Enterococcus faecalis Antagonizes Pseudomonas aeruginosa Growth in Mixed-Species Interactions

Pseudomonas aeruginosa alkyl quinolone response is dampened by Enterococcus faecalis

The bacterium Pseudomonas aeruginosa is an opportunistic pathogen that can cause lung, skin, wound, joint, urinary tract, and eye infections. While P. aeruginosa is known to exhibit a robust competitive response toward other bacterial species, this bacterium is frequently identified in polymicrobial infections where multiple species survive. For example, in prosthetic joint infections, P. aeruginosa can be identified along with other pathogenic bacteria including Staphylococcus aureus, Enterococcus faecalis, and Corynebacterium striatum. Here, we have explored the survival and behavior of such microbes and find that E. faecalis readily survives culturing with P. aeruginosa while other tested species do not. In each of the tested conditions, E. faecalis growth remained unchanged by the presence of P. aeruginosa, indicating a unique mutualistic interaction between the two species. We find that E. faecalis proximity leads P. aeruginosa to attenuate competitive behaviors as exemplified by reduced production of Pseudomonas quinolone signal and pyocyanin. Reduced alkyl quinolones are important to E. faecalis as these will grow in supernatant from a quinolone mutant but not P. aeruginosa wild-type in planktonic culture. The reduced pyocyanin production of P. aeruginosa is attributable to production of ornithine by E. faecalis, which we recapitulate by adding exogenous ornithine to P. aeruginosa monocultures. Similarly, co-culture with an ornithine-deficient strain of E. faecalis leads P. aeruginosa to yield near monoculture amounts of pyocyanin. Here, we directly demonstrate how notorious pathogens such as P. aeruginosa might persist in polymicrobial infections under the influence of metabolites produced by other bacterial species.

IMPORTANCE

While we now appreciate that many infections are polymicrobial, we understand little of the specific actions between a given set of microbes to enable combinatorial survival and pathogenesis. The bacteria Pseudomonas aeruginosa and Enterococcus faecalis are both prevalent pathogens in wound, urinary tract, and bacteremic infections. While P. aeruginosa often kills other species in standard laboratory culture conditions, we present here that E. faecalis can be reliably co-cultured with P. aeruginosa. We specifically detail that ornithine produced by E. faecalis reduces the Pseudomonas quinolone signal response of P. aeruginosa. This reduction of the Pseudomonas quinolone signal response aids E. faecalis growth.

Insights into ecology, pathogenesis, and biofilm formation of Enterococcus faecalis from functional genomics

SUMMARYEnterococcus faecalis is a significant resident of the gastrointestinal tract of most animals, including humans. Although generally non-pathogenic in healthy hosts, this microbe is adept at the exploitation of compromises in host immune functions, resulting in life-threatening opportunistic infections whose treatments are complicated by a high degree of intrinsic and acquired resistance to antimicrobial chemotherapy. Historically, progress in enterococcal research was limited by a lack of experimental models that replicate natural infection pathways and the relevance of in vitro studies to the natural biology of the organism. In this review, we summarize the history of enterococcal research during the 20th and early 21st centuries and describe more recent genetic and genomic tools and screens developed to address challenges in the field. We also describe how the results of recent studies reveal the importance of previously uncharacterized enterococcal genes, and we provide examples of interesting determinants that have emerged as important contributors to enterococcal biology. These factors may also serve as targets for future vaccines and chemotherapeutic agents to combat life-threatening hospital infections.

Pseudomonas aeruginosa Alkyl Quinolone Response is dampened by Enterococcus faecalis

The bacterium Pseudomonas aeruginosa is an opportunistic pathogen that can cause lung, skin, wound, joint, urinary tract, and eye infections. While P. aeruginosa is known to exhibit a robust competitive response towards other bacterial species, this bacterium is frequently identified in polymicrobial infections where multiple species survive. For example, in prosthetic joint infections (PJIs), P. aeruginosa can be identified along with other pathogenic bacteria including Staphylococcus aureus, Enterococcus faecalis, and Corynebacterium striatum. Here we have explored the survival and behavior of such microbes and find that E. faecalis readily survives culturing with P. aeruginosa while other tested species do not. In each of the tested conditions, E. faecalis growth remained unchanged by the presence of P. aeruginosa, indicating a unique mutualistic interaction between the two species. We find that E. faecalis proximity leads P. aeruginosa to attenuate competitive behaviors as exemplified by reduced production of Pseudomonas quinolone signal (PQS) and pyocyanin. Reduced alkyl quinolones is important to E. faecalis as it will grow in supernatant from a quinolone mutant but not P. aeruginosa wildtype in planktonic culture. The reduced pyocyanin production of P. aeruginosa is attributable to production of ornithine by E. faecalis, which we recapitulate by adding exogenous ornithine to P. aeruginosa monocultures. Similarly, co-culture with an ornithine-deficient strain of E. faecalis leads P. aeruginosa to yield near mono-culture amounts of pyocyanin. Here, we directly demonstrate how notorious pathogens such as P. aeruginosa might persist in polymicrobial infections under the influence of metabolites produced by other bacterial species.

The role of exopolysaccharides Psl and Pel in resistance of Pseudomonas aeruginosa to the oxidative stressors sodium hypochlorite and hydrogen peroxide

Pseudomonas aeruginosa is well-known for its antimicrobial resistance and the ability to survive in harsh environmental conditions due to an abundance of resistance mechanisms, including the formation of biofilms and the production of exopolysaccharides. Exopolysaccharides are among the major components of the extracellular matrix in biofilms and aggregates of P. aeruginosa. Although their contribution to antibiotic resistance has been previously shown, their roles in resistance to oxidative stressors remain largely elusive. Here, we studied the function of the exopolysaccharides Psl and Pel in the resistance of P. aeruginosa to the commonly used disinfectants and strong oxidizing agents NaOCl and H2O2. We observed that the simultaneous inactivation of Psl and Pel in P. aeruginosa PAO1 mutant strain ∆pslA pelF resulted in a significant increase in susceptibility to both NaOCl and H2O2. Further analyses revealed that Pel is more important for oxidative stress resistance in P. aeruginosa and that the form of Pel (i.e., cell-associated or cell-free) did not affect NaOCl susceptibility. Additionally, we show that Psl/Pel-negative strains are protected against oxidative stress in co-culture biofilms with P. aeruginosa PAO1 WT. Taken together, our results demonstrate that the EPS matrix and, more specifically, Pel exhibit protective functions against oxidative stressors such as NaOCl and H2O2 in P. aeruginosa.

IMPORTANCE

Biofilms are microbial communities of cells embedded in a self-produced polymeric matrix composed of polysaccharides, proteins, lipids, and extracellular DNA. Biofilm bacteria have been shown to possess unique characteristics, including increased stress resistance and higher antimicrobial tolerance, leading to failures in bacterial eradication during chronic infections or in technical settings, including drinking and wastewater industries. Previous studies have shown that in addition to conferring structure and stability to biofilms, the polysaccharides Psl and Pel are also involved in antibiotic resistance. This work provides evidence that these biofilm matrix components also contribute to the resistance of Pseudomonas aeruginosa to oxidative stressors including the widely used disinfectant NaOCl. Understanding the mechanisms by which bacteria escape antimicrobial agents, including strong oxidants, is urgently needed in the fight against antimicrobial resistance and will help in developing new strategies to eliminate resistant strains in any environmental, industrial, and clinical setting.

