Topical antimicrobials for burn wound infections

Abstract

Throughout most of history, serious burns occupying a large percentage of body surface area were an almost certain death sentence because of subsequent infection. A number of factors such as disruption of the skin barrier, ready availability of bacterial nutrients in the burn milieu, destruction of the vascular supply to the burned skin, and systemic disturbances lead to immunosuppression combined together to make burns particularly susceptible to infection. In the 20th century the introduction of antibiotic and antifungal drugs, the use of topical antimicrobials that could be applied to burns, and widespread adoption of early excision and grafting all helped to dramatically increase survival. However the relentless increase in microbial resistance to antibiotics and other antimicrobials has led to a renewed search for alternative approaches to prevent and combat burn infections. This review will cover patented strategies that have been issued or filed with regard to new topical agents, preparations, and methods of combating burn infections. Animