factors for pathogenesis is the meningococcal lipo-oligosaccharide capsule. Role of capsules in antimicrobial resistance and persistence is not yet established. In this study, different virulence factors of N. meningitidis were examined in presence of sub-MIC of four antibiotics: penicillin, ciprofloxacin, erythromycin and chloramphenicol. We observed increased production of the capsule by N. meningitidis when grown in the presence of penicillin, erythromycin, and chloramphenicol at sub-inhibitory concentration. Capsular production increase concurrently with increased resistance to inducing antibiotic which also confers increased survival in human serum. Finally, we show that increased capsule production in response to antibiotic exposure is aided by siaC, ctrB, lipA gene expression. These findings show that capsule synthesis, a major pathogenicity determinant, is regulated in response to antibiotic stress. Our findings support a model in which gene expression changes caused by ineffective antibiotic treatment cause N. meningitidis transition between states of low and high virulence potential, contributing to pathogen's opportunistic nature.

Bacterial resistance to antibacterial agents: Mechanisms, control strategies, and implications for global health

The spread of bacterial drug resistance has posed a severe threat to public health globally. Here, we cover bacterial resistance to current antibacterial drugs, including traditional herbal medicines, conventional antibiotics, and antimicrobial peptides. We summarize the influence of bacterial drug resistance on global health and its economic burden while highlighting the resistance mechanisms developed by bacteria. Based on the One Health concept, we propose 4A strategies to combat bacterial resistance, including prudent Application of antibacterial agents, Administration, Assays, and Alternatives to antibiotics. Finally, we identify several opportunities and unsolved questions warranting future exploration for combating bacterial resistance, such as predicting genetic bacterial resistance through the use of more effective techniques, surveying both genetic determinants of bacterial resistance and the transmission dynamics of antibiotic resistance genes (ARGs).

Susceptible bacteria survive antibiotic treatment in the mammalian gastrointestinal tract without evolving resistance

In vitro systems have provided great insight into the mechanisms of antibiotic resistance. Yet, in vitro approaches cannot reflect the full complexity of what transpires within a host. As the mammalian gut is host to trillions of resident bacteria and thus a potential breeding ground for antibiotic resistance, we sought to better understand how gut bacteria respond to antibiotic treatment in vivo . Here, we colonized germ-free mice with a genetically barcoded antibiotic pan-susceptible Escherichia coli clinical isolate and then administered the antibiotic cefepime via programmable subcutaneous pumps which allowed for closer emulation of human parenteral antibiotic pharmacokinetics/dynamics. After seven days of antibiotics, we were unable to culture E. coli from feces. We were, however, able to recover barcoded E. coli from harvested gastrointestinal (GI) tissue, despite high GI tract and plasma cefepime concentrations. Strikingly, these E. coli isolates were not resistant to cefepime but had acquired mutations â€" most notably in the wbaP gene, which encodes an enzyme required for the initiation of the synthesis of the polysaccharide capsule and lipopolysaccharide O antigen - that increased their ability to invade and survive within intestinal cells, including cultured human colonocytes. Further, these E. coli mutants exhibited a persister phenotype when exposed to cefepime, allowing for greater survival to pulses of cefepime treatment when compared to the wildtype strain. Our findings highlight a mechanism by which bacteria in the gastrointestinal tract can adapt to antibiotic treatment by increasing their ability to persist during antibiotic treatment and invade intestinal epithelial cells where antibiotic concentrations are substantially reduced.

Interactions between host epithelial cells and Acinetobacter baumannii promote the emergence of highly antibiotic resistant and highly mucoid strains

Acinetobacter baumannii is an important nosocomial pathogen. Upon colonizing a host, A. baumannii are subjected to selective pressure by immune defenses as they adapt to the host environment. However, the mechanism of this pathoadaptation is unknown. Here, we established an in vitro system to evolve A. baumannii driven by the continuous selective pressure exerted by epithelial cells, and we used a combination of experimental evolution, phenotypic characterization and multi-omics analysis to address the underlying mechanism. When continuously exposed to selective pressure by pulmonary epithelial cells, A. baumannii showed ptk mutation-mediated mucoid conversion (reduced adhesion and increased anti-phagocytic ability) by enhancement of capsular exopolysaccharide chain length; rsmG mutation-mediated deficiency of 7-methylguanosine modification in the 524th nucleotide of 16S rRNA, which increased ribosome translation efficiency; and rnaseI mutation-mediated changes in outer membrane permeability and efflux pump expression. Together, these mutations altered susceptibility to a variety of antimicrobial agents, including the novel antibiotic cefiderocol, by regulating siderophore and siderophore-receptor biosynthesis. In conclusion, pulmonary epithelial cells modulate A. baumannii pathoadaptation, implicating the host–microbe interaction in the survival and persistence of A. baumannii.

Pathogenomic in silico approach identifies NSP-A and Fe-IIISBP as possible drug targets in Neisseria Meningitidis MC58 and development of pharmacophores as novel therapeutic candidates

Meningitis creates a life-threatening clinical crisis. Moreover, the administered antibiotics result into multi-drug resistance, thereby necessitating development of alternative therapeutic strategies. This study aimed at identifying novel-drug targets in Neisseria meningitidis and therapeutic molecules which can be exploited for the treatment of meningitis. Novel targets were identified by applying a pathogenomic approach involving protein data-set mining, subtractive channel analysis and subsequent qualitative analysis comprising of in silico pharmacokinetics, molecular docking and pharmacophore generation. Pathogenomic studies revealed Neisserial Surface Protein A (NSP-A) and Iron-III-Substrate Binding Protein (Fe-IIISBP) as potential targets. Two pharmacophore models comprising of 2-(biaryl) carbapenems, efavirenz, praziquantel and pyrimethamine for NSP-A and 2-(biaryl) carbapenems, trimipramine and pyrimethamine for Fe-IIISBP, showed successful docking, followed drug-likeness criteria and generated pharmacophore model with a score of 8.08 and 8.818, respectively, which had further been docked to the target stably. Thus, our study identifies NSP-A and Fe-IIISBP as novel targets in Neisseria meningitidis for which 2-(biaryl) carbapenems, efavirenz, praziquantel, trimipramine and pyrimethamine may be employed for effective treatment of meningitis.

The role of bacterial transport systems in the removal of host antimicrobial peptides in Gram-negative bacteria

Antibiotic resistance is a global issue that threatens our progress in healthcare and life expectancy. In recent years, antimicrobial peptides (AMPs) have been considered as promising alternatives to the classic antibiotics. AMPs are potentially superior due to their lower rate of resistance development, since they primarily target the bacterial membrane ('Achilles' heel' of the bacteria). However, bacteria have developed mechanisms of AMP resistance, including the removal of AMPs to the extracellular space by efflux pumps such as the MtrCDE or AcrAB-TolC systems, and the internalization of AMPs to the cytoplasm by the Sap transporter, followed by proteolytic digestion. In this review, we focus on AMP transport as a resistance mechanism compiling all the experimental evidence for the involvement of efflux in AMP resistance in Gram-negative bacteria and combine this information with the analysis of the structures of the efflux systems involved. Finally, we expose some open questions with the aim of arousing the interest of the scientific community towards the AMPs-efflux pumps interactions. All the collected information broadens our understanding of AMP removal by efflux pumps and gives some clues to assist the rational design of AMP-derivatives as inhibitors of the efflux pumps.

