Inhibition of virulence factor expression in Staphylococcus aureus by the Staphylococcus epidermidis agr pheromone and derivatives

Microbe Interactions within the Skin Microbiome

The skin is the largest human organ and is responsible for many important functions, such as temperature regulation, water transport, and protection from external insults. It is colonized by several microorganisms that interact with each other and with the host, shaping the microbial structure and community dynamics. Through these interactions, the skin microbiota can inhibit pathogens through several mechanisms such as the production of bacteriocins, proteases, phenol soluble modulins (PSMs), and fermentation. Furthermore, these commensals can produce molecules with antivirulence activity, reducing the potential of these pathogens to adhere to and invade human tissues. Microorganisms of the skin microbiota are also able to sense molecules from the environment and shape their behavior in response to these signals through the modulation of gene expression. Additionally, microbiota-derived compounds can affect pathogen gene expression, including the expression of virulence determinants. Although most studies related to microbial interactions in the skin have been directed towards elucidating competition mechanisms, microorganisms can also use the products of other species to their benefit. In this review, we will discuss several mechanisms through which microorganisms interact in the skin and the biotechnological applications of products originating from the skin microbiota that have already been reported in the literature.

Heterogeneous Regulation of StaphylococcusAureus by Different StaphylococcusEpidermidisagr Types in Atopic Dermatitis

The skin commensal Staphylococcus epidermidis exhibits a protective role in skin inflammation; however, the exact functions of S. epidermidis and their mechanisms in atopic dermatitis (AD) are not fully understood. Here, whole-genome sequencing was conducted on strains of S. epidermidis isolated from pediatric patients with AD and revealed significant strain-level heterogeneity in functional genes. Specific sequence analysis of S. epidermidis identified four types of accessory gene regulator (agr) according to locus variations in the agr operon, which was consistent with the metagenomic data of the contextual microbiota. The number of S. epidermidisagr type I was slightly decreased among AD isolates, whereas agr type IV was hardly detected in AD isolates. Functional experiments showed that strains of S. epidermidisagr types I and IV, but not types II and III, inhibited the expression of S. aureusagr-mediated virulence factors in vitro, suppressed S. aureus epidermal colonization, and attenuated skin inflammation in a mouse model. The delineation of genome signatures of S. epidermidis at the strain level in AD and the quorum-sensing interference between S. epidermidisagr type IV and S. aureus provide a foundation for the modulation of the skin microbiota and the treatment of AD.

Quorum-sensing, intra- and inter-species competition in the staphylococci

In Gram-positive bacteria such as Staphylococcus aureus and the coagulase-negative staphylococci (CoNS), the accessory gene regulator (agr) is a highly conserved but polymorphic quorum-sensing system involved in colonization, virulence and biofilm development. Signalling via agr depends on the interaction of an autoinducing peptide (AIP) with AgrC, a transmembrane sensor kinase that, once phosphorylated activates the response regulator AgrA. This in turn autoinduces AIP biosynthesis and drives target gene expression directly via AgrA or via the post-transcriptional regulator, RNAIII. In this review we describe the molecular mechanisms underlying the agr-mediated generation of, and response to, AIPs and the molecular basis of AIP-dependent activation and inhibition of AgrC. How the environment impacts on agr functionality is considered and the consequences of agr dysfunction for infection explored. We also discuss the concept of AIP-driven competitive interference between S. aureus and the CoNS and its anti-infective potential.

Cinnamaldehyde Targets the LytTR DNA-Binding Domain of the Response Regulator AgrA to Attenuate Biofilm Formation of Listeria monocytogenes

The Agr quorum sensing (QS) system is known to contribute to biofilm formation in Listeria monocytogenes. Cinnamaldehyde, a natural food preservative, is considered an inhibitor of Agr-mediated QS in L. monocytogenes. However, the exact mechanism by which cinnamaldehyde acts on Agr remains unclear. In this study, we assessed the effects of cinnamaldehyde on the histidine kinase AgrC and the response regulator AgrA in the Agr system. AgrC kinase activity was not influenced by cinnamaldehyde, and binding between AgrC and cinnamaldehyde was not observed when microscale thermophoresis (MST) was performed, indicating that AgrC was not the target of cinnamaldehyde. AgrA is specifically bound to the agr promoter (P2) to activate the transcription of the Agr system. However, AgrA-P2 binding was prevented by cinnamaldehyde. The interaction between cinnamaldehyde and AgrA was further confirmed with MST. Two conserved amino acids, Asn-178 and Arg-179, located in the LytTR DNA-binding domain of AgrA, were identified as the key sites for cinnamaldehyde-AgrA binding by alanine mutagenesis and MST. Coincidentally, Asn-178 was also involved in the AgrA-P2 interaction. Taken together, these results suggest that cinnamaldehyde acts as a competitive inhibitor of AgrA in AgrA-P2 binding, which leads to suppressed transcription of the Agr system and reduced biofilm formation in L. monocytogenes. IMPORTANCE Listeria monocytogenes can form biofilms on various food contact surfaces, posing a serious threat to food safety. Biofilm formation of L. monocytogenes is positively regulated by the Agr quorum sensing system. Thus, an alternative strategy for controlling L. monocytogenes biofilms is interfering with the Agr system. Cinnamaldehyde is considered an inhibitor of the L. monocytogenes Agr system; however, its exact mechanism of action is still unclear. Here, we found that AgrA (response regulator), rather than AgrC (histidine kinase), was the target of cinnamaldehyde. The conserved Asn-178 in the LytTR DNA-binding domain of AgrA was involved in cinnamaldehyde-AgrA and AgrA-P2 binding. Therefore, the occupation of Asn-178 by cinnamaldehyde suppressed transcription of the Agr system and reduced biofilm formation in L. monocytogenes. Our findings could provide a better understanding of the mechanism by which cinnamaldehyde inhibits L. monocytogenes biofilm formation.

Critical Assessment of the Prospects of Quorum-Quenching Therapy for Staphylococcus aureus Infection

Staphylococcus aureus is an important pathogen that causes a high number of infections and is one of the leading causes of death in hospitalized patients. Widespread antibiotic resistance such as in methicillin-resistant S. aureus (MRSA) has prompted research into potential anti-virulence-targeted approaches. Targeting the S. aureus accessory gene regulator (Agr) quorum-sensing system, a master regulator of virulence, is the most frequently proposed anti-virulence strategy for S. aureus. While much effort has been put into the discovery and screening for Agr inhibitory compounds, in vivo analysis of their efficacy in animal infection models is still rare and reveals various shortcomings and problems. These include (i) an almost exclusive focus on topical skin infection models, (ii) technical problems that leave doubt as to whether observed in vivo effects are due to quorum-quenching, and (iii) the discovery of counterproductive biofilm-increasing effects. Furthermore, potentially because of the latter, invasive S. aureus infection is associated with Agr dysfunctionality. Altogether, the potential of Agr inhibitory drugs is nowadays seen with low enthusiasm given the failure to provide sufficient in vivo evidence for their potential after more than two decades since the initiation of such efforts. However, current Agr inhibition-based probiotic approaches may lead to a new application of Agr inhibition strategies in preventing S. aureus infections by targeting colonization or for otherwise difficult-to-treat skin infections such as atopic dermatitis.

Mechanism of Action of Ribosomally Synthesized and Post-Translationally Modified Peptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a natural product class that has undergone significant expansion due to the rapid growth in genome sequencing data and recognition that they are made by biosynthetic pathways that share many characteristic features. Their mode of actions cover a wide range of biological processes and include binding to membranes, receptors, enzymes, lipids, RNA, and metals as well as use as cofactors and signaling molecules. This review covers the currently known modes of action (MOA) of RiPPs. In turn, the mechanisms by which these molecules interact with their natural targets provide a rich set of molecular paradigms that can be used for the design or evolution of new or improved activities given the relative ease of engineering RiPPs. In this review, coverage is limited to RiPPs originating from bacteria.

Staphylococcus epidermidis Controls Opportunistic Pathogens in the Nose, Could It Help to Regulate SARS-CoV-2 (COVID-19) Infection?

Staphylococcus epidermidis is more abundant in the anterior nares than internal parts of the nose, but its relative abundance changes along with age; it is more abundant in adolescents than in children and adults. Various studies have shown that S. epidermidis is the guardian of the nasal cavity because it prevents the colonization and infection of respiratory pathogens (bacteria and viruses) through the secretion of antimicrobial molecules and inhibitors of biofilm formation, occupying the space of the membrane mucosa and through the stimulation of the host's innate and adaptive immunity. There is a strong relationship between the low number of S. epidermidis in the nasal cavity and the increased risk of serious respiratory infections. The direct application of S. epidermidis into the nasal cavity could be an effective therapeutic strategy to prevent respiratory infections and to restore nasal cavity homeostasis. This review shows the mechanisms that S. epidermidis uses to eliminate respiratory pathogens from the nasal cavity, also S. epidermidis is proposed to be used as a probiotic to prevent the development of COVID-19 because S. epidermidis induces the production of interferon type I and III and decreases the expression of the entry receptors of SARS-CoV-2 (ACE2 and TMPRSS2) in the nasal epithelial cells.

Secondary Metabolites Governing Microbiome Interaction of Staphylococcal Pathogens and Commensals

Various Staphylococcus species colonize skin and upper airways of warm-blooded animals. They compete successfully with many other microorganisms under the hostile and nutrient-poor conditions of these habitats using mechanisms that we are only beginning to appreciate. Small-molecule mediators, whose biosynthesis requires complex enzymatic cascades, so-called secondary metabolites, have emerged as crucial components of staphylococcal microbiome interactions. Such mediators belong to a large variety of compound classes and several of them have attractive properties for future drug development. They include, for instance, bacteriocins such as lanthipeptides, thiopeptides, and fibupeptides that inhibit bacterial competitor species; signaling molecules such as thiolactone peptides that induce or inhibit sensory cascades in other bacteria; or metallophores such as staphyloferrins and staphylopine that scavenge scant transition metal ions. For some secondary metabolites such as the aureusimines, the exact function remains to be elucidated. How secondary metabolites shape the fitness of Staphylococcus species in the complex context of other microbial and host defense factors remains a challenging field of future research. A detailed understanding will help to harness staphylococcal secondary metabolites for excluding the pathogenic species Staphylococcus aureus from the nasal microbiomes of at-risk patients, and it will be instrumental for the development of advanced anti-infective interventions.

Beneficial effects of coagulase-negative Staphylococci on Staphylococcus aureus skin colonization

Our skin is constantly exposed to a large number of pathogens while at the same time undergoing selective colonization by commensal microorganisms such as coagulase-negative Staphylococci. Staphylococcus aureus, however, is a facultative pathogen that is usually absent from healthy skin but frequently colonizes the inflamed skin of atopic dermatitis patients, where it further promotes inflammation. Enhanced S. aureus skin colonization was shown to correlate with a loss of microbiome diversity indicating a role for skin commensals to shape pathogen colonization. Together, keratinocytes and immune cells in the skin need to discriminate commensals from pathogens and orchestrate subsequent immune reactions in response to colonizing microbes. However, the mechanisms how individual commensals cooperate with keratinocytes and the immune system of the skin to prevent pathogen colonization are barely understood. In this review, we discuss the current knowledge on the functional effects of coagulase-negative staphylococci, the most frequently isolated skin commensals, on S. aureus skin colonization.

Human skin microbiota-friendly lysostaphin

Growing antibiotic resistance of bacteria is a burning problem of human and veterinary medicine. Expansion and introduction of novel microbicidal therapeutics is highly desirable. However, antibiotic treatment disturbs the balance of physiological microbiota by changing its qualitative and/or quantitative composition, resulting in a number of adverse effects that include secondary infections. Although such dysbiosis may be reversed by the treatment with probiotics, a more attractive alternative is the use of antibiotics that target only pathogens, while sparing the commensals. Here, we describe lysostaphin LSp222, an enzyme produced naturally by Staphylococcus pseudintermedius 222. LSp222 is highly effective against S. aureus, including its multi-drug resistant strains. Importantly, the inhibitory concentration for S. epidermidis, the predominant commensal in healthy human skin, is at least two orders of magnitude higher compared to S. aureus. Such significant therapeutic window makes LSp222 a microbiota-friendly antibacterial agent with a potential application in the treatment of S. aureus-driven skin infections.

Cyclic Peptide Inhibitors of the Tsg101 UEV Protein Interactions Refined through Global Docking and Gaussian Accelerated Molecular Dynamics Simulations

Tsg101 UEV domain proteins are potential targets for virus infection therapy, especially for HIV and Ebola viruses. Peptides are key in curbing virus transmission, and cyclic peptides have a greater survival time than their linear peptides. To date, the accurate prediction of cyclic peptide-protein receptors binding conformations still is challenging because of high peptide flexibility. Here, a useful approach combined the global peptide docking, Gaussian accelerated molecular dynamics (GaMD), two-dimensional (2D) potential of mean force (PMF), normal molecular dynamics (cMD), and solvated interaction energy (SIE) techniques. Then we used this approach to investigate the binding conformations of UEV domain proteins with three cyclic peptides inhibitors. We reported the possible cyclic peptide-UEV domain protein binding conformations via 2D PMF free energy profiles and SIE free energy calculations. The residues Trp145, Tyr147, and Trp148 of the native cyclic peptide (CP1) indeed play essential roles in the cyclic peptides-UEV domain proteins interactions. Our findings might increase the accuracy of cyclic peptide-protein conformational prediction, which may facilitate cyclic peptide inhibitor design. Our approach is expected to further aid in addressing the challenges in cyclic peptide inhibitor design.

Psoriasis and Antimicrobial Peptides

Psoriasis is a systemic inflammatory disease caused by crosstalk between various cells such as T cells, neutrophils, dendritic cells, and keratinocytes. Antimicrobial peptides (AMPs) such as β-defensin, S100, and cathelicidin are secreted from these cells and activate the innate immune system through various mechanisms to induce inflammation, thus participating in the pathogenesis of psoriasis. In particular, these antimicrobial peptides enhance the binding of damage-associated molecular patterns such as self-DNA and self-RNA to their receptors and promote the secretion of interferon from activated plasmacytoid dendritic cells and keratinocytes to promote inflammation in psoriasis. Neutrophil extracellular traps (NETs), complexes of self-DNA and proteins including LL-37 released from neutrophils in psoriatic skin, induce Th17. Activated myeloid dendritic cells secrete a mass of inflammatory cytokines such as IL-12 and IL-23 in psoriasis, which is indispensable for the proliferation and survival of T cells that produce IL-17. AMPs enhance the production of some of Th17 and Th1 cytokines and modulate receptors and cellular signaling in psoriasis. Inflammation induced by DAMPs, including self-DNA and RNA released due to microinjuries or scratches, and the enhanced recognition of DAMPs by AMPs, may be involved in the mechanism underlying the Köbner phenomenon in psoriasis.

Dolosigranulum pigrum Cooperation and Competition in Human Nasal Microbiota

Staphylococcus aureus and Streptococcus pneumoniae infections cause significant morbidity and mortality in humans. For both, nasal colonization is a risk factor for infection. Studies of nasal microbiota identify Dolosigranulum pigrum as a benign bacterium present when adults are free of S. aureus or when children are free of S. pneumoniae. Here, we validated these in vivo associations with functional assays. We found that D. pigrum inhibited S. aureusin vitro and, together with a specific nasal Corynebacterium species, also inhibited S. pneumoniae. Furthermore, genomic analysis of D. pigrum indicated that it must obtain key nutrients from other nasal bacteria or from humans. These phenotypic interactions support the idea of a role for microbe-microbe interactions in shaping the composition of human nasal microbiota and implicate D. pigrum as a mutualist of humans. These findings support the feasibility of future development of microbe-targeted interventions to reshape nasal microbiota composition to exclude S. aureus and/or S. pneumoniae.

Multiple epidemiological studies identify Dolosigranulum pigrum as a candidate beneficial bacterium based on its positive association with health, including negative associations with nasal/nasopharyngeal colonization by the pathogenic species Staphylococcus aureus and Streptococcus pneumoniae. Using a multipronged approach to gain new insights into D. pigrum function, we observed phenotypic interactions and predictions of genomic capacity that support the idea of a role for microbe-microbe interactions involving D. pigrum in shaping the composition of human nasal microbiota. We identified in vivo community-level and in vitro phenotypic cooperation by specific nasal Corynebacterium species. Also, D. pigrum inhibited S. aureus growth in vitro, whereas robust inhibition of S. pneumoniae required both D. pigrum and a nasal Corynebacterium together. D. pigruml-lactic acid production was insufficient to account for these inhibitions. Genomic analysis of 11 strains revealed that D. pigrum has a small genome (average 1.86 Mb) and multiple predicted auxotrophies consistent with D. pigrum relying on its human host and on cocolonizing bacteria for key nutrients. Further, the accessory genome of D. pigrum harbored a diverse repertoire of biosynthetic gene clusters, some of which may have a role in microbe-microbe interactions. These new insights into D. pigrum’s functions advance the field from compositional analysis to genomic and phenotypic experimentation on a potentially beneficial bacterial resident of the human upper respiratory tract and lay the foundation for future animal and clinical experiments.

IMPORTANCEStaphylococcus aureus and Streptococcus pneumoniae infections cause significant morbidity and mortality in humans. For both, nasal colonization is a risk factor for infection. Studies of nasal microbiota identify Dolosigranulum pigrum as a benign bacterium present when adults are free of S. aureus or when children are free of S. pneumoniae. Here, we validated these in vivo associations with functional assays. We found that D. pigrum inhibited S. aureusin vitro and, together with a specific nasal Corynebacterium species, also inhibited S. pneumoniae. Furthermore, genomic analysis of D. pigrum indicated that it must obtain key nutrients from other nasal bacteria or from humans. These phenotypic interactions support the idea of a role for microbe-microbe interactions in shaping the composition of human nasal microbiota and implicate D. pigrum as a mutualist of humans. These findings support the feasibility of future development of microbe-targeted interventions to reshape nasal microbiota composition to exclude S. aureus and/or S. pneumoniae.

Skin microbiota analysis-inspired development of novel anti-infectives

BACKGROUND

The alarming spread of antimicrobial resistance requires the development of novel anti-infective drugs. Despite the recent research focus on the human microbiome and its likely value to understand and exploit inter-bacterial inhibitory phenomena as a source for antimicrobial strategies, the human microbiota has barely been investigated for the purpose of drug development.

RESULTS

We performed a large screen analyzing over 3000 human skin isolates to evaluate bacterial competition within the human skin microbiota as a basis for the development of anti-infective therapeutics. We discovered a Staphylococcus hominis strain with strong and broad activity against Gram-positive pathogens that was mediated by the bacteriocin micrococcin P1 (MP1). In "probiotic" approaches, this strain led to reduced Staphylococcus aureus infection and accelerated closure of S. aureus-infected wounds. Furthermore, we used a nanoparticle strategy to overcome the physico-chemical limitations often encountered with natural substances such as MP1 and demonstrate a significant reduction of S. aureus infection by MP1-loaded nanoparticles.

CONCLUSIONS

Our study gives examples of how analysis of bacterial interactions in the human microbiota can be explored for the development of novel, effective anti-infective strategies. Video Abstract.

Computational modeling of cyclic peptide inhibitor-MDM2/MDMX binding through global docking and Gaussian accelerated molecular dynamics simulations

MDM2 and MDMX are potential targets for p53-dependent cancer therapy. Peptides are key in cellular immunology and oncology, and cyclic peptides generally have higher half-life than their linear counterparts. However, prediction of cyclic peptide-protein binding is challenging with normal molecular simulation approaches because of high peptide flexibility. Here, we used global peptide docking, normal molecular dynamics, Gaussian accelerated molecular dynamics (GaMD), two-dimensional (2D) potential of mean force (PMF) profiles, and solvated interaction energy (SIE) techniques to investigate the interactions of MDM2/MDMX with three N-to-C-terminal cyclic peptide-based inhibitors. We determined the possible cyclic peptide-MDM2/MDMX complex structures via 2D PMF profiles and SIE calculations. Our findings increase the accuracy of peptide-protein structural prediction, which may facilitate cyclic peptide drug design. Advancements in the computational methods and computing power may further aid in addressing the challenges in cyclic peptide drug design. Communicated by Ramaswamy H. Sarma.

Synthetic strategies to access staphylococcus auto-inducing peptides as quorum sensing modulators

The accessory gene regulator (agr) quorum-sensing system is arguably the most important regulator of staphylococcus virulence and has been the focus of tremendous interest in the development of effective therapies for pathogenic bacterial infections. With regards to chemotherapeutic based strategies, the significant proportion of currently reported agr-system modulating molecules are mimics of the native ArgC substrate, which is a thioester-based macrocyclic peptide know as the auto-inducing peptide. Over the past two decades, more than two-hundred synthetic analogues have been reported. This review traces the development of the synthetic strategies employed to synthesise these analogues with a particular focus on macrocyclisation. At present these synthetic approaches can be clustered into five broad categories (1) solution-phase cyclisation, (2) immobilised carbodiimide assisted cyclisation, (3) concomitant on-resin cleavage and macrocyclisation, (4) Boc-compatible chemoselective thioesterification, and (5) Fmoc-compatible chemoselective thioesterification. The advantages and limitation provided by each of the approaches are compared and contrasted with a view towards potential reaction scale-up.

Staphylococci in the human microbiome: the role of host and interbacterial interactions

Staphylococci are common commensals on human epithelial surfaces. Some species, most notably Staphylococcus aureus, have considerable pathogenic potential and can cause severe and sometimes fatal infections. Despite the long-known fact that staphylococcal infection arises from colonizing isolates, research on staphylococcal colonization has been limited, in particular regarding interactions with the colonizing microbiota. However, several recent studies are beginning to decipher such interactions, which range from bacteriocin-based or signaling interference-mediated inhibitory interactions to cooperation with host defenses to outcompete co-colonizers. This review will give an outline of recent research on the mechanistic underpinnings of staphylococcal interference with other members of the colonizing microbiota, some of which suggest new avenues for the development of novel anti-infectives or decolonization strategies.

Biotherapeutic Approaches in Atopic Dermatitis

The skin microbiome plays a central role in inflammatory skin disorders such as atopic dermatitis (AD). In AD patients, an imbalance between pathogenic Staphylococcus aureus (S. aureus) and resident skin symbionts creates a state of dysbiosis which induces immune dysregulation and impairs skin barrier function. There are now exciting new prospects for microbiome-based interventions for AD prevention. In the hopes of achieving sustained control and management of disease in AD patients, current emerging biotherapeutic strategies aim to harness the skin microbiome associated with health by restoring a more diverse symbiotic skin microbiome, while selectively removing pathogenic S. aureus. Examples of such strategies are demonstrated in skin microbiome transplants, phage-derived anti-S. aureus endolysins, monoclonal antibodies, and quorum sensing (QS) inhibitors. However, further understanding of the skin microbiome and its role in AD pathogenesis is still needed to understand how these biotherapeutics alter the dynamics of the microbiome community; to optimize patient selection, drug delivery, and treatment duration; overcome rapid recolonization upon treatment cessation; and improve efficacy to allow these therapeutic options to eventually reach routine clinical practice.

Novel Research Models for Staphylococcus aureus Small Colony Variants (SCV) Development: Co-pathogenesis and Growth Rate

Staphylococcus aureus remains a great burden on the healthcare system. Despite prescribed treatments often seemingly to be successful, S. aureus can survive and cause a relapsing infection which cannot be cleared. These infections are in part due to quasi-dormant sub-population which is tolerant to antibiotics and able to evade the host immune response. These include Small Colony Variants (SCVs). Because SCVs readily revert to non-SCV cell types under laboratory conditions, the characterization of SCVs has been problematic. This mini-review covers the phenotypic and genetic changes in stable SCVs including the selection of SCVs by and interactions with other bacterial species.

Structure-Activity Relationship Studies of Small Molecule Modulators of the Staphylococcal Accessory Gene Regulator

The accessory gene regulator (agr) quorum-sensing system is arguably the most important regulator of Staphylococcus virulence. The agr-system serves a crucial role in pathogenesis by triggering substantive gene expression alterations to up-regulate the production of a wide variety of virulence determinants such as exoenzymes (proteases, lipases, nucleases) and downregulate the expression of surface binding proteins. Accordingly, the agr-system represents a compelling target for the development of antivirulence therapeutics as potential adjuncts, or alternatives, to conventional bactericidal and bacteriostatic antibiotics. Despite this potential, to date, no agr-system inhibitors have progressed to the clinic; however, several promising lead compounds have been identified through screens of synthetic and natural product libraries. On the basis of the molecular components within the agr-system, the current contingent of regulating compounds can be clustered into three broad groups, AgrA-P3 activation inhibitors, AgrB-AgrD processing inhibitors, and AgrC-AIP interaction inhibitors. This review aims to provide an overview of the development, structure-activity-relationships, and limitations of compounds within each of these groups in addition to the current opportunities for developing next-generation anologs.

Effect of Co-inhabiting Coagulase Negative Staphylococci on S. aureus agr Quorum Sensing, Host Factor Binding, and Biofilm Formation

Staphylococcus aureus is a commensal colonizer of both humans and animals, but also an opportunistic pathogen responsible for a multitude of diseases. In recent years, colonization of pigs by methicillin resistant S. aureus has become a problem with increasing numbers of humans being infected by livestock strains. In S. aureus colonization and virulence factor expression is controlled by the agr quorum sensing system, which responds to and is activated by self-generated, autoinducing peptides (AIPs). AIPs are also produced by coagulase negative staphylococci (CoNS) commonly found as commensals in both humans and animals, and interestingly, some of these inhibit S. aureus agr activity. Here, we have addressed if cross-communication occurs between S. aureus and CoNS strains isolated from pig nares, and if so, how properties such as host factor binding and biofilm formation are affected. From 25 pig nasal swabs we obtained 54 staphylococcal CoNS isolates belonging to 8 different species. Of these, none were able to induce S. aureus agr as monitored by reporter gene fusions to agr regulated genes but a number of agr-inhibiting species were identified including Staphylococcus hyicus, Staphylococcus simulans, Staphylococcus arlettae, Staphylococcus lentus, and Staphylococcus chromogenes. After establishing that the inhibitory activity was mediated via AgrC, the receptor of AIPs, we synthesized selective AIPs to explore their effect on adhesion of S. aureus to fibronectin, a host factor involved in S. aureus colonization. Here, we found that the CoNS AIPs did not affect adhesion of S. aureus except for strain 8325-4. When individual CoNS strains were co-cultured together with S. aureus we observed variable degrees of biofilm formation which did not correlate with agr interactions. Our results show that multiple CoNS species can be isolated from pig nares and that the majority of these produce AIPs that inhibit S. aureus agr. Further they show that the consequences of the interactions between CoNS and S. aureus are complex and highly strain dependent.

