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.

Secretory IgA impacts the microbiota density in the human nose

BACKGROUND

Respiratory mucosal host defense relies on the production of secretory IgA (sIgA) antibodies, but we currently lack a fundamental understanding of how sIgA is induced by contact with microbes and how such immune responses may vary between humans. Defense of the nasal mucosal barrier through sIgA is critical to protect from infection and to maintain homeostasis of the microbiome, which influences respiratory disorders and hosts opportunistic pathogens.

METHODS

We applied IgA-seq analysis to nasal microbiota samples from male and female healthy volunteers, to identify which bacterial genera and species are targeted by sIgA on the level of the individual host. Furthermore, we used nasal sIgA from the same individuals in sIgA deposition experiments to validate the IgA-seq outcomes.

CONCLUSIONS

We observed that the amount of sIgA secreted into the nasal mucosa by the host varied substantially and was negatively correlated with the bacterial density, suggesting that nasal sIgA limits the overall bacterial capacity to colonize. The interaction between mucosal sIgA antibodies and the nasal microbiota was highly individual with no obvious differences between potentially invasive and non-invasive bacterial species. Importantly, we could show that for the clinically relevant opportunistic pathogen and frequent nasal resident Staphylococcus aureus, sIgA reactivity was in part the result of epitope-independent interaction of sIgA with the antibody-binding protein SpA through binding of sIgA Fab regions. This study thereby offers a first comprehensive insight into the targeting of the nasal microbiota by sIgA antibodies. It thereby helps to better understand the shaping and homeostasis of the nasal microbiome by the host and may guide the development of effective mucosal vaccines against bacterial pathogens. Video Abstract.

Host-microbiome protein-protein interactions capture disease-relevant pathways

Background

Host-microbe interactions are crucial for normal physiological and immune system development and are implicated in a variety of diseases, including inflammatory bowel disease (IBD), colorectal cancer (CRC), obesity, and type 2 diabetes (T2D). Despite large-scale case-control studies aimed at identifying microbial taxa or genes involved in pathogeneses, the mechanisms linking them to disease have thus far remained elusive.

Results

To identify potential pathways through which human-associated bacteria impact host health, we leverage publicly-available interspecies protein-protein interaction (PPI) data to find clusters of microbiome-derived proteins with high sequence identity to known human-protein interactors. We observe differential targeting of putative human-interacting bacterial genes in nine independent metagenomic studies, finding evidence that the microbiome broadly targets human proteins involved in immune, oncogenic, apoptotic, and endocrine signaling pathways in relation to IBD, CRC, obesity, and T2D diagnoses.

Conclusions

This host-centric analysis provides a mechanistic hypothesis-generating platform and extensively adds human functional annotation to commensal bacterial proteins.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02643-9.

Synergistic effect of immunomodulatory S100A8/A9 and ciprofloxacin against Pseudomonas aeruginosa biofilm in a murine chronic wound model

The majority of chronic wounds are associated with bacterial biofilms recalcitrant to antibiotics and host responses. Immunomodulatory S100A8/A9 is suppressed in Pseudomonas aeruginosa biofilm infected wounds. We aimed at investigating a possible additive effect between S100A8/A9 and ciprofloxacin against biofilms.

MATERIALS/METHODS

Thirty-two mice were injected with alginate-embedded P. aeruginosa following a third-degree burn. The mice were randomized into four groups receiving combination ciprofloxacin and S100A8/A9 or monotherapy ciprofloxacin, S100A8/A9 or a placebo and evaluated by host responses and quantitative bacteriology in wounds. In addition, in vitro checkerboard analysis was performed, with P. aeruginosa and ascending S100A8/A9 and ciprofloxacin concentrations.

RESULTS

S100A8/A9 augmented the effect of ciprofloxacin in vivo by lowering the bacterial quantity compared to the placebo arm and the two monointervention groups (P < 0.0001). S100A8 and 100A9 were increased in the double-treated group as compared to the monointervention groups (P = 0.032, P = 0.0023). Tissue inhibitor of metalloproteinases-1 and keratinocyte\chemokine chemoattractant-1 were increased in the double-intervention group compared to the S100A8/A9 group (P = 0.050, P = 0.050). No in vitro synergism was detected.

CONCLUSION

The observed ciprofloxacin-augmenting effect of S100A8/A9 in vivo was not confirmed by checkerboard analysis, indicating dependence on host cells for the S100A8/A9 effect. S100A8/A9 and ciprofloxacin is a promising therapy for optimizing chronic wound treatment.

Finegoldia magna, an Anaerobic Gram-Positive Bacterium of the Normal Human Microbiota, Induces Inflammation by Activating Neutrophils

The Gram-positive anaerobic commensal Finegoldia magna colonizes the skin and other non-sterile body surfaces, and is an important opportunistic pathogen. Here we analyzed the effect of F. magna on human primary neutrophils. F. magna strains ALB8 (expressing protein FAF), 312 (expressing protein L) and 505 (naturally lacking both protein FAF and L) as well as their associated proteins activate neutrophils to release reactive oxygen species, an indication for neutrophil oxidative burst. Co-incubation of neutrophils with the bacteria leads to a strong increase of CD66b surface expression, another indicator for neutrophil activation. Furthermore, all tested stimuli triggered the release of NETs from the activated neutrophils, pointing to a host defense mechanism in response to the tested stimuli. This phenotype is dependent on actin rearrangement, NADPH oxidases and the ERK1/2 pathway. Proteins FAF and L also induced the secretion of several pro-inflammatory neutrophil proteins; HBP, IL-8 and INFγ. This study shows for the first time a direct interaction of F. magna with human neutrophils and suggests that the activation of neutrophils plays a role in F. magna pathogenesis.

Antimicrobial action of calprotectin that does not involve metal withholding

Calprotectin is a potent antimicrobial that inhibits the growth of pathogens by tightly binding transition metals such as Mn and Zn, thereby preventing their uptake and utilization by invading microbes. At sites of infection, calprotectin is abundantly released from neutrophils, but calprotectin is also present in non-neutrophil cell types that may be relevant to infections. We show here that in patients infected with the Lyme disease pathogen Borreliella (Borrelia) burgdorferi, calprotectin is produced in neutrophil-free regions of the skin, in both epidermal keratinocytes and in immune cells infiltrating the dermis, including CD68 positive macrophages. In culture, B. burgdorferi's growth is inhibited by calprotectin, but surprisingly, the mechanism does not involve the classical withholding of metal nutrients. B. burgdorferi cells exposed to calprotectin cease growth with no reduction in intracellular Mn and no loss in activity of Mn enzymes including the SodA superoxide dismutase. Additionally, there is no obvious loss in intracellular Zn. Rather than metal depletion, we find that calprotectin inhibits B. burgdorferi growth through a mechanism that requires physical association of calprotectin with the bacteria. By comparison, calprotectin inhibited E. coli growth without physically interacting with the microbe, and calprotectin effectively depleted E. coli of intracellular Mn and Zn. Our studies with B. burgdorferi demonstrate that the antimicrobial capacity of calprotectin is complex and extends well beyond simple withholding of metal micronutrients.

Immune Modulating Topical S100A8/A9 Inhibits Growth of Pseudomonas aeruginosa and Mitigates Biofilm Infection in Chronic Wounds

Pseudomonas aeruginosa biofilm maintains and perturbs local host defense, hindering timely wound healing. Previously, we showed that P. aeruginosa suppressed S100A8/A9 of the murine innate host defense. We assessed the potential antimicrobial effect of S100A8/A9 on biofilm-infected wounds in a murine model and P. aeruginosa growth in vitro. Seventy-six mice, inflicted with a full-thickness burn wound were challenged subcutaneously (s.c.) by 10⁶ colony-forming units (CFUs) of P. aeruginosa biofilm. Mice were subsequently randomized into two treatment groups, one group receiving recombinant murine S100A8/A9 and a group of vehicle controls (phosphate-buffered saline, PBS) all treated with s.c. injections daily for up to five days. Wounds were analyzed for quantitative bacteriology and contents of key inflammatory markers. Count of blood polymorphonuclear leukocytes was included. S100A8/A9-treatment ameliorated wound infection, as evaluated by quantitative bacteriology (p ≤ 0.05). In vitro, growth of P. aeruginosa was inhibited dose-dependently by S100A8/A9 in concentrations from 5 to 40 μg/mL, as determined by optical density-measurement (OD-measurement) and quantitative bacteriology. Treatment slightly augmented key inflammatory cytokine Tumor Necrosis Factor-α (TNF-α), but dampened interferon-γ (IFN-γ) levels and blood polymorphonuclear count. In conclusion, topical S100A8/A9 displays remarkable novel immune stimulatory and anti-infective properties in vivo and in vitro. Importantly, treatment by S100A8/A9 provides local infection control. Implications for a role as adjunctive treatment in healing of chronic biofilm-infected wounds are discussed.

MRP8/14 enhances corneal susceptibility to Pseudomonas aeruginosa Infection by amplifying inflammatory responses

PURPOSE

We explored the role of myeloid-related protein 8 and 14 (MRP8/14) in Pseudomonas aeruginosa (PA) keratitis.

METHODS

MRP8/14 mRNA levels in human corneal scrapes and mouse corneas infected by PA were tested using real-time PCR. MRP8/14 protein expression in C57BL/6 (B6) corneas was confirmed using Western blot assay and immunohistochemistry. B6 mice were injected subconjunctivally with siRNA for MRP8/14, and then infected with PA. Bacterial plate counts and myeloperoxidase assays were used to determine the bacterial load and polymorphonuclear neutrophil (PMN) infiltration in infected B6 corneas. Pro-inflammatory cytokine levels in vivo and in vitro were examined with PCR and ELISA. In murine macrophage-like RAW264.7 cells, phagocytosis and bacterial killing were assessed using plate count assays, and reactive oxygen species (ROS) and nitric oxide (NO) levels were tested with flow cytometry and Griess assay, respectively.

RESULTS

MRP8/14 expression levels were increased significantly in human corneal scrapes and B6 corneas after PA infection. Silencing of MRP8/14 in B6 corneas significantly reduced the severity of corneal disease, bacterial clearance, PMN infiltration, and pro-inflammatory cytokine expression after PA infection. In vitro studies demonstrated further that silencing of MRP8/14 suppressed pro-inflammatory cytokine production, bacterial killing, and ROS production, but not phagocytosis or NO production.

CONCLUSIONS

Our study demonstrated a dual role for MRP8/14 in bacterial keratitis. Although MRP8/14 promotes bacterial clearance by enhancing ROS production, it functions more importantly as an inflammatory amplifier at the ocular surface by enhancing pro-inflammatory cytokine expression, thus contributing to the corneal susceptibility.

Gram-positive anaerobic cocci--commensals and opportunistic pathogens

Among the Gram-positive anaerobic bacteria associated with clinical infections, the Gram-positive anaerobic cocci (GPAC) are the most prominent and account for approximately 25-30% of all isolated anaerobic bacteria from clinical specimens. Still, routine culture and identification of these slowly growing anaerobes to the species level has been limited in the diagnostic laboratory, mainly due to the requirement of prolonged incubation times and time-consuming phenotypic identification. In addition, GPAC are mostly isolated from polymicrobial infections with known pathogens and therefore their relevance has often been overlooked. However, through improvements in diagnostic and in particular molecular techniques, the isolation and identification of individual genera and species of GPAC associated with specific infections have been enhanced. Furthermore, the taxonomy of GPAC has undergone considerable changes over the years, mainly due to the development of molecular identification methods. Existing species have been renamed and novel species have been added, resulting in changes of the nomenclature. As the abundance and significance of GPAC in clinical infections grow, knowledge of virulence factors and antibiotic resistance patterns of different species becomes more important. The present review describes recent advances of GPAC and what is known of the biology and pathogenic effects of Anaerococcus, Finegoldia, Parvimonas, Peptoniphilus and Peptostreptococcus, the most important GPAC genera isolated from human infections.

Metal ion acquisition in Staphylococcus aureus: overcoming nutritional immunity

Transition metals are essential nutrients to virtually all forms of life, including bacterial pathogens. In Staphylococcus aureus, metal ions participate in diverse biochemical processes such as metabolism, DNA synthesis, regulation of virulence factors, and defense against oxidative stress. As an innate immune response to bacterial infection, vertebrate hosts sequester transition metals in a process that has been termed "nutritional immunity." To successfully infect vertebrates, S. aureus must overcome host sequestration of these critical nutrients. The objective of this review is to outline the current knowledge of staphylococcal metal ion acquisition systems, as well as to define the host mechanisms of nutritional immunity during staphylococcal infection.

The B-cell superantigen Finegoldia magna protein L causes pulmonary inflammation by a mechanism dependent on MyD88 but not B cells or immunoglobulins

OBJECTIVE AND DESIGN

To determine whether Finegoldia magna protein L (PL) causes lung inflammation and, if so, whether the response is dependent on its immunoglobulin (Ig)-binding B-cell superantigenic property.

MATERIAL

Pulmonary inflammatory reactions were analyzed at various time points after intratracheal administration of PL to various strains of mice.

RESULTS

PL caused peribronchial and perivascular inflammation that peaked at 18-24 h. Polymorphonuclear cells (PMNs) began to accumulate in bronchoalveolar lavage fluid (BALF) of PL-challenged mice by 4 h and accounted for >90% of leukocytes by 18-24 h. Inflammation was marked by the appearance of MIP-2, KC, TNF-α, and IL-6 in the BALF with peak levels attained 4 h after PL administration. PL-induced pulmonary inflammation was associated with increased airway hyper-reactivity following inhalation of methacholine. The inflammatory reaction was unabated in mice lacking B cells and immunoglobulins. In contrast, PL-induced inflammation was abrogated in MyD88-deficient mice. PL-induced responses required alveolar macrophages.

CONCLUSIONS

These results strongly suggest that PL-induced lung inflammation is dependent on an innate MyD88-dependent pathway rather than the Ig-binding properties of this microbial B cell superantigen. We propose that this pulmonary inflammatory reaction is caused by the interaction of PL with a Toll-like receptor expressed on alveolar macrophages.