Neisseria genes required for persistence identified via in vivo screening of a transposon mutant library

The mechanisms used by human adapted commensal Neisseria to shape and maintain a niche in their host are poorly defined. These organisms are common members of the mucosal microbiota and share many putative host interaction factors with Neisseria meningitidis and Neisseria gonorrhoeae. Evaluating the role of these shared factors during host carriage may provide insight into bacterial mechanisms driving both commensalism and asymptomatic infection across the genus. We identified host interaction factors required for niche development and maintenance through in vivo screening of a transposon mutant library of Neisseria musculi, a commensal of wild-caught mice which persistently and asymptomatically colonizes the oral cavity and gut of CAST/EiJ and A/J mice. Approximately 500 candidate genes involved in long-term host interaction were identified. These included homologs of putative N. meningitidis and N. gonorrhoeae virulence factors which have been shown to modulate host interactions in vitro. Importantly, many candidate genes have no assigned function, illustrating how much remains to be learned about Neisseria persistence. Many genes of unknown function are conserved in human adapted Neisseria species; they are likely to provide a gateway for understanding the mechanisms allowing pathogenic and commensal Neisseria to establish and maintain a niche in their natural hosts. Validation of a subset of candidate genes confirmed a role for a polysaccharide capsule in N. musculi persistence but not colonization. Our findings highlight the potential utility of the Neisseria musculi-mouse model as a tool for studying the pathogenic Neisseria; our work represents a first step towards the identification of novel host interaction factors conserved across the genus.

The Neisseria genus contains many genetically related commensals of animals and humans, and two human pathogens, Neisseria gonorrhoeae and Neisseria meningitidis. The mechanisms allowing commensal Neisseria to maintain a niche in their host is little understood. To identify genes required for persistence, we screened a library of transposon mutants of Neisseria musculi, a commensal of wild-caught mice, in CAST/EiJ mice, which persistently and asymptomatically colonizes. Approximately 500 candidate host interaction genes were identified. A subset of these are homologs of N. meningitidis and N. gonorrhoeae genes known to modulate pathogen-host interactions in vitro. Many candidate genes have no known function, demonstrating how much remains to be learned about N. musculi niche maintenance. As many genes of unknown function are conserved in human adapted Neisseria, they provide a gateway for understanding Neisseria persistence mechanisms in general.

Capsule-dependent impact of MAPK signalling on host cell invasion and immune response during infection of the choroid plexus epithelium by Neisseria meningitidis

Background

The Gram-negative bacterium Neisseria meningitidis (Nm) can cause meningitis in humans, but the host signalling pathways manipulated by Nm during central nervous system (CNS) entry are not completely understood.

Methods

We investigate the role of the mitogen-activated protein kinases (MAPK) Erk1/2 and p38 in an in vitro model of the blood-cerebrospinal fluid barrier (BCSFB) based on human epithelial choroid plexus (CP) papilloma (HIBCPP) cells during infection with Nm serogroup B (NmB) and serogroup C (NmC) strains. A transcriptome analysis of HIBCPP cells following infection with Nm by massive analysis of cDNA ends (MACE) was done to further characterize the cellular response to infection of the barrier.

Results

Interestingly, whereas NmB and NmC wild type strains required active Erk1/2 and p38 pathways for infection, invasion by capsule-deficient mutants was independent of Erk1/2 and, in case of the NmB strain, of p38 activity. The transcriptome analysis of HIBCPP cells following infection with Nm demonstrated specific regulation of genes involved in the immune response dependent on Erk1/2 signalling. Gene ontology (GO) analysis confirmed loss of MAPK signalling after Erk1/2 inhibition and revealed an additional reduction of cellular responses including NFκB and JAK-STAT signalling. Interestingly, GO terms related to TNF signalling and production of IL6 were lost specifically following Erk1/2 inhibition during infection with wild type Nm, which correlated with the reduced infection rates by the wild type in absence of Erk1/2 signalling.

Conclusion

Our data point towards a role of MAPK signalling during infection of the CP epithelium by Nm, which is strongly influenced by capsule expression, and affects infection rates as well as the host cell response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-021-00288-7.

Tofacitinib Suppresses IL-10/IL-10R Signaling and Modulates Host Defense Responses in Human Macrophages

Jak inhibitors are increasingly used in dermatology. Despite broad inhibitory effects on cytokine signaling cascades, they only modestly increase the risk for infectious diseases. To address the molecular mechanisms underlying this unexpected clinical observation, we investigated how tofacintib (tofa), a first-in-class Jak inhibitor, regulates host defense responses in toll-like receptor 4-activated human macrophages. Specifically, we asked whether tofa inhibits anti-inflammatory IL-10 signaling, thereby counteracting the downregulation of inflammatory, host-protective pathways. We found that tofa blocked macrophage responses to IL-10 at the level of signal transducer and activator of transcription 3 phosphorylation. Furthermore, toll-like receptor 4-induced, autocrine/paracrine IL-10/IL-10R activation promoted the expression of hepcidin, the master regulator of iron metabolism, resulting in intracellular iron sequestration. In contrast, autocrine/paracrine IL-10/IL-10R activation repressed the expression of cathelicidin antimicrobial peptide as well as antigen-presenting molecules, thus together, inducing a pathogen-favoring environment. Although tofa further repressed cathelicidin, it prevented the induction of intracellular HAMP and restored the expression of antigen-presentation molecules in toll-like receptor 4-activated macrophages. Our study supports the concept that induction of IL-10/IL-10R signaling drives a complex immune evasion strategy of intracellular microbes. Moreover, we conclude that tofa has diverging effects on macrophage host response pathways, and we identify the toll-like receptor 4-IL-10-signal transducer and activator of transcription 3-HAMP axis as a potential therapeutic target to counteract immune evasion.

DNA Blocks the Lethal Effect of Human Beta-Defensin 2 Against Neisseria meningitidis

Neisseria meningitidis is a gram-negative bacterium that often asymptomatically colonizes the human nasopharyngeal tract. These bacteria cross the epithelial barrier can cause life-threatening sepsis and/or meningitis. Antimicrobial peptides are one of the first lines of defense against invading bacterial pathogens. Human beta-defensin 2 (hBD2) is an antimicrobial peptide with broad antibacterial activity, although its mechanism of action is poorly understood. Here, we investigated the effect of hBD2 on N. meningitidis. We showed that hBD2 binds to and kills actively growing meningococcal cells. The lethal effect was evident after 2 h incubation with the peptide, which suggests a slow killing mechanism. Further, the membrane integrity was not changed during hBD2 treatment. Incubation with lethal doses of hBD2 decreased the presence of diplococci; the number and size of bacterial microcolonies/aggregates remained constant, indicating that planktonic bacteria may be more susceptible to the peptide. Meningococcal DNA bound hBD2 in mobility shift assays and inhibited the lethal effect of hBD2 in a dose-dependent manner both in suspension and biofilms, supporting the interaction between hBD2 and DNA. Taken together, the ability of meningococcal DNA to bind hBD2 opens the possibility that extracellular DNA due to bacterial lysis may be a means of N. meningitidis to evade immune defenses.

Lactate-induced dispersal of Neisseria meningitidis microcolonies is mediated by changes in cell density and pilus retraction and is influenced by temperature change

Neisseria meningitidis is the etiologic agent of meningococcal meningitis and sepsis. Initial colonization of meningococci to the upper respiratory tract epithelium is crucial for disease development. The colonization occurs in several steps and expression of type IV pili (Tfp) is essential for both attachment and microcolony formation of encapsulated bacteria. Previously, we have shown that host-derived lactate induces synchronized dispersal of meningococcal microcolonies. In this study, we demonstrated that lactate-induced dispersal is dependent on bacterial concentration but not on the quorum sensing system autoinducer-2 or the two-component systems NarP/NarQ, PilR/PilS, NtrY/NtrX, and MisR/MisS. Further, there were no changes in expression of genes related to assembly, elongation, retraction, and modification of Tfp throughout the time course of lactate induction. By using pilT and pptB mutants, however, we found that lactate-induced dispersal was dependent on PilT-retraction but not on phosphoglycerol-modification of Tfp even though the PptB activity was important for preventing re-aggregation post-dispersal. Furthermore, protein synthesis was required for lactate-induced dispersal. Finally, we found that at a lower temperature, lactate-induced dispersal was delayed and unsynchronized, and bacteria reformed microcolonies. We conclude that lactate-induced microcolony dispersal is dependent on bacterial concentration, PilT-dependent Tfp retraction, and protein synthesis and influenced by environmental temperature.

Neisseria meningitidis Urethritis Outbreak Isolates Express a Novel Factor H Binding Protein Variant That Is a Potential Target of Group B-Directed Meningococcal (MenB) Vaccines

Factor H binding protein (FHbp) is an important Neisseria meningitidis virulence factor that binds a negative regulator of the alternative complement pathway, human factor H (FH). Binding of FH increases meningococcal resistance to complement-mediated killing. FHbp also is reported to prevent interaction of the antimicrobial peptide (AMP) LL-37 with the meningococcal surface and meningococcal killing. FHbp is a target of two licensed group B-directed meningococcal (MenB) vaccines. We found a new FHbp variant, peptide allele identification no. 896 (ID 896), was highly expressed by an emerging meningococcal pathotype, the nonencapsulated urethritis clade (US_NmUC). This clade has been responsible for outbreaks of urethritis in multiple U.S. cities since 2015, other mucosal infections, and cases of invasive meningococcal disease. FHbp ID 896 is a member of the variant group 1 (subfamily B), bound protective anti-FHbp monoclonal antibodies, bound high levels of human FH, and enhanced the resistance of the clade to complement-mediated killing in low levels of human complement likely present at human mucosal surfaces. Interestingly, expression of FHbp ID 896 resulted in augmented killing of the clade by LL-37. FHbp ID 896 of the clade was recognized by antibodies elicited by FHbp in MenB vaccines.

Antimicrobial Peptide JH-3 Effectively Kills Salmonella enterica Serovar Typhimurium Strain CVCC541 and Reduces Its Pathogenicity in Mice

Salmonella is an important zoonotic pathogen and is a major cause of gastrointestinal diseases worldwide. The current serious problem of antibiotic abuse has prompted the search for new substitutes for antibiotics. JH-3 is a small antimicrobial peptide with broad-spectrum bactericidal activity. In this study, we showed that JH-3 has good bactericidal activity towards the clinical isolate Salmonella enterica serovar Typhimurium strain CVCC541. The minimum inhibitory concentration (MIC) of JH-3 against this bacterium was determined to be 100 μg/mL, which could decrease the number of CVCC541 cells by 1000-fold in vitro within 5 h. The transmission electron microscopy (TEM) results showed that JH-3 can damage the cell wall and membrane of CVCC541, leading to the leakage of cell contents and subsequent cell death. To measure the bactericidal activity of CVCC541-infected mice were treated intraperitoneally 40 or 10 mg/kg JH-3 at 2 h or 3 days postinfection. Our results showed that treatment with 40 mg/kg JH-3 at 2 h postinfection had the best therapeutic effect and could significantly protect mice from a lethal dose of CVCC541. Furthermore, the clinical symptoms, bacterial burden in blood and organs, and intestinal pathological changes were all decreased and were close to normal. This study examined the therapeutic effect of the antimicrobial peptide JH-3 against S. enterica CVCC541 infection for the first time and determined the therapeutic effect of different JH-3 doses and treatment times, laying the foundation for studies of new antimicrobial agents.

The Role of Bacterial Membrane Vesicles in the Dissemination of Antibiotic Resistance and as Promising Carriers for Therapeutic Agent Delivery

The rapid emergence and spread of antibiotic-resistant bacteria continues to be an issue difficult to deal with, especially in the clinical, animal husbandry, and food fields. The occurrence of multidrug-resistant bacteria renders treatment with antibiotics ineffective. Therefore, the development of new therapeutic methods is a worthwhile research endeavor in treating infections caused by antibiotic-resistant bacteria. Recently, bacterial membrane vesicles (BMVs) have been investigated as a possible approach to drug delivery and vaccine development. The BMVs are released by both pathogenic and non-pathogenic Gram-positive and Gram-negative bacteria, containing various components originating from the cytoplasm and the cell envelope. The BMVs are able to transform bacteria with genes that encode enzymes such as proteases, glycosidases, and peptidases, resulting in the enhanced antibiotic resistance in bacteria. The BMVs can increase the resistance of bacteria to antibiotics. However, the biogenesis and functions of BMVs are not fully understood in association with the bacterial pathogenesis. Therefore, this review aims to discuss BMV-associated antibiotic resistance and BMV-based therapeutic interventions.

Coevolution of Resistance Against Antimicrobial Peptides

Antimicrobial peptides (AMPs) are produced by all forms of life, ranging from eukaryotes to prokaryotes, and they are a crucial component of innate immunity, involved in clearing infection by inhibiting pathogen colonization. In the recent past, AMPs received high attention due to the increase of extensive antibiotic resistance by these pathogens. AMPs exhibit a diverse spectrum of activity against bacteria, fungi, parasites, and various types of cancer. AMPs are active against various bacterial pathogens that cause disease in animals and plants. However, because of the coevolution of host and pathogen interaction, bacteria have developed the mechanisms to sense and exhibit an adaptive response against AMPs. These resistance mechanisms are playing an important role in bacterial virulence within the host. Here, we have discussed the different resistance mechanisms used by gram-positive and gram-negative bacteria to sense and combat AMP actions. Understanding the mechanism of AMP resistance may provide directions toward the development of novel therapeutic strategies to control multidrug-resistant pathogens.