Transcriptomic profiling of interacting nasal staphylococci species reveals global changes in gene and non-coding RNA expression

Interspecies interactions between bacterial pathogens and the commensal microbiota can influence disease outcome. In the nasal cavities, Staphylococcus epidermidis has been shown to be a determining factor for Staphylococcus aureus colonization and biofilm formation. However, the interaction between S. epidermidis and S. aureus has mainly been described by phenotypic analysis, and little is known about how this interaction modulates gene expression. This study aimed to determine the interactome of nasal S. aureus and S. epidermidis isolates to understand the molecular effect of interaction. After whole-genome sequencing of two nasal staphylococcal isolates, an agar-based RNA sequencing setup was utilized to identify interaction-induced transcriptional alterations in surface-associated populations. Our results revealed differential expression of several virulence genes in both species. We also identified putative non-coding RNAs (ncRNAs) and, interestingly, detected a putative ncRNA transcribed antisense to esp, the serine protease of S. epidermidis, that has previously been shown to inhibit nasal colonization of S. aureus. In our study, the gene encoding Esp and the antisense ncRNA are both downregulated during interaction with S. aureus. Our findings contribute to a better understanding of pathogen physiology in the context of interactions with the commensal microbiota, and may provide targets for future therapeutics.

Identification of autoinducing thiodepsipeptides from staphylococci enabled by native chemical ligation

Staphylococci secrete autoinducing peptides (AIPs) as signalling molecules to regulate population-wide behaviour. AIPs from non-Staphylococcus aureus staphylococci have received attention as potential antivirulence agents to inhibit quorum sensing and virulence gene expression in the human pathogen Staphylococcus aureus. However, only a limited number of AIP structures from non-S. aureus staphylococci have been identified to date, as the minute amounts secreted in complex media render it difficult. Here, we report a method for the identification of AIPs by exploiting their thiolactone functionality for chemoselective trapping and enrichment of the compounds from the bacterial supernatant. Standard liquid chromatography mass spectrometry analysis, guided by genome sequencing data, then readily provides the AIP identities. Using this approach, we confirm the identity of five known AIPs and identify the AIPs of eleven non-S. aureus species, and we expect that the method should be extendable to AIP-expressing Gram-positive bacteria beyond the Staphylococcus genus.

Microbiome Imbalances: An Overlooked Potential Mechanism in Chronic Nonhealing Wounds

Chronic nonhealing wounds are a severe burden to health care systems worldwide, causing millions of patients to have lengthy hospital stays, high health care costs, periods of unemployment, and reduced quality of life. Moreover, treating chronic nonhealing wounds effectively and reasonably in countries with limited medical resources can be extremely challenging. With many outstanding questions surrounding chronic nonhealing wounds, in this review, we offer changes to the microbiome as a potentially ignored mechanism important in the formation and treatment of chronic wounds. Our analysis helps bring a whole new understanding to wound formation and healing and provides a potential breakthrough in the treatment of chronic nonhealing wounds in the future.

Commensal Staphylococci Influence Staphylococcus aureus Skin Colonization and Disease

Commensal organisms that constitute the skin microbiota play a pivotal role in the orchestration of cutaneous homeostasis and immune competence. This balance can be promptly offset by the expansion of the opportunistic pathogen Staphylococcus aureus, which is responsible for the majority of bacterial skin infections. S. aureus carriage is also known to be a precondition for its transmission and pathogenesis. Recent reports suggest that skin-dwelling coagulase-negative staphylococci (CoNS) can prime the skin immune system to limit the colonization potential of invaders, and they can directly compete through production of antimicrobial molecules or through signaling antagonism. We review recent advances in these CoNS colonization resistance mechanisms, which may serve to aid development of pharmacologic and probiotic intervention strategies to limit S. aureus skin colonization and disease.

Treating Polymicrobial Infections in Chronic Diabetic Wounds

This review provides a comprehensive summary of issues associated with treating polyclonal bacterial biofilms in chronic diabetic wounds. We use this as a foundation and discuss the alternatives to conventional antibiotics and the emerging need for suitable drug delivery systems. In recent years, extraordinary advances have been made in the field of nanoparticle synthesis and packaging. However, these systems have not been incorporated into the clinic for treatments other than for cancer or severe genetic diseases. We present a unifying perspective on how the field is evolving and the need for an early amalgamation of engineering principles and a biological understanding of underlying phenomena in order to develop a therapy that is translatable to the clinic in a shorter time.

Cyclic Peptides that Govern Signal Transduction Pathways: From Prokaryotes to Multi-Cellular Organisms

Cyclic peptide scaffolds are key components of signal transduction pathways in both prokaryotic and eukaryotic organisms since they act as chemical messengers that activate or inhibit specific cognate receptors. In prokaryotic organisms these peptides are utilized in non-essential pathways, such as quorum sensing, that are responsible for virulence and pathogenicity. In the more evolved eukaryotic systems, cyclic peptide hormones play a key role in the regulation of the overall function of multicellular organisms, mainly through the endocrine system. This review will highlight several prokaryote and eukaryote systems that use cyclic peptides as their primary signals and the potential associated with utilizing these scaffolds for the discovery of novel therapeutics for a wide range of diseases and illnesses.

Phylogenomic Based Comparative Studies on Indian and American Commensal Staphylococcus epidermidis Isolates

Staphylococcus epidermidis is a prominent commensal member of human skin microbiome and an emerging nosocomial pathogen, making it a good model organism to provide genomic insights, correlating its transition between commensalism and pathogenicity. While there are numerous studies to understand differences in commensal and pathogenic isolates, systematic efforts to understand variation and evolutionary pattern in multiple strains isolated from healthy individuals are lacking. In the present study, using whole genome sequencing and analysis, we report presence of diverse lineages of S. epidermidis isolates in healthy individuals from two geographically diverse locations of India and North America. Further, there is distinct pattern in the distribution of candidate gene(s) for pathogenicity and commensalism. The pattern is not only reflected in lineages but is also based on geographic origin of the isolates. This is evident by the fact that North American isolates under this study are more genomically dynamic and harbor pathogenicity markers in higher frequency. On the other hand, isolates of Indian origin are less genomically dynamic, harbor less pathogenicity marker genes and possess two unique antimicrobial peptide gene clusters. This study provides a basis to understand the nature of selection pressure in a key human skin commensal bacterium with implications in its management as an opportunistic pathogen.

