T3SS and alginate biosynthesis of Pseudomonas aeruginosa impair healing of infected rabbit wounds

Pseudomonas aeruginosa (a Gram-negative bacterium) is an opportunistic pathogen found in many infected wounds and is known to impair healing. To test the hypothesis that knocking out P. aeruginosa genes that are overexpressed during wound infection can cripple a pathogen's ability to impair healing, we assessed two pathways: the Type III secretion system (T3SS) and alginate biosynthesis. We generated single- and double-mutant strains of ExsA (T3SS activator), AlgD (GDP- mannose 6-dehydrogenase of alginate biosynthesis) and their complemented strains and evaluated their pathogenicity in a rabbit ear full-thickness excision-wound infection model. Wounds were inoculated with different strains (wild type, mutants, and complementary strains) at 10⁶ CFU/wound on post-wounding day 3. After 24 h, 5 days and 9 days post-infection, wounds were harvested for measuring bacterial counts (viable and total) and wound healing (epithelial gap). On day 9 post-infection, the viable counts of the double mutant, (exsA/algD)‾ were 100-fold lower than the counts of the wild type (PAO1), single mutants, or the complement double-mutant, (exsA/algD)‾^/+. Also, when compared to wounds infected with wild type or control strains, wounds infected with the double-knockout mutant was less inhibitory to wound healing (p < 0.05). Additionally, the double mutant showed greater susceptibility to macrophage phagocytosis in vitro than all other strains (p < 0.001). In conclusion, compared to single gene knockouts, double knockout of virulence genes in T3SS pathway and alginate biosynthesis pathway is more effective in reducing P. aeruginosa pathogenicity and its ability to impair wound healing. This study highlights the necessity of a dual-targeted anti-virulence strategy to improve healing outcomes of P. aeruginosa-infected wounds.

Cerium Nitrate Treatment Provides Eschar Stabilization through Reduction in Bioburden, DAMPs, and Inflammatory Cytokines in a Rat Scald Burn Model

In this study, we used a clinically relevant rat scald burn model to determine the treatment effects of cerium nitrate (CN) for stabilizing burn eschars through reduction of damage-associated molecular patterns (DAMPs), inflammatory cytokines, and bioburden. Forty-two male Sprague-Dawley rats were anesthetized before undergoing a scald burn at 99°C for 6 seconds to create a 10% full-thickness burn. The test groups included sham burn, burn with water bathing, and burn with CN bathing. End point parameters included circulating DAMPs, proinflammatory cytokines, tissue myeloperoxidase activity, and quantification of resident flora in burn skin. The high mobility group protein box 1 was found to be elevated in burn animals at postoperative days (POD) 1 and 7. CN significantly alleviated the increase (P < .05 at POD 1 and P < .01 at POD 7). CN also lessened the heightened levels of hyaluronan in burn animals (P < .05 at POD 7). Additionally, CN significantly reduced the burn-induced increases in interleukin-1β, growth-regulated oncogene/keratinocyte chemoattractant, and macrophage inflammatory protein-1α in burn wounds. The anti-inflammatory effect of CN was also demonstrated in its ability to mitigate the upregulated circulatory xanthine oxidase/dehydrogenase and increased tissue neutrophil infiltration in burn animals. Last, CN suppressed postburn proliferation of resident skin microbes, resulting in a significant 2-log reduction by POD 7. In conclusion, these results suggest that CN attenuates the burn-induced DAMPs, tissue inflammatory responses, and regrowth of resident skin flora, all of which collectively could improve the quality of burn eschar when applied at the point of injury in prolonged field care situations.

Development of Pseudomonas aeruginosa Biofilms in Partial-Thickness Burn Wounds Using a Sprague-Dawley Rat Model

We used a modified Walker–Mason scald burn rat model to demonstrate that Pseudomonas aeruginosa, a common opportunistic pathogen in the burn ward and notable biofilm former, establishes biofilms within deep partial-thickness burn wounds in rats.

Deep partial-thickness burn wounds, ~10% of the TBSA, were created in anesthetized male Sprague-Dawley rats (350–450 g; n = 84). Immediately post-burn, 100 µl of P. aeruginosa in phosphate-buffered saline at 1 × 10³, 1 × 10⁴, or 1 × 10⁵ cells/wound was spread over the burn surface . At 1, 3, 7, and 11 days post-burn, animals were euthanized and blood and tissue were collected for complete blood counts, colony-forming unit (CFU) counts, biofilm gene expression, histology, scanning electron microscopy (SEM), and myeloperoxidase activity in the burn eschar.

P. aeruginosa developed robust biofilm wound infections, plateauing at ~1 × 10⁹ CFU/g burn tissue within 7 days regardless of inoculum size. Expression of Pseudomonas alginate genes and other virulence factors in the infected wound indicated formation of mature P. aeruginosa biofilm within the burn eschar. Compared to un-inoculated wounds, P. aeruginosa infection caused both local and systemic immune responses demonstrated by changes in systemic neutrophil counts, histology, and myeloperoxidase activity within the burn wound. Additionally, SEM showed P. aeruginosa enmeshed within an extracellular matrix on the burn surface as well as penetrating 500–600 µm deep into the eschar.

P. aeruginosa establishes biofilms within deep partial-thickness burn wounds and invades deep into the burned tissue. This new in vivo biofilm infection model is valuable for testing novel anti-biofilm agents to advance burn care.

Formation of Pseudomonas aeruginosa Biofilms in Full-thickness Scald Burn Wounds in Rats

Using Sprague-Dawley rats (350-450 g; n = 61) and the recently updated Walker-Mason rat scald burn model, we demonstrated that Pseudomonas aeruginosa readily formed biofilms within full-thickness burn wounds. Following the burn, wounds were surface-inoculated with P. aeruginosa in phosphate-buffered saline (PBS), while sterile PBS was used for controls. On post-burn days 1, 3, 7, and 11, animals were euthanized and samples collected for quantitative bacteriology, bacterial gene expression, complete blood cell counts, histology, and myeloperoxidase activity. Robust biofilm infections developed in the full-thickness burn wounds inoculated with 1 × 10⁴ CFU of P. aeruginosa. Both histology and scanning electron microscopy showed the pathogen throughout the histologic cross-sections of burned skin. Quantigene analysis revealed significant upregulation of alginate and pellicle biofilm matrix genes of P. aeruginosa within the burn eschar. Additionally, expression of P. aeruginosa proteases and siderophores increased significantly in the burn wound environment. Interestingly, the host's neutrophil response to the pathogen was not elevated in either the eschar or circulating blood when compared to the control burn. This new full-thickness burn biofilm infection model will be used to test new anti-biofilm therapies that may be deployed with soldiers in combat for immediate use at the site of burn injury on the battlefield.

Antimicrobial and Antibiofilm Activity of Synergistic Combinations of a Commercially Available Small Compound Library With Colistin Against Pseudomonas aeruginosa

Biofilm-associated Pseudomonas aeruginosa infections remain a significant clinical challenge since the conventional antibiotic treatment or combination therapies are largely ineffective; and new approaches are needed. To circumvent the major challenges associated with discovery of new antimicrobials, we have screened a library of compounds that are commercially available and approved by the FDA (Prestwick Chemical Library) against P. aeruginosa for effective antimicrobial and anti-biofilm activity. A preliminary screen of the Prestwick Chemical Library alone did not yield any repositionable candidates, but in a screen of combinations with a fixed sub-inhibitory concentration of the antibiotic colistin we observed 10 drugs whose bacterial inhibiting activity was reproducibly enhanced, seven of which were enhanced by more than 50%. We performed checkerboard assays of these seven drugs in combination with colistin against planktonic cells, and analysis of their interactions over the complete combination matrix using the Zero Interaction Potency (ZIP) model revealed interactions that varied from highly synergistic to completely antagonistic. Of these, five combinations that showed synergism were down-selected and tested against preformed biofilms of P. aeruginosa. Two of the five combinations were active against preformed biofilms of both laboratory and clinical strain of P. aeruginosa, resulting in a 2-log reduction in culturable cells. In summary, we have identified synergistic combinations of five commercially available, FDA-approved drugs and colistin that show antimicrobial activity against planktonic P. aeruginosa (Clomiphene Citrate, Mitoxantrone Dihydrochloride, Methyl Benzethonium Chloride, Benzethonium Chloride, and Auranofin) as well as two combinations (Auranofin and Clomiphene Citrate) with colistin that show antibiofilm activity.

Control of Oral Biofilm Formation by an Antimicrobial Decapeptide

Oral biofilms are mixed-species microbial communities, and their uncontrolled outgrowth can express as oral diseases. Antimicrobial peptides represent alternative classes of antimicrobials that exhibit selectivity for prokaryotes. We wanted to test the effect of a synthetic decapeptide antimicrobial, KSL, on the development of oral biofilms formed by isolated human salivary bacteria. We used differential interference contrast microscopy, coupled with a dual-flow cell system, to determine the effect of KSL on oral biofilm development. We used reductions of viable counts and confocal microscopy to assess the bactericidal activity of KSL on mature oral biofilms. KSL effectively blocked biofilm development. A significant effect on the viability of mature biofilms was observed when KSL was used in the presence of a surface-active agent, or after biofilms were mechanically disrupted. This study shows that KSL may be a useful adjunct for conventional oral hygiene to prevent plaque-mediated dental diseases.

RNA-Seq Transcriptomic Responses of Full-Thickness Dermal Excision Wounds to Pseudomonas aeruginosa Acute and Biofilm Infection

Pseudomonas aeruginosa infections of wounds in clinical settings are major complications whose outcomes are influenced by host responses that are not completely understood. Herein we evaluated transcriptomic changes of wounds as they counter P. aeruginosa infection—first active infection, and then chronic biofilm infection. We used the dermal full-thickness, rabbit ear excisional wound model. We studied the wound response: towards acute infection at 2, 6, and 24 hrs after inoculating 10⁶ bacteria into day-3 wounds; and, towards more chronic biofilm infection of wounds similarly infected for 24 hrs but then treated with topical antibiotic to coerce biofilm growth and evaluated at day 5 and 9 post-infection. The wounds were analyzed for bacterial counts, expression of P. aeruginosa virulence and biofilm-synthesis genes, biofilm morphology, infiltrating immune cells, re-epithelialization, and genome-wide gene expression (RNA-Seq transcriptome). This analysis revealed that 2 hrs after bacterial inoculation into day-3 wounds, the down-regulated genes (infected vs. non-infected) of the wound edge were nearly all non-coding RNAs (ncRNAs), comprised of snoRNA, miRNA, and RNU6 pseudogenes, and their down-regulation preceded a general down-regulation of skin-enriched coding gene expression. As the active infection intensified, ncRNAs remained overrepresented among down-regulated genes; however, at 6 and 24 hrs they changed to a different set, which overlapped between these times, and excluded RNU6 pseudogenes but included snRNA components of the major and minor spliceosomes. Additionally, the raw counts of multiple types of differentially-expressed ncRNAs increased on post-wounding day 3 in control wounds, but infection suppressed this increase. After 5 and 9 days, these ncRNA counts in control wounds decreased, whereas they increased in the infected, healing-impaired wounds. These data suggest a sequential and coordinated change in the levels of transcripts of multiple major classes of ncRNAs in wound cells transitioning from inflammation to the proliferation phase of healing.

Antimicrobial peptides containing unnatural amino acid exhibit potent bactericidal activity against ESKAPE pathogens

A series of 36 synthetic antimicrobial peptides containing unnatural amino acids were screened to determine their effectiveness to treat Enterococcus faecium, Staphylococcus aureus, Klebsiella pnemoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species (ESKAPE) pathogens, which are known to commonly infect chronic wounds. The primary amino acid sequences of these peptides incorporate either three or six dipeptide units consisting of the unnatural amino acids Tetrahydroisoquinolinecarboxylic acid (Tic) and Octahydroindolecarboxylic acid (Oic). The Tic-Oic dipeptide units are separated by SPACER amino acids with specific physicochemical properties that control how these peptides interact with bacterial cell membranes of different chemical compositions. These peptides exhibited minimum inhibitory concentrations (MIC) against these pathogens in the range from >100 to 6.25 μg/mL. The observed diversity of MIC values for these peptides against the various bacterial strains are consistent with our hypothesis that the complementarity of the physicochemical properties of the peptide and the lipid of the bacteria's cell membrane determines the resulting antibacterial activity of the peptide.

Synthetic decapeptide reduces bacterial load and accelerates healing in the wounds of restraint-stressed mice

Wound healing is a complex process involving four transitional yet concurrent stages: coagulation, inflammation, cell proliferation/epithelialization and remodeling. These overlapping stages occur uneventfully in normal physiology. However, during psychological stress, the inflammatory response can become dysregulated and result in increased susceptibility to bacterial infection and delayed wound closure. In our restraint stress model, cutaneous wounds of stressed SKH-1 mice demonstrate significantly higher levels of bacterial load, and healing progresses at a rate 30% slower, than in non-stressed mice. The purpose of this study was to test the hypothesis that a synthetic antimicrobial decapeptide (KSLW) enhances bacterial clearance during stress-impaired healing in mice. Here, using a Pluronic block copolymer nanocarrier, we endeavored to identify an efficient drug delivery system for KSLW, which would enhance the stability, substantivity and function of the cationic peptide in delayed-healing wounds. In this study, intradermal treatment of excisional wounds of stressed mice with 2mg/ml KSLW loaded in Pluronic F68, resulted in a sustained antimicrobial effect through post-operative day 5, with a 2-log (p<0.01) reduction in bacterial load compared with other stressed mice. The demonstrated bacterial reduction in KSLW-treated stressed mice did not approach the levels observed among control mice. Furthermore, treatment of stressed mice with KSLW improved healing, resulting in significantly faster (p<0.05) wound closure from days 2 to 5 post-wounding, relative to untreated stressed mice and stressed mice treated with Pluronic alone. These findings suggest that Pluronic F68 is an efficient carrier for KSLW, which improves its stability and activity in impaired dermal wounds.

An improved medium for growing Staphylococcus aureus biofilm

A medium (Brain Heart Infusion plus 10% human plasma) was developed, tested, and validated for growing Staphylococcus aureus biofilm in vitro. With this medium, S. aureus forms reproducible and robust biofilms in flow chambers under controlled shear flow and with increased viability recovery in static well plates.

Susceptibility of oral bacteria to an antimicrobial decapeptide

Naturally occurring antimicrobial peptides have emerged as alternative classes of antimicrobials. In general, these antimicrobial peptides exhibit selectivity for prokaryotes and minimize the problems of engendering microbial resistance. As an alternative method to search for more effective broad-spectrum peptide antimicrobials, investigators have developed peptide libraries by using synthetic combinatorial technology. A novel decapeptide, KKVVFKVKFK (KSL), has been identified that shows a broad range of antibacterial activity. The purpose of this study was to test the efficacy of this antimicrobial peptide in killing selected strains of oral pathogens and resident saliva bacteria collected from human subjects. Cytotoxic activity of KSL against mammalian cells and the structural features of this decapeptide were also investigated, the latter by using two-dimensional NMR in aqueous and DMSO solutions. MICs of KSL for the majority of oral bacteria tested in vitro ranged from 3 to 100 microg ml(-1). Minimal bactericidal concentrations of KSL were, in general, within one to two dilutions of the MICs. KSL exhibited an ED(99) (the dose at which 99 % killing was observed after 15 min at 37 degrees C) of 6.25 microg ml(-1) against selected strains of Lactobacillus salivarius, Streptococcus mutans, Streptococcus gordonii and Actinobacillus actinomycetemcomitans. In addition, KSL damaged bacterial cell membranes and caused 1.05 log units reduction of viability counts of saliva bacteria. In vitro toxicity studies showed that KSL, at concentrations up to 1 mg ml(-1), did not induce cell death or compromise the membrane integrity of human gingival fibroblasts. NMR studies suggest that KSL adopts an alpha-helical structure in DMSO solution, which mimics the polar aprotic membrane environment, whereas it remains unstructured in aqueous medium. This study shows that KSL may be a useful antimicrobial agent for inhibiting the growth of oral bacteria that are associated with caries development and early plaque formation.