Antimicrobial Peptides: Virulence and Resistance Modulation in Gram-Negative Bacteria

Growing resistance to antibiotics is one of the biggest threats to human health. One of the possibilities to overcome this resistance is to use and develop alternative molecules such as antimicrobial peptides (AMPs). However, an increasing number of studies have shown that bacterial resistance to AMPs does exist. Since AMPs are immunity molecules, it is important to ensure that their potential therapeutic use is not harmful in the long term. Recently, several studies have focused on the adaptation of Gram-negative bacteria to subinhibitory concentrations of AMPs. Such concentrations are commonly found in vivo and in the environment. It is therefore necessary to understand how bacteria detect and respond to low concentrations of AMPs. This review focuses on recent findings regarding the impact of subinhibitory concentrations of AMPs on the modulation of virulence and resistance in Gram-negative bacteria.

Virulence Traits of a Serogroup C Meningococcus and Isogenic cssA Mutant, Defective in Surface-Exposed Sialic Acid, in a Murine Model of Meningitis

In serogroup C Neisseria meningitidis, the cssA (siaA) gene codes for an UDP-N-acetylglucosamine 2-epimerase that catalyzes the conversion of UDP-N-acetyl-α-d-glucosamine into N-acetyl-d-mannosamine and UDP in the first step in sialic acid biosynthesis. This enzyme is required for the biosynthesis of the (α2→9)-linked polysialic acid capsule and for lipooligosaccharide (LOS) sialylation. In this study, we have used a reference serogroup C meningococcal strain and an isogenic cssA knockout mutant to investigate the pathogenetic role of surface-exposed sialic acids in a model of meningitis based on intracisternal inoculation of BALB/c mice. Results confirmed the key role of surface-exposed sialic acids in meningococcal pathogenesis. The 50% lethal dose (LD50) of the wild-type strain 93/4286 was about four orders of magnitude lower than that of the cssA mutant. Compared to the wild-type strain, the ability of this mutant to replicate in brain and spread systemically was severely impaired. Evaluation of brain damage evidenced a significant reduction in cerebral hemorrhages in mice infected with the mutant in comparison with the levels in those challenged with the wild-type strain. Histological analysis showed the typical features of bacterial meningitis, including inflammatory cells in the subarachnoid, perivascular, and ventricular spaces especially in animals infected with the wild type. Noticeably, 80% of mice infected with the wild-type strain presented with massive bacterial localization and accompanying inflammatory infiltrate in the corpus callosum, indicating high tropism of meningococci exposing sialic acids toward this brain structure and a specific involvement of the corpus callosum in the mouse model of meningococcal meningitis.

Deletion of D-Lactate Dehydrogenase A in Neisseria meningitidis Promotes Biofilm Formation Through Increased Autolysis and Extracellular DNA Release

Neisseria meningitidis is a Gram-negative bacterium that asymptomatically colonizes the human nasopharyngeal mucosa. Pilus-mediated initial adherence of N. meningitidis to the epithelial mucosa is followed by the formation of three-dimensional aggregates, called microcolonies. Dispersal from microcolonies contributes to the transmission of N. meningitidis across the epithelial mucosa. We have recently discovered that environmental concentrations of host cell-derived lactate influences N. meningitidis microcolony dispersal. Here, we examined the ability of N. meningitidis mutants deficient in lactate metabolism to form biofilms. A lactate dehydrogenease A (ldhA) mutant had an increased level of biofilm formation. Deletion of ldhA increased the N. meningitidis cell surface hydrophobicity and aggregation. In this study, we used FAM20, which belongs to clonal complex ST-11 that forms biofilms independently of extracellular DNA (eDNA). However, treatment with DNase I abolished the increased biofilm formation and aggregation of the ldhA-deficient mutant, suggesting a critical role for eDNA. Compared to wild-type, the ldhA-deficient mutant exhibited an increased autolytic rate, with significant increases in the eDNA concentrations in the culture supernatants and in biofilms. Within the ldhA mutant biofilm, the transcription levels of the capsule, pilus, and bacterial lysis genes were downregulated, while norB, which is associated with anaerobic respiration, was upregulated. These findings suggest that the absence of ldhA in N. meningitidis promotes biofilm formation and aggregation through autolysis-mediated DNA release.

Heteroresistance to the model antimicrobial peptide polymyxin B in the emerging Neisseria meningitidis lineage 11.2 urethritis clade: mutations in the pilMNOPQ operon

Clusters of Neisseria meningitidis (Nm) urethritis among primarily heterosexual males in multiple US cities have been attributed to a unique non-encapsulated meningococcal clade (the US Nm urethritis clade, US_NmUC) within the hypervirulent clonal complex 11. Resistance to antimicrobial peptides (AMPs) is a key feature of urogenital pathogenesis of the closely related species, Neisseria gonorrhoeae. The US_NmUC isolates were found to be highly resistant to the model AMP, polymyxin B (PmB, MICs 64-256 µg ml^-1 ). The isolates also demonstrated stable subpopulations of heteroresistant colonies that showed near total resistant to PmB (MICs 384-1024 µg ml^-1 ) and colistin (MIC 256 µg ml^-1 ) as well as enhanced LL-37 resistance. This is the first observation of heteroresistance in N. meningitidis. Consistent with previous findings, overall PmB resistance in US_NmUC isolates was due to active Mtr efflux and LptA-mediated lipid A modification. However, whole genome sequencing, variant analyses and directed mutagenesis revealed that the heteroresistance phenotypes and very high-level AMP resistance were the result of point mutations and IS1655 element movement in the pilMNOPQ operon, encoding the type IV pilin biogenesis apparatus. Cross-resistance to other classes of antibiotics was also observed in the heteroresistant colonies. High-level resistance to AMPs may contribute to the pathogenesis of US_NmUC.

Antimicrobial Peptides: Features, Action, and Their Resistance Mechanisms in Bacteria

In recent years, because of increased resistance to conventional antimicrobials, many researchers have started to study the synthesis of new antibiotics to control the disease-causing effects of infectious pathogens. Antimicrobial peptides (AMPs) are among the newest antibiotics; these peptides are integral compounds in all kinds of organisms and play a significant role in microbial ecology, and critically contribute to the innate immunity of organisms by destroying invading microorganisms. Moreover, AMPs may encourage cells to produce chemokines, stimulate angiogenesis, accelerate wound healing, and influence programmed cell death in multicellular organisms. Bacteria differ in their inherent susceptibility and resistance mechanisms to these peptides when responding to the antimicrobial effects of AMPs. Generally, the development of AMP resistance mechanisms is driven by direct competition between bacterial species, and host and pathogen interactions. Several studies have shown diverse mechanisms of bacterial resistance to AMPs, for example, some bacteria produce proteases and trapping proteins; some modify cell surface charge, change membrane fluidity, and activate efflux pumps; and some species make use of biofilms and exopolymers, and develop sensing systems by selective gene expression. A closer understanding of bacterial resistance mechanisms may help in developing novel therapeutic approaches for the treatment of infections caused by pathogenic organisms that are successful in developing extensive resistance to AMPs. Based on these observations, this review discusses the properties of AMPs, their targeting mechanisms, and bacterial resistance mechanisms against AMPs.