Methicillin-resistant Staphylococcus aureus causes sustained collecting lymphatic vessel dysfunction

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of morbidity and mortality worldwide and is a frequent cause of skin and soft tissue infections (SSTIs). Lymphedema-fluid accumulation in tissue caused by impaired lymphatic vessel function-is a strong risk factor for SSTIs. SSTIs also frequently recur in patients and sometimes lead to acquired lymphedema. However, the mechanism of how SSTIs can be both the consequence and the cause of lymphatic vessel dysfunction is not known. Intravital imaging in mice revealed an acute reduction in both lymphatic vessel contractility and lymph flow after localized MRSA infection. Moreover, chronic lymphatic impairment is observed long after MRSA is cleared and inflammation is resolved. Associated with decreased collecting lymphatic vessel function was the loss and disorganization of lymphatic muscle cells (LMCs), which are critical for lymphatic contraction. In vitro, incubation with MRSA-conditioned supernatant led to LMC death. Proteomic analysis identified several accessory gene regulator (agr)-controlled MRSA exotoxins that contribute to LMC death. Infection with agr mutant MRSA resulted in sustained lymphatic function compared to animals infected with wild-type MRSA. Our findings suggest that agr is a promising target to preserve lymphatic vessel function and promote immunity during SSTIs.

Coagulase-Negative Staphylococcal Strain Prevents Staphylococcus aureus Colonization and Skin Infection by Blocking Quorum Sensing

Coagulase-negative staphylococci (CoNS) and Staphylococcus aureus are part of the natural flora of humans and other mammals. We found that spent media from the CoNS species Staphylococcus caprae can inhibit agr-mediated quorum sensing by all classes of S. aureus. A biochemical assessment of the inhibitory activity suggested that the S. caprae autoinducing peptide (AIP) was responsible, and mass spectrometric analysis identified the S. caprae AIP as an eight-residue peptide (YSTCSYYF). Using a murine model of intradermal MRSA infection, the therapeutic efficacy of synthetic S. caprae AIP was evident by a dramatic reduction in both dermonecrotic injury and cutaneous bacterial burden relative to controls. Competition experiments between S. caprae and MRSA demonstrated a significant reduction in MRSA burden using murine models of both skin colonization and intradermal infection. Our findings indicate that important interactions occur between commensals that can impact disease outcomes and potentially shape the composition of the natural flora.

Skin Colonization by Staphylococcus aureus Precedes the Clinical Diagnosis of Atopic Dermatitis in Infancy

Atopic dermatitis (AD) has a well-established association with skin colonization or infection by Staphylococcus aureus, which can exacerbate the disease. However, a causal relationship between specific changes in skin colonization during the first years of life and AD development still remains unclear. In this prospective birth cohort study, we aimed to characterize the association between skin colonization and AD development in 149 white infants with or without a family history of atopy. We assessed infants clinically and collected axillary and antecubital fossa skin swabs for culture-based analysis at birth and at seven time points over the first 2 years of life. We found that at age 3 months, S. aureus was more prevalent on the skin of infants who developed AD later on. S. aureus prevalence was increased on infants' skin at the time of AD onset and also 2 months before it, when compared with age-matched, unaffected infants. Furthermore, at AD onset, infants testing positive for S. aureus were younger than uncolonized subjects. In conclusion, our results suggest that specific changes in early-life skin colonization may actively contribute to clinical AD onset in infancy.

Signal Biosynthesis Inhibition with Ambuic Acid as a Strategy To Target Antibiotic-Resistant Infections

There has been major interest by the scientific community in antivirulence approaches against bacterial infections. However, partly due to a lack of viable lead compounds, antivirulence therapeutics have yet to reach the clinic. Here we investigate the development of an antivirulence lead targeting quorum sensing signal biosynthesis, a process that is conserved in Gram-positive bacterial pathogens. Some preliminary studies suggest that the small molecule ambuic acid is a signal biosynthesis inhibitor. To confirm this, we constructed a methicillin-resistant Staphylococcus aureus (MRSA) strain that decouples autoinducing peptide (AIP) production from regulation and demonstrate that AIP production is inhibited in this mutant. Quantitative mass spectrometric measurements show that ambuic acid inhibits signal biosynthesis (50% inhibitory concentration [IC₅₀] of 2.5 ± 0.1 μM) against a clinically relevant USA300 MRSA strain. Quantitative real-time PCR confirms that this compound selectively targets the quorum sensing regulon. We show that a 5-μg dose of ambuic acid reduces MRSA-induced abscess formation in a mouse model and verify its quorum sensing inhibitory activity in vivo Finally, we employed mass spectrometry to identify or confirm the structure of quorum sensing signaling peptides in three strains each of S. aureus and Staphylococcus epidermidis and single strains of Enterococcus faecalis, Listeria monocytogenes, Staphylococcus saprophyticus, and Staphylococcus lugdunensis By measuring AIP production by these strains, we show that ambuic acid possesses broad-spectrum efficacy against multiple Gram-positive bacterial pathogens but does not inhibit quorum sensing in some commensal bacteria. Collectively, these findings demonstrate the promise of ambuic acid as a lead for the development of antivirulence therapeutics.

Novel bioactive from Lactobacillus brevis DSM17250 to stimulate the growth of Staphylococcus epidermidis: a pilot study

Commensal skin microbiota plays an important role in both influencing the immune response of the skin and acting as a barrier against colonisation of potentially pathogenic microorganisms and overgrowth of opportunistic pathogens. Staphylococcus epidermidis is a key constituent of the normal microbiota on human skin. It balances the inflammatory response after skin injury and produces antimicrobial molecules that selectively inhibit skin pathogens. Here we describe Lactobacillus brevis DSM17250 that was identified among hundreds of Lactobacillus strains to exhibit an anti-inflammatory effect in human keratinocytes in vitro and specific stimulatory impact on the growth of S. epidermidis. The aqueous cell-free extract of L. brevis DSM17250 was used in an ointment formulation and tested in a randomized placebo-controlled double blinded human pilot study. Healthy volunteers with diagnosed dry skin were treated for four weeks. The study data shows that L. brevis DSM17250 extract induces re-colonisation of the skin by protective commensal microorganisms as judged from selective bacterial cultivation of surface-associated skin microorganism of the lower leg. Furthermore, the 4 week administration of the L. brevis DSM17250 extract significantly improved the transepidermal water loss value (TEWL), reduced the xerosis cutis symptoms and stinging. The data shows that daily application of L. brevis DSM17250 extract in a topical product significantly improves the microbial skin microbiota by promoting the growth of species which possess beneficial regulatory and protective properties such as S. epidermidis. Restoring the natural skin microbiota leads to significantly improved skin barrier function (as transepidermal water loss) and decrease of xeroderma (xerosis cutis) symptoms (as measured by dry skin area and severity index, DASI). We propose that improving and stabilizing the natural skin microbiota by specifically stimulating the growth of S. epidermidis is an important and novel concept to manage skin diseases associated with microbiota dysbiosis.

Skin Biomes

The cutaneous microbiome has been investigated broadly in recent years and some traditional perspectives are beginning to change. A diverse microbiome exists on human skin and has a potential to influence pathogenic microbes and modulate the course of skin disorders, e.g. atopic dermatitis. In addition to the known dysfunctions in barrier function of the skin and immunologic disturbances, evidence is rising that frequent skin disorders, e.g. atopic dermatitis, might be connected to a dysbiosis of the microbial community and changes in the skin microbiome. As a future perspective, examining the skin microbiome could be seen as a potential new diagnostic and therapeutic target in inflammatory skin disorders.

Bactericidal, quorum quenching and anti-biofilm nanofactories: a new niche for nanotechnologists

Despite several conventional potent antibacterial therapies, bacterial infections pose a significant threat to human health because they are emerging as the leading cause of death worldwide. Due to the development of antibiotic resistance in bacteria, there is a pressing demand to discover novel approaches for developing more effective therapies to treat multidrug-resistant bacterial strains and biofilm-associated infections. Therefore, attention has been especially devoted to a new and emerging branch of science "nanotechnology" to design non-conventional antimicrobial chemotherapies. A range of nanomaterials and nano-sized carriers for conventional antimicrobial agents have fully justified their potential to combat bacterial diseases by reducing cell viability, by attenuating quorum sensing, and by inhibiting/or eradicating biofilms. This communication summarizes emerging nano-antimicrobial therapies in treating bacterial infections, particularly using antibacterial, quorum quenching, and anti-biofilm nanomaterials as new approaches to tackle the current challenges in combating infectious diseases.

Structure-Function Analyses of a Staphylococcus epidermidis Autoinducing Peptide Reveals Motifs Critical for AgrC-type Receptor Modulation

Staphylococcus epidermidis is frequently implicated in human infections associated with indwelling medical devices due to its ubiquity in the skin flora and formation of robust biofilms. The accessory gene regulator (agr) quorum sensing (QS) system plays a prominent role in the establishment of biofilms and infection by this bacterium. Agr activation is mediated by the binding of a peptide signal (or autoinducing peptide, AIP) to its cognate AgrC receptor. Many questions remain about the role of QS in S. epidermidis infections, as well as in mixed-microbial populations on a host, and chemical modulators of its agr system could provide novel insights into this signaling network. The AIP ligand provides an initial scaffold for the development of such probes; however, the structure-activity relationships (SARs) for activation of S. epidermidis AgrC receptors by AIPs are largely unknown. Herein, we report the first SAR analyses of an S. epidermidis AIP by performing systematic alanine and d-amino acid scans of the S. epidermidis AIP-I. On the basis of these results, we designed and identified potent, pan-group inhibitors of the AgrC receptors in the three S. epidermidis agr groups, as well as a set of AIP-I analogs capable of selective AgrC inhibition in either specific S. epidermidis agr groups or in another common staphylococcal species, S. aureus. In addition, we uncovered a non-native peptide agonist of AgrC-I that can strongly inhibit S. epidermidis biofilm growth. Together, these synthetic analogs represent new and readily accessible probes for investigating the roles of QS in S. epidermidis colonization and infections.

Specificity and complexity in bacterial quorum-sensing systems

Quorum sensing (QS) is a microbial cell-to-cell communication process that relies on the production and detection of chemical signals called autoinducers (AIs) to monitor cell density and species complexity in the population. QS allows bacteria to behave as a cohesive group and coordinate collective behaviors. While most QS receptors display high specificity to their AI ligands, others are quite promiscuous in signal detection. How do specific QS receptors respond to their cognate signals with high fidelity? Why do some receptors maintain low signal recognition specificity? In addition, many QS systems are composed of multiple intersecting signaling pathways: what are the benefits of preserving such a complex signaling network when a simple linear ‘one-to-one’ regulatory pathway seems sufficient to monitor cell density? Here, we will discuss different molecular mechanisms employed by various QS systems that ensure productive and specific QS responses. Moreover, the network architectures of some well-characterized QS circuits will be reviewed to understand how the wiring of different regulatory components achieves different biological goals.

This review focuses on the specificity and complexity of quorum-sensing circuits in both Gram-negative and Gram-positive bacterial species.

Quorum Quenching Strategy Targeting Gram-Positive Pathogenic Bacteria

Quorum sensing (QS) is a cell density-dependent regulatory system that orchestrates the group behavior of unicellular organisms by synchronizing the expression of certain gene(s) within the clonal community of same species. Bacterial pathogens often employ QS system to establish efficiently an infection. A large part of low GC Gram-positive bacteria belonging to phylum Firmicutes use thiolactone/lactone peptides as communication signals so-called autoinducing peptides (AIPs) to coordinate QS circuit. In particular, QS of staphylococci, enterococci, and clostridia have been intensively studied in terms of alternative target of anti-pathogenic chemotherapy independent of bactericidal antibiotics. Thus far, a number of quorum quenching (QQ) agents that targeting the QS circuit of these Gram-positive pathogens have been developed by random screening of natural compounds or rationale design of AIP antagonists. This review summarizes those QQ agents and previews their potential as post-antibiotic drugs.

Emerging concepts promising new horizons for marine biodiscovery and synthetic biology

The vast oceans of the world, which comprise a huge variety of unique ecosystems, are emerging as a rich and relatively untapped source of novel bioactive compounds with invaluable biotechnological and pharmaceutical potential. Evidence accumulated over the last decade has revealed that the diversity of marine microorganisms is enormous with many thousands of bacterial species detected that were previously unknown. Associated with this diversity is the production of diverse repertoires of bioactive compounds ranging from peptides and enzymes to more complex secondary metabolites that have significant bioactivity and thus the potential to be exploited for innovative biotechnology. Here we review the discovery and functional potential of marine bioactive peptides such as lantibiotics, nanoantibiotics and peptidomimetics, which have received particular attention in recent years in light of their broad spectrum of bioactivity. The significance of marine peptides in cell-to-cell communication and how this may be exploited in the discovery of novel bioactivity is also explored. Finally, with the recent advances in bioinformatics and synthetic biology, it is becoming clear that the integration of these disciplines with genetic and biochemical characterization of the novel marine peptides, offers the most potential in the development of the next generation of societal solutions.