Arginine impacts aggregation, biofilm formation, and antibiotic susceptibility in Enterococcus faecalis

Enterococcus faecalis is a commensal bacterium in the gastrointestinal (GI) tract of humans and other organisms. E. faecalis also causes infections in root canals, wounds, the urinary tract, and on heart valves. E. faecalis metabolizes arginine through the arginine deiminase pathway, which converts arginine to ornithine and releases ATP, ammonia, and CO2. E. faecalis arginine metabolism also affects virulence of other pathogens during co-culture. E. faecalis may encounter elevated levels of arginine in the GI tract or the oral cavity, where arginine is used as a dental therapeutic. Little is known about how E. faecalis responds to growth in arginine in the absence of other bacteria. To address this, we used RNAseq and additional assays to measure growth, gene expression, and biofilm formation in E. faecalis OG1RF grown in arginine. We demonstrate that arginine decreases E. faecalis biofilm production and causes widespread differential expression of genes related to metabolism, quorum sensing, and polysaccharide synthesis. Growth in arginine also increases aggregation of E. faecalis and promotes decreased susceptibility to the antibiotics ampicillin and ceftriaxone. This work provides a platform for understanding how the presence of arginine in biological niches affects E. faecalis physiology and virulence of surrounding microbes.

Pseudomonas aeruginosa: metabolic allies and adversaries in the world of polymicrobial infections

Pseudomonas aeruginosa (PA), an opportunistic human pathogen that is frequently linked with chronic infections in immunocompromised individuals, is also metabolically versatile, and thrives in diverse environments. Additionally, studies report that PA can interact with other microorganisms, such as bacteria, and fungi, producing unique metabolites that can modulate the host immune response, and contribute to disease pathogenesis. This review summarizes the current knowledge related to the metabolic interactions of PA with other microorganisms (Staphylococcus, Acinetobacter, Klebsiella, Enterococcus, and Candida) and human hosts, and the importance of these interactions in a polymicrobial context. Further, we highlight the potential applications of studying these metabolic interactions toward designing better diagnostic tools, and therapeutic strategies to prevent, and treat infections caused by this pathogen.

The wound microbiota: microbial mechanisms of impaired wound healing and infection

The skin barrier protects the human body from invasion by exogenous and pathogenic microorganisms. A breach in this barrier exposes the underlying tissue to microbial contamination, which can lead to infection, delayed healing, and further loss of tissue and organ integrity. Delayed wound healing and chronic wounds are associated with comorbidities, including diabetes, advanced age, immunosuppression and autoimmune disease. The wound microbiota can influence each stage of the multi-factorial repair process and influence the likelihood of an infection. Pathogens that commonly infect wounds, such as Staphylococcus aureus and Pseudomonas aeruginosa, express specialized virulence factors that facilitate adherence and invasion. Biofilm formation and other polymicrobial interactions contribute to host immunity evasion and resistance to antimicrobial therapies. Anaerobic organisms, fungal and viral pathogens, and emerging drug-resistant microorganisms present unique challenges for diagnosis and therapy. In this Review, we explore the current understanding of how microorganisms present in wounds impact the process of skin repair and lead to infection through their actions on the host and the other microbial wound inhabitants.

Arginine impacts aggregation, biofilm formation, and antibiotic susceptibility in Enterococcus faecalis

Enterococcus faecalis is a commensal bacterium in the gastrointestinal tract (GIT) of humans and other organisms. E. faecalis also causes infections in root canals, wounds, the urinary tract, and on heart valves. E. faecalis metabolizes arginine through the arginine deiminase (ADI) pathway, which converts arginine to ornithine and releases ATP, ammonia, and CO2. E. faecalis arginine metabolism also affects virulence of other pathogens during co-culture. E. faecalis may encounter elevated levels of arginine in the GIT or the oral cavity, where arginine is used as a dental therapeutic. Little is known about how E. faecalis responds to growth in arginine in the absence of other bacteria. To address this, we used RNAseq and additional assays to measure growth, gene expression, and biofilm formation in E. faecalis OG1RF grown in arginine. We demonstrate that arginine decreases E. faecalis biofilm production and causes widespread differential expression of genes related to metabolism, quorum sensing, and polysaccharide synthesis. Growth in arginine also increases aggregation of E. faecalis and promotes decreased susceptibility to the antibiotics ampicillin and ceftriaxone. This work provides a platform for understanding of how the presence of arginine in biological niches affects E. faecalis physiology and virulence of surrounding microbes.

Decoding the complexity of delayed wound healing following Enterococcus faecalis infection

Wound infections are highly prevalent and can lead to delayed or failed healing, causing significant morbidity and adverse economic impacts. These infections occur in various contexts, including diabetic foot ulcers, burns, and surgical sites. Enterococcus faecalis is often found in persistent non-healing wounds, but its contribution to chronic wounds remains understudied. To address this, we employed single-cell RNA sequencing (scRNA-seq) on infected wounds in comparison to uninfected wounds in a mouse model. Examining over 23,000 cells, we created a comprehensive single-cell atlas that captures the cellular and transcriptomic landscape of these wounds. Our analysis revealed unique transcriptional and metabolic alterations in infected wounds, elucidating the distinct molecular changes associated with bacterial infection compared to the normal wound healing process. We identified dysregulated keratinocyte and fibroblast transcriptomes in response to infection, jointly contributing to an anti-inflammatory environment. Notably, E. faecalis infection prompted a premature, incomplete epithelial-mesenchymal transition in keratinocytes. Additionally, E. faecalis infection modulated M2-like macrophage polarization by inhibiting pro-inflammatory resolution in vitro, in vivo, and in our scRNA-seq atlas. Furthermore, we discovered macrophage crosstalk with neutrophils, which regulates chemokine signaling pathways, while promoting anti-inflammatory interactions with endothelial cells. Overall, our findings offer new insights into the immunosuppressive role of E. faecalis in wound infections.

Genomic and phenotypic characterisation of Enterococcus mundtii AM_AQ_BC8 for its anti-biofilm, antimicrobial and probiotic potential

Enterococcus mundtii AM_AQ_BC8 isolated from biofouled filtration membrane was characterised as a potential probiotic bacterium showing strong L-lactic acid-producing capability. Experimental studies revealed that E. mundtii AM_AQ_BC8 possess antibiofilm and antimicrobial ability too, as tested against strong biofilm-forming bacteria like Pseudomonas spp. The present study has evaluated the genetic potential of E. mundtii AM_AQ_BC8 through genome sequencing. Whole genome analysis revealed the presence of key genes like ldh_1 and ldh_2 responsible for lactic acid production along with genes encoding probiotic features such as acid and bile salt resistance (dnaK, dnaJ, argS), fatty acid synthesis (fabD, fabE) and lactose utilisation (lacG, lacD). The phylogenomic analysis based on OrthoANI (99.85%) and dDDH (96.8%) values revealed that the strain AM_AQ_BC8 shared the highest homology with E. mundtii. The genome sequence of strain AM_AQ_BC8 has been deposited to NCBI and released with GenBank accession no. SAMN32531201. The study primarily demonstrated the probiotic potential of E. mundtii AM_AQ_BC8 isolate, for L-lactate synthesis in high concentration (8.98 g/L/day), which also showed anti-biofilm and antimicrobial activities.

Bacterial synergies and antagonisms affecting Pseudomonas aeruginosa virulence in the human lung, skin and intestine

Pseudomonas aeruginosa requires a significant breach in the host defense to cause an infection. While its virulence factors are well studied, its tropism cannot be explained only by studying its interaction with the host. Why are P. aeruginosa infections so rare in the intestine compared with the lung and skin? There is not enough evidence to claim specificity in virulence factors deployed by P. aeruginosa in each anatomical site, and host physiology differences between the lung and the intestine cannot easily explain the observed differences in virulence. This perspective highlights a relatively overlooked parameter in P. aeruginosa virulence, namely, potential synergies with bacteria found in the human skin and lung, as well as antagonisms with bacteria of the human intestine.