Host cell-derived lactate functions as an effector molecule in Neisseria meningitidis microcolony dispersal

The development of meningococcal disease, caused by the human pathogen Neisseria meningitidis, is preceded by the colonization of the epithelial layer in the nasopharynx. After initial adhesion to host cells meningococci form aggregates, through pilus-pilus interactions, termed microcolonies from which the bacteria later detach. Dispersal from microcolonies enables access to new colonization sites and facilitates the crossing of the cell barrier; however, this process is poorly understood. In this study, we used live-cell imaging to investigate the process of N. meningitidis microcolony dispersal. We show that direct contact with host cells is not required for microcolony dispersal, instead accumulation of a host-derived effector molecule induces microcolony dispersal. By using a host-cell free approach, we demonstrated that lactate, secreted from host cells, initiate rapid dispersal of microcolonies. Interestingly, metabolic utilization of lactate by the bacteria was not required for induction of dispersal, suggesting that lactate plays a role as a signaling molecule. Furthermore, Neisseria gonorrhoeae microcolony dispersal could also be induced by lactate. These findings reveal a role of host-secreted lactate in microcolony dispersal and virulence of pathogenic Neisseria.

The human restricted pathogen Neisseria meningitidis is a major cause of bacterial meningitis and sepsis worldwide. Colonization of the mucosal layer in the upper respiratory tract is essential to establish invasive disease. The initial interaction with host cells is characterized by bacterial proliferation and adhesion as aggregates, called microcolonies. Detachment from microcolonies in the nasopharyngeal epithelium facilitates crossing of the cell barrier that can result in invasive disease, yet this process is poorly understood. Here we demonstrate that lactate, an abundant molecule in host mucosal environments, induces N. meningitidis microcolony dispersal. Interestingly, metabolic utilization of lactate by the bacteria was not required for the process, suggesting that lactate play a role as a signaling molecule in pathogenic Neisseria. We propose that the microcolony dispersal in pathogenic Neisseria is influenced by environmental concentrations of lactate. These findings will assist in better understanding the transition from asymptomatic carriage to invasive disease.

Thymol kills bacteria, reduces biofilm formation, and protects mice against a fatal infection of Actinobacillus pleuropneumoniae strain L20

Actinobacillus pleuropneumoniae is the causative agent of the highly contagious and deadly respiratory infection porcine pleuropneumonia, resulting in serious losses to the pig industry worldwide. Alternative to antibiotics are urgently needed due to the serious increase in antimicrobial resistance. Thymol is a monoterpene phenol and efficiently kills a variety of bacteria. This study found that thymol has strong bactericidal effects on the A. pleuropneumoniae 5b serotype strain, an epidemic strain in China. Sterilization occurred rapidly, and the minimum inhibitory concentration (MIC) is 31.25μg/mL; the A. pleuropneumoniae density was reduced 1000 times within 10min following treatment with 1 MIC. Transmission electron microscopy (TEM) analysis revealed that thymol could rapidly disrupt the cell walls and cell membranes of A. pleuropneumoniae, causing leakage of cell contents and cell death. In addition, treatment with thymol at 0.5 MIC significantly reduced the biofilm formation of A. pleuropneumoniae. Quantitative RT-PCR results indicated that thymol treatment significantly increased the expression of the virulence genes purC, tbpB1 and clpP and down-regulated ApxI, ApxII and Apa1 expression in A. pleuropneumoniae. Therapeutic analysis of a murine model showed that thymol (20mg/kg) protected mice from a lethal dose of A. pleuropneumoniae, attenuated lung pathological lesions. This study is the first to report the use of thymol to treat A. pleuropneumoniae infection, establishing a foundation for the development of new antimicrobials.

Antimicrobial Peptides from Fruits and Their Potential Use as Biotechnological Tools-A Review and Outlook

Bacterial resistance is a major threat to plant crops, animals and human health, and over the years this situation has increasingly spread worldwide. Due to their many bioactive compounds, plants are promising sources of antimicrobial compounds that can potentially be used in the treatment of infections caused by microorganisms. As well as stem, flowers and leaves, fruits have an efficient defense mechanism against pests and pathogens, besides presenting nutritional and functional properties due to their multifunctional molecules. Among such compounds, the antimicrobial peptides (AMPs) feature different antimicrobials that are capable of disrupting the microbial membrane and of acting in binding to intra-cytoplasmic targets of microorganisms. They are therefore capable of controlling or halting the growth of microorganisms. In summary, this review describes the major classes of AMPs found in fruits, their possible use as biotechnological tools and prospects for the pharmaceutical industry and agribusiness.

Bacterial Evasion of Host Antimicrobial Peptide Defenses

Antimicrobial peptides (AMPs), also known as host defense peptides, are small naturally occurring microbicidal molecules produced by the host innate immune response that function as a first line of defense to kill pathogenic microorganisms by inducing deleterious cell membrane damage. AMPs also possess signaling and chemoattractant activities and can modulate the innate immune response to enhance protective immunity or suppress inflammation. Human pathogens have evolved defense molecules and strategies to counter and survive the AMPs released by host immune cells such as neutrophils and macrophages. Here, we review the various mechanisms used by human bacterial pathogens to resist AMP-mediated killing, including surface charge modification, active efflux, alteration of membrane fluidity, inactivation by proteolytic digestion, and entrapment by surface proteins and polysaccharides. Enhanced understanding of AMP resistance at the molecular level may offer insight into the mechanisms of bacterial pathogenesis and augment the discovery of novel therapeutic targets and drug design for the treatment of recalcitrant multidrug-resistant bacterial infections.

Regulation of capsule in Neisseria meningitidis

Neisseria meningitidis, a devastating pathogen exclusive to humans, expresses capsular polysaccharides that are the major meningococcal virulence determinants and the basis for successful meningococcal vaccines. With rare exceptions, the expression of capsule (serogroups A, B, C, W, X, Y) is required for systemic invasive meningococcal disease. Changes in capsule expression or structure (e.g. hypo- or hyper-encapsulation, capsule "switching", acetylation) can influence immunologic diagnostic assays or lead to immune escape. The loss or down-regulation of capsule is also critical in meningococcal biology facilitating meningococcal attachment, microcolony formation and the carriage state at human mucosal surfaces. Encapsulated meningococci contain a cps locus with promoters located in an intergenic region between the biosynthesis and the conserved capsule transport operons. The cps intergenic region is transcriptionally regulated (and thus the amount of capsule expressed) by IS element insertion, by a two-component system, MisR/MisS and through sequence changes that result in post-transcriptional RNA thermoregulation. Reversible on-off phase variation of capsule expression is controlled by slipped strand mispairing of homo-polymeric tracts and by precise insertion and excision of IS elements (e.g. IS1301) in the biosynthesis operon. Capsule structure can be altered by phase-variable expression of capsular polymer modification enzymes or "switched" through transformation and homologous recombination of different polymerases. Understanding the complex regulation of meningococcal capsule has important implications for meningococcal biology, pathogenesis, diagnostics, current and future vaccine development and vaccine strategies.

Bacterial strategies of resistance to antimicrobial peptides

Antimicrobial peptides (AMPs) are a key component of the host's innate immune system, targeting invasive and colonizing bacteria. For successful survival and colonization of the host, bacteria have a series of mechanisms to interfere with AMP activity, and AMP resistance is intimately connected with the virulence potential of bacterial pathogens. In particular, because AMPs are considered as potential novel antimicrobial drugs, it is vital to understand bacterial AMP resistance mechanisms. This review gives a comparative overview of Gram-positive and Gram-negative bacterial strategies of resistance to various AMPs, such as repulsion or sequestration by bacterial surface structures, alteration of membrane charge or fluidity, degradation and removal by efflux pumps.This article is part of the themed issue 'Evolutionary ecology of arthropod antimicrobial peptides'.

Bacterial strategies of resistance to antimicrobial peptides

Antimicrobial peptides (AMPs) are a key component of the host's innate immune system, targeting invasive and colonizing bacteria. For successful survival and colonization of the host, bacteria have a series of mechanisms to interfere with AMP activity, and AMP resistance is intimately connected with the virulence potential of bacterial pathogens. In particular, because AMPs are considered as potential novel antimicrobial drugs, it is vital to understand bacterial AMP resistance mechanisms. This review gives a comparative overview of Gram-positive and Gram-negative bacterial strategies of resistance to various AMPs, such as repulsion or sequestration by bacterial surface structures, alteration of membrane charge or fluidity, degradation and removal by efflux pumps.This article is part of the themed issue 'Evolutionary ecology of arthropod antimicrobial peptides'.

Neisseria meningitidis Polynucleotide Phosphorylase Affects Aggregation, Adhesion, and Virulence

Neisseria meningitidis autoaggregation is an important step during attachment to human cells. Aggregation is mediated by type IV pili and can be modulated by accessory pilus proteins, such as PilX, and posttranslational modifications of the major pilus subunit PilE. The mechanisms underlying the regulation of aggregation remain poorly characterized. Polynucleotide phosphorylase (PNPase) is a 3'-5' exonuclease that is involved in RNA turnover and the regulation of small RNAs. In this study, we biochemically confirm that NMC0710 is the N. meningitidis PNPase, and we characterize its role in N. meningitidis pathogenesis. We show that deletion of the gene encoding PNPase leads to hyperaggregation and increased adhesion to epithelial cells. The aggregation induced was found to be dependent on pili and to be mediated by excessive pilus bundling. PNPase expression was induced following bacterial attachment to human cells. Deletion of PNPase led to global transcriptional changes and the differential regulation of 469 genes. We also demonstrate that PNPase is required for full virulence in an in vivo model of N. meningitidis infection. The present study shows that PNPase negatively affects aggregation, adhesion, and virulence in N. meningitidis.

Mechanisms of Antimicrobial Peptide Resistance in Gram-Negative Bacteria

Cationic antimicrobial peptides (CAMPs) are important innate immune defenses that inhibit colonization by pathogens and contribute to clearance of infections. Gram-negative bacterial pathogens are a major target, yet many of them have evolved mechanisms to resist these antimicrobials. These resistance mechanisms can be critical contributors to bacterial virulence and are often crucial for survival within the host. Here, we summarize methods used by Gram-negative bacteria to resist CAMPs. Understanding these mechanisms may lead to new therapeutic strategies against pathogens with extensive CAMP resistance.

Potential Use of Antimicrobial Peptides as Vaginal Spermicides/Microbicides

The concurrent increases in global population and sexually transmitted infection (STI) demand a search for agents with dual spermicidal and microbicidal properties for topical vaginal application. Previous attempts to develop the surfactant spermicide, nonoxynol-9 (N-9), into a vaginal microbicide were unsuccessful largely due to its inefficiency to kill microbes. Furthermore, N-9 causes damage to the vaginal epithelium, thus accelerating microbes to enter the women's body. For this reason, antimicrobial peptides (AMPs), naturally secreted by all forms of life as part of innate immunity, deserve evaluation for their potential spermicidal effects. To date, twelve spermicidal AMPs have been described including LL-37, magainin 2 and nisin A. Human cathelicidin LL-37 is the most promising spermicidal AMP to be further developed for vaginal use for the following reasons. First, it is a human AMP naturally produced in the vagina after intercourse. Second, LL-37 exerts microbicidal effects to numerous microbes including those that cause STI. Third, its cytotoxicity is selective to sperm and not to the female reproductive tract. Furthermore, the spermicidal effects of LL-37 have been demonstrated in vivo in mice. Therefore, the availability of LL-37 as a vaginal spermicide/microbicide will empower women for self-protection against unwanted pregnancies and STI.

Defensive remodeling: How bacterial surface properties and biofilm formation promote resistance to antimicrobial peptides

Multidrug resistance bacteria are a major concern worldwide. These pathogens cannot be treated with conventional antibiotics and thus alternative therapeutic agents are needed. Antimicrobial peptides (AMPs) are considered to be good candidates for this purpose. Most AMPs are short and positively charged amphipathic peptides, which are found in all known forms of life. AMPs are known to kill bacteria by binding to the negatively charged bacterial surface, and in most cases cause membrane disruption. Resistance toward AMPs can be developed, by modification of bacterial surface molecules, secretion of protective material and up-regulation or elimination of specific proteins. Because of the general mechanisms of attachment and action of AMPs, bacterial resistance to AMPs often involves biophysical and biochemical changes such as surface rigidity, cell wall thickness, surface charge, as well as membrane and cell wall modification. Here we focus on the biophysical, surface and surrounding changes that bacteria undergo in acquiring resistance to AMPs. In addition we discuss the question of whether bacterial resistance to administered AMPs might compromise our innate immunity to endogenous AMPs. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

Antimicrobial peptide resistance in Neisseria meningitidis

Antimicrobial peptides (AMPs) play an important role as a host defense against microbial pathogens and are key components of the human innate immune response. Neisseria meningitidis frequently colonizes the human nasopharynx as a commensal but also is a worldwide cause of epidemic meningitis and rapidly fatal sepsis. In the human respiratory tract, the only known reservoir of N. meningitidis, meningococci are exposed to human endogenous AMPs. Thus, it is not surprising that meningococci have evolved effective mechanisms to confer intrinsic and high levels of resistance to the action of AMPs. This article reviews the current knowledge about AMP resistance mechanisms employed by N. meningitidis. Two major resistance mechanisms employed by meningococci are the constitutive modification of the lipid A head groups of lipooligosaccharides by phosphoethanolamine and the active efflux pump mediated excretion of AMPs. Other factors influencing AMP resistance, such as the major porin PorB, the pilin biogenesis apparatus, and capsular polysaccharides, have also been identified. Even with an inherently high intrinsic resistance, several AMP resistance determinants can be further induced upon exposure to AMPs. Many well-characterized AMP resistance mechanisms in other Gram-negative bacteria are not found in meningococci. Thus, N. meningitidis utilizes a limited but highly effective set of molecular mechanisms to mediate antimicrobial peptide resistance. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

Characterization of motility and piliation in pathogenic Neisseria

Background

The type IV pili (Tfp) of pathogenic Neisseria (i.e., N. gonorrhoeae and N. meningitidis) are essential for twitching motility. Tfp retraction, which is dependent on the ATPase PilT, generates the forces that move bacteria over surfaces. Neisseria motility has mainly been studied in N. gonorrhoeae whereas the motility of N. meningitidis has not yet been characterized.

Results

In this work, we analyzed bacterial motility and monitored Tfp retraction using live-cell imaging of freely moving bacteria. We observed that N. meningitidis moved over surfaces at an approximate speed of 1.6 μm/s, whereas N. gonorrhoeae moved with a lower speed (1.0 μm/s). An alignment of the meningococcal and gonococcal pilT promoters revealed a conserved single base pair variation in the −10 promoter element that influence PilT expression. By tracking mutants with altered pilT expression or pilE sequence, we concluded that the difference in motility speed was independent of both. Live-cell imaging using total internal reflection fluorescence microscopy demonstrated that N. gonorrhoeae more often moved with fewer visible retracting filaments when compared to N. meningitidis. Correspondingly, meningococci also displayed a higher level of piliation in transmission electron microscopy. Nevertheless, motile gonococci that had the same number of filaments as N. meningitidis still moved with a lower speed.

Conclusions

These data reveal differences in both speed and piliation between the pathogenic Neisseria species during twitching motility, suggesting a difference in Tfp-dynamics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0424-6) contains supplementary material, which is available to authorized users.

Antibiotic modulation of capsular exopolysaccharide and virulence in Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic pathogen of increasing importance due to its propensity for intractable multidrug-resistant infections in hospitals. All clinical isolates examined contain a conserved gene cluster, the K locus, which determines the production of complex polysaccharides, including an exopolysaccharide capsule known to protect against killing by host serum and to increase virulence in animal models of infection. Whether the polysaccharides determined by the K locus contribute to intrinsic defenses against antibiotics is unknown. We demonstrate here that mutants deficient in the exopolysaccharide capsule have lowered intrinsic resistance to peptide antibiotics, while a mutation affecting sugar precursors involved in both capsule and lipopolysaccharide synthesis sensitizes the bacterium to multiple antibiotic classes. We observed that, when grown in the presence of certain antibiotics below their MIC, including the translation inhibitors chloramphenicol and erythromycin, A. baumannii increases production of the K locus exopolysaccharide. Hyperproduction of capsular exopolysaccharide is reversible and non-mutational, and occurs concomitantly with increased resistance to the inducing antibiotic that is independent of the presence of the K locus. Strikingly, antibiotic-enhanced capsular exopolysaccharide production confers increased resistance to killing by host complement and increases virulence in a mouse model of systemic infection. Finally, we show that augmented capsule production upon antibiotic exposure is facilitated by transcriptional increases in K locus gene expression that are dependent on a two-component regulatory system, bfmRS. These studies reveal that the synthesis of capsule, a major pathogenicity determinant, is regulated in response to antibiotic stress. Our data are consistent with a model in which gene expression changes triggered by ineffectual antibiotic treatment cause A. baumannii to transition between states of low and high virulence potential, which may contribute to the opportunistic nature of the pathogen.

Acinetobacter baumannii has gained notoriety as a cause of hospital-acquired infections that are difficult to treat due to extensive antibiotic resistance. While the microorganism rarely causes disease in the community, it commonly infects patients receiving antibiotics. The factors intrinsic to the bacterium that enable growth in the presence of antibiotics are not well characterized. Furthermore, the consequences of subinhibitory antibiotic concentrations on A. baumannii disease are unknown. Here we examined the K locus, a bacterial disease determinant responsible for the production of protective surface polysaccharides, and asked whether this determinant also contributes to antibiotic resistance. We found that K locus polysaccharides facilitate resistance to multiple antibiotics, and, unexpectedly, that the bacterium responds to certain antibiotics at subinhibitory concentrations by increasing production of capsule, the principal K locus polysaccharide. This augmented production of capsule, which is mediated by upregulation of K locus gene expression, increased the ability of the bacterium to overcome attack by the complement system, an important anti-pathogen host defense, and result in lethal disease during experimental bloodstream infection in mice. Our studies indicate that A. baumannii increases its disease-causing potential in the setting of inadequate antibiotic treatment, which may promote the development of opportunistic infections.

The specificity of protection against cationic antimicrobial peptides by lactoferrin binding protein B

A variety of Gram-negative pathogens possess host-specific lactoferrin (Lf) receptors that mediate the acquisition of iron from host Lf. The integral membrane protein component of the receptor, lactoferrin binding protein A specifically binds host Lf and is required for acquisition of iron from Lf. In contrast, the role of the bi-lobed surface lipoprotein, lactoferrin binding protein B (LbpB), in Lf binding and iron acquisition is uncertain. A common feature of LbpBs from most species is the presence of clusters of negatively charged amino acids in the protein's C-terminal lobe. Recently it has been shown that the negatively charged regions from the Neisseria meningitidis LbpB are responsible for protecting against an 11 amino acid cationic antimicrobial peptide (CAP), lactoferricin (Lfcin), derived from human Lf. In this study we investigated whether the LbpB confers resistance to other CAPs since N. meningitidis is likely to encounter other CAPs from the host. LbpB provided protection against the cathelicidin derived peptide, cathelicidin related antimicrobial peptide (mCRAMP), but did not confer protection against Tritrp 1 or LL37 under our experimental conditions. When tested against a range of rationally designed synthetic peptides, LbpB was shown to protect against IDR-1002 and IDR-0018 but not against HH-2 or HHC10.

Role of factor H binding protein in Neisseria meningitidis virulence and its potential as a vaccine candidate to broadly protect against meningococcal disease

Neisseria meningitidis is a Gram-negative microorganism that exists exclusively in humans and can cause devastating invasive disease. Although capsular polysaccharide-based vaccines against serogroups A, C, Y, and W135 are widely available, the pathway to a broadly protective vaccine against serogroup B has been more complex. The last 11 years has seen the discovery and development of the N. meningitidis serogroup B (MnB) outer membrane protein factor H binding protein (fHBP) as a vaccine component. Since the initial discovery of fHBP, a tremendous amount of work has accumulated on the diversity, structure, and regulation of this important protein. fHBP has proved to be a virulence factor for N. meningitidis and a target for functional bactericidal antibodies. fHBP is critical for survival of meningococci in the human host, as it is responsible for the primary interaction with human factor H (fH). Binding of hfH by the meningococcus serves to downregulate the host alternative complement pathway and helps the organism evade host innate immunity. Preclinical studies have shown that an fHBP-based vaccine can elicit serum bactericidal antibodies capable of killing MnB, and the vaccine has shown very encouraging results in human clinical trials. This report reviews our current knowledge of fHBP. In particular, we discuss the recent advances in our understanding of fHBP, its importance to N. meningitidis, and its potential role as a vaccine for preventing MnB disease.

Antibiotic development challenges: the various mechanisms of action of antimicrobial peptides and of bacterial resistance

Antimicrobial peptides (AMPs) are natural antibiotics produced by various organisms such as mammals, arthropods, plants, and bacteria. In addition to antimicrobial activity, AMPs can induce chemokine production, accelerate angiogenesis, and wound healing and modulate apoptosis in multicellular organisms. Originally, their antimicrobial mechanism of action was thought to consist solely of an increase in pathogen cell membrane permeability, but it has already been shown that several AMPs do not modulate membrane permeability in the minimal lethal concentration. Instead, they exert their effects by inhibiting processes such as protein and cell wall synthesis, as well as enzyme activity, among others. Although resistance to these molecules is uncommon several pathogens developed different strategies to overcome AMPs killing such as surface modification, expression of efflux pumps, and secretion of proteases among others. This review describes the various mechanisms of action of AMPs and how pathogens evolve resistance to them.

Serogroup-specific interaction of Neisseria meningitidis capsular polysaccharide with host cell microtubules and effects on tubulin polymerization

We have previously shown that during late stages of the infectious process, serogroup B meningococci (MenB) are able to escape the phagosome of in vitro-infected human epithelial cells. They then multiply in the cytosolic environment and spread intracellularly and to surrounding cells by exploiting the microtubule cytoskeleton, as suggested by results of infections in the presence of microtubule inhibitors and evidence of nanotubes connecting neighboring cells. In this study, by using microtubule binding assays with purified microtubule asters and bundles and microtubule bundles synthesized in vitro, we demonstrate that the MenB capsule directly mediates the interaction between bacteria and microtubules. The direct interaction between the microtubules and the MenB capsular polysaccharide was confirmed by coimmunoprecipitation experiments. Unexpectedly, serogroup C meningococci (MenC), which have a capsular polysaccharide that differs from that of MenB only by its anomeric linkage, α(2→9) instead of α(2→8), were not able to interact with the microtubules, and the lack of interaction was not due to capsular polysaccharide O-acetylation that takes place in most MenC strains but not in MenB strains. Moreover, we demonstrate that the MenB capsular polysaccharide inhibits tubulin polymerization in vitro. Thus, at variance with MenC, MenB may interfere with microtubule dynamics during cell infection.

Meningococcal resistance to antimicrobial peptides is mediated by bacterial adhesion and host cell RhoA and Cdc42 signalling

Antimicrobial peptides (AMPs) constitute an essential part of the innate immune defence. Pathogenic bacteria have evolved numerous strategies to withstand AMP-mediated killing. The influence of host epithelia on bacterial AMP resistance is, however, still largely unknown. We found that adhesion to pharyngeal epithelial cells protected Neisseria meningitidis, a leading cause of meningitis and sepsis, from the human cathelicidin LL-37, the cationic model amphipathic peptide (MAP) and the peptaibol alamethicin, but not from polymyxin B. Adhesion to primary airway epithelia resulted in a similar increase in LL-37 resistance. The inhibition of selective host cell signalling mediated by RhoA and Cdc42 was found to abolish the adhesion-induced LL-37 resistance by a mechanism unrelated to the actin cytoskeleton. Moreover, N. meningitidis triggered the formation of cholesterol-rich membrane microdomains in pharyngeal epithelial cells, and host cell cholesterol proved to be essential for adhesion-induced resistance. Our data highlight the importance of Rho GTPase-dependent host cell signalling for meningococcal AMP resistance. These results indicate that N. meningitidis selectively exploits the epithelial microenvironment in order to protect itself from LL-37.

In vitro resistance mechanisms of Neisseria meningitidis against neutrophil extracellular traps

Neisseria meningitidis (Nm) is a leading cause of septicemia in childhood. Nm septicemia is unique with respect to very quick disease progression, high in vivo bacterial replication rate and its considerable mortality. Nm circumvents major mechanisms of innate immunity such as complement system and phagocytosis. Neutrophil extracellular traps (NETs) are formed from neutrophils during systemic infection and are suggested to contain invading microorganisms. Here, we investigated the interaction of Nm with NETs. Both, meningococci and spontaneously released outer membrane vesicles (SOMVs) were potent NET inducers. NETs were unable to kill NET bound meningococci, but slowed down their proliferation rate. Using Nm as model organism we identified three novel mechanisms how bacteria can evade NET-mediated killing: (i) modification of lipid A of meningococcal LPS with phosphoethanolamine protected Nm from NET-bound cathepsin G; (ii) expression of the high-affinity zinc uptake receptor ZnuD allowed Nm to escape NET-mediated nutritional immunity; (iii) binding of SOMVs to NETs saved Nm from NET binding and the consequent bacteriostatic effect. Escape from NETs may contribute to the most rapid progression of meningococcal disease. The induction of NET formation by Nm in vivo might aggravate thrombosis in vessels ultimately directing to disseminated intravascular coagulation (DIC).

Identification of cell-penetrating peptides that are bactericidal to Neisseria meningitidis and prevent inflammatory responses upon infection

Meningococcal disease is characterized by a fast progression and a high mortality rate. Cell-penetrating peptides (CPPs), developed as vectors for cargo delivery into eukaryotic cells, share structural features with antimicrobial peptides. A screen identified two CPPs, transportan-10 (TP10) and model amphipathic peptide (MAP), with bactericidal action against Neisseria meningitidis. Both peptides were active in human whole blood at micromolar concentrations, while hemolysis remained negligible. Additionally, TP10 exhibited significant antibacterial activity in vivo. Uptake of SYTOX green into live meningococci was observed within minutes after TP10 treatment, suggesting that TP10 may act by membrane permeabilization. Apart from its bactericidal activity, TP10 suppressed inflammatory cytokine release from macrophages infected with N. meningitidis as well as from macrophages stimulated with enterobacterial and meningococcal lipopolysaccharide (LPS). Finally, incubation with TP10 reduced the binding of LPS to macrophages. This novel endotoxin-inhibiting property of TP10, together with its antimicrobial activity in vivo, indicates the possibility to design peptide-based therapies for infectious diseases.