Investigational therapies targeting quorum-sensing for the treatment of Staphylococcus aureus infections

INTRODUCTION

Antibiotic resistance is a serious global health concern for developed and developing nations. MRSA represents a particularly severe public health threat that is associated with high morbidity and mortality. The lack of novel antibiotics has led scientists to explore therapies targeting bacterial virulence mechanisms and virulence regulators, including those controlling cell-cell communication.

AREAS COVERED

The authors discuss the role of quorum-sensing in Staphylococcus aureus infections and components of the system that are being targeted using novel investigational drugs. In particular, the authors examine the role of the accessory gene regulator (Agr) system in virulence regulation of S. aureus pathogenesis. Finally, the authors present and compare natural and synthetic compounds that have been found to interfere with Agr functionality.

EXPERT OPINION

There is a great need to develop new therapeutic methods to combat S. aureus infections. These include anti-virulence therapies that target key global regulators involved with the establishment and propagation of infection. Several molecules have been found to interfere with S. aureus virulence regulation, especially those targeting the Agr quorum-sensing signaling molecule. These preliminary findings warrant further investigation and validation, with the goal of refining a compound that has broad-spectrum inhibitory effects on most S. aureus strains and Agr subtypes.

ω-Hydroxyemodin limits staphylococcus aureus quorum sensing-mediated pathogenesis and inflammation

Antibiotic-resistant pathogens are a global health threat. Small molecules that inhibit bacterial virulence have been suggested as alternatives or adjuncts to conventional antibiotics, as they may limit pathogenesis and increase bacterial susceptibility to host killing. Staphylococcus aureus is a major cause of invasive skin and soft tissue infections (SSTIs) in both the hospital and community settings, and it is also becoming increasingly antibiotic resistant. Quorum sensing (QS) mediated by the accessory gene regulator (agr) controls virulence factor production essential for causing SSTIs. We recently identified ω-hydroxyemodin (OHM), a polyhydroxyanthraquinone isolated from solid-phase cultures of Penicillium restrictum, as a suppressor of QS and a compound sought for the further characterization of the mechanism of action. At concentrations that are nontoxic to eukaryotic cells and subinhibitory to bacterial growth, OHM prevented agr signaling by all four S. aureus agr alleles. OHM inhibited QS by direct binding to AgrA, the response regulator encoded by the agr operon, preventing the interaction of AgrA with the agr P2 promoter. Importantly, OHM was efficacious in a mouse model of S. aureus SSTI. Decreased dermonecrosis with OHM treatment was associated with enhanced bacterial clearance and reductions in inflammatory cytokine transcription and expression at the site of infection. Furthermore, OHM treatment enhanced the immune cell killing of S. aureus in vitro in an agr-dependent manner. These data suggest that bacterial disarmament through the suppression of S. aureus QS may bolster the host innate immune response and limit inflammation.

Defensive symbioses of animals with prokaryotic and eukaryotic microorganisms

Many organisms team up with symbiotic microbes for defense against predators, parasites, parasitoids, or pathogens. Here we review the known defensive symbioses in animals and the microbial secondary metabolites responsible for providing protection to the host.

Bacterial skin commensals and their role as host guardians

Recent years' investigations of the co-evolution and functional integration of the human body and its commensal microbiota have disclosed that the microbiome has a major impact on physiological functions including protection against infections, reaction patterns in the immune system, and disposition for inflammation-mediated diseases. Two ubiquitous members of the skin microbiota, the Gram-positive bacteria Staphylococcus epidermidis and Propionibacterium acnes, are predominant on human epithelia and in sebaceous follicles, respectively. Their successful colonisation is a result of a commensal or even mutualistic lifestyle, favouring traits conferring persistency over aggressive host-damaging properties. Some bacterial properties suggest an alliance with the host to keep transient, potential pathogens at bay, such as the ability of S. epidermidis to produce antimicrobials, or the production of short-chain fatty acids by P. acnes. These features can function together with host-derived components of the innate host defence to establish and maintain the composition of a health-associated skin microbiota. However, depending largely on the host status, the relationship between the human host and S. epidermidis/P. acnes can also have parasitic features. Both microorganisms are frequently isolated from opportunistic infections. S. epidermidis is a causative agent of hospital-acquired infections, mostly associated with the use of medical devices. P. acnes is suspected to be of major importance in the pathogenesis of acne and also in a number of other opportunistic infections. In this review we will present bacterial factors and traits of these two key members of our skin microbiota and discuss how they contribute to mutualistic and parasitic properties. The elucidation of their roles in health-promoting or disease-causing processes could lead to new prophylactic and therapeutic strategies against skin disorders and other S. epidermidis/P. acnes-associated diseases, and increase our understanding of the delicate interplay of the skin microbiota with the human host.

Probiotics for human lactational mastitis

The use of culture-dependent and -independent techniques to study the human milk microbiota and microbiome has revealed a complex ecosystem with a much greater diversity than previously anticipated. The potential role of the milk microbiome appears to have implications not only for short- and long-term infant health but also for mammary health. In fact, mammary disbiosis, which may be triggered by a variety of host, microbial and medical factors, often leads to acute, subacute or subclinical mastitis, a condition that represents the first medical cause for undesired weaning. Multiresistance to antibiotics, together with formation of biofilms and mechanisms for evasion of the host immune response, is a common feature among the bacterial agents involved. This explains why this condition uses to be elusive to antibiotic therapy and why the development of new strategies for mastitis management based on probiotics is particularly appealing. In fact, selected lactobacilli strains isolated from breast milk have already shown a high efficacy for treatment.

The origin of human milk bacteria: is there a bacterial entero-mammary pathway during late pregnancy and lactation?

Human milk is a source of bacteria to the infant gut; however, the origin of milk bacteria, as well as their impact on neonatal gut microbiota establishment, remains largely unknown. In the past years, results provided by different research groups suggest that certain bacteria from the maternal gastrointestinal tract could translocate through a mechanism involving mononuclear immune cells, migrate to the mammary glands via an endogenous cellular route (the bacterial entero-mammary pathway), and subsequently colonize the gastrointestinal tract of the breast-fed neonate. If such findings are confirmed in the future, we could exert a positive influence on infant health by modulating the maternal gut microbiota.