Solving polymicrobial puzzles: evolutionary dynamics and future directions

Polymicrobial infections include various microorganisms, often necessitating different treatment methods than a monomicrobial infection. Scientists have been puzzled by the complex interactions within these communities for generations. The presence of specific microorganisms warrants a chronic infection and impacts crucial factors such as virulence and antibiotic susceptibility. Game theory is valuable for scenarios involving multiple decision-makers, but its relevance to polymicrobial infections is limited. Eco-evolutionary dynamics introduce causation for multiple proteomic interactions like metabolic syntropy and niche segregation. The review culminates both these giants to form evolutionary dynamics (ED). There is a significant amount of literature on inter-bacterial interactions that remain unsynchronised. Such raw data can only be moulded by analysing the ED involved. The review culminates the inter-bacterial interactions in multiple clinically relevant polymicrobial infections like chronic wounds, CAUTI, otitis media and dental carries. The data is further moulded with ED to analyse the niche colonisation of two notoriously competitive bacteria: S.aureus and P.aeruginosa. The review attempts to develop a future trajectory for polymicrobial research by following recent innovative strategies incorporating ED to curb polymicrobial infections.

Isolation, structure elucidation, and biological activities of sesquiterpenes and phthalides from two edible mushrooms Pleurotus species

Antimicrobial compounds were purified from culture filtrates from 2 edible Pleurotus species. Using a bioassay-guided fractionation of the culture filtrate extracts, 3 compounds (1-3) were obtained from Pleurotus ostreatus, and another compound (4) was obtained from Pleurotus pulmonarius. Spectroscopic analysis revealed that 1-3 was identified as 5,7-dimethoxyphthalide, 4,6-dimethoxyphthalide, and cheimonophyllon E, respectively, while 4 were identified as pleuroton A. The minimum inhibitory concentration and minimum bactericidal concentration of these compounds were determined against 6 pathogenic bacterial species, Enterococcus faecalis, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter cloacae. Compounds 2 and 4 were inhibitory against all tested bacteria, while 1 and 4 were inhibitory against 3 and 2 species, respectively. In addition, 1-4 inhibited tyrosinase, with IC50 values of 0.10-0.30 mg/mL, and α-glucosidase, with IC50 values of 0.12-0.54 mg/mL. However, their antioxidant capacities were marginal.

Streptococcus mutans inhibits the growth of Enterococcus via the non-ribosomal cyclic peptide mutanobactin

Enterococcus faecalis is a Gram-positive commensal bacterium in the gastrointestinal tract and an opportunistic pathogen. Enterococci are a leading cause of nosocomial infections, treatment of which is complicated by intrinsic and acquired antibiotic resistance mechanisms. Additionally, E. faecalis has been associated with various oral diseases, and it is frequently implicated in the failure of endodontic treatment. For establishment and persistence in a microbial community, E. faecalis must successfully compete against other bacteria. Streptococcal species play an important role in the establishment of the oral microbiome and co-exist with Enterococcus in the small intestine, yet the nature of interactions between E. faecalis and oral streptococci remains unclear. Here, we describe a mechanism by which Streptococcus mutans inhibits the growth of E. faecalis and other Gram-positive pathogens through the production of mutanobactin, a cyclic lipopeptide. Mutanobactin is produced by a polyketide synthase-nonribosomal peptide synthetase hybrid system encoded by the mub locus. Mutanobactin-producing S. mutans inhibits planktonic and biofilm growth of E. faecalis and is also active against other Enterococcus species and Staphylococcus aureus. Mutanobactin damages the cell envelope of E. faecalis, similar to other lipopeptide antibiotics like daptomycin. E. faecalis resistance to mutanobactin is mediated by the virulence factor gelatinase, a secreted metalloprotease. Our results highlight the anti-biofilm potential of the microbial natural product mutanobactin, provide insight into how E. faecalis interacts with other organisms in the human microbiome, and demonstrate the importance of studying E. faecalis dynamics within polymicrobial communities.

Interspecies interactions in mixed-species biofilms formed by Enterococcus faecalis and gram-negative bacteria isolated from polymicrobial diabetic foot ulcers

Diabetic foot ulcers (DFU) are exacerbated by bacterial colonisation. Here, a high prevalence of Enterococcus faecalis was observed in DFU patients from an Argentinean hospital. E. faecalis was frequently co-isolated with Escherichia coli, Morganella morganii, and Pseudomonas aeruginosa. The effect of interspecies interactions on bacterial growth was investigated in mixed-species macrocolony biofilms developed in Lubbock-Glc-agar. Similar cell counts were found for E. faecalis and M. morganii growing in mixed and single-species biofilms. An E. faecalis strain showed 1 Log higher cell counts in mixed biofilms with E. coli. Remarkably, E. faecalis strains showed 2 to 4 Log higher cell counts in mixed biofilms with P. aeruginosa. This effect was not observed in planktonic growth or biofilms developed in tryptic soy agar. The present findings reveal bacterial interactions that benefit E. faecalis in mixed-species biofilms, mainly with P. aeruginosa, in a medium that partially mimics the nutrients found in DFU.

Efflux, Signaling and Warfare in a Polymicrobial World

The discovery void of antimicrobial development has occurred at a time when the world has seen a rapid emergence and spread of antimicrobial resistance, the 'perfect storm' as it has often been described. While the discovery and development of new antibiotics has continued in the research sphere, the pipeline to clinic has largely been fed by derivatives of existing classes of antibiotics, each prone to pre-existing resistance mechanisms. A novel approach to infection management has come from the ecological perspective whereby microbial networks and evolved communities already possess small molecular capabilities for pathogen control. The spatiotemporal nature of microbial interactions is such that mutualism and parasitism are often two ends of the same stick. Small molecule efflux inhibitors can directly target antibiotic efflux, a primary resistance mechanism adopted by many species of bacteria and fungi. However, a much broader anti-infective capability resides within the action of these inhibitors, borne from the role of efflux in key physiological and virulence processes, including biofilm formation, toxin efflux, and stress management. Understanding how these behaviors manifest within complex polymicrobial communities is key to unlocking the full potential of the advanced repertoires of efflux inhibitors.

Role of Enterococcus faecalis in refractory apical periodontitis: from pathogenicity to host cell response

BACKGROUND

Refractory apical periodontitis (RAP) is an oral infectious disease characterised by persistent inflammation, progressive alveolar bone destruction, and delayed bone healing. RAP has received increasing attention, because it cannot be cured after repeated root canal therapies. The aetiology of RAP is related to the complex interplay between the pathogen and its host. However, the exact pathogenesis of RAP remains unclarified and includes several factors, such as microorganism immunogenicity, host immunity and inflammation, and tissue destruction and repair. Enterococcus faecalis is the dominant pathogen involved in RAP, and has evolved multiple strategies to ensure survival, which cause persistent intraradicular and extraradicular infections.

OBJECTIVE

To review the crucial role of E. faecalis in the pathogenesis of RAP, and open new avenues for prevention and treatment of RAP.

METHODS

The PubMed and Web of Science databases were searched for pertinent publications, employing the search terms "Enterococcus faecalis", "refractory apical periodontitis", "persistent periapical periodontitis", "pathogenicity", "virulence", "biofilm formation", "dentine tubule", "immune cell", "macrophage", and "osteoblast".

RESULTS AND CONCLUSION

Besides its high pathogenicity due to various virulence mechanisms, E. faecalis modulates the macrophage and osteoblast responses, including regulated cell death, cell polarisation, cell differentiation, and inflammatory response. An in-depth understanding of the multifaceted host cell responses modulated by E. faecalis will help to design potential future therapeutic strategies and overcome the challenges of sustained infection and delayed tissue healing in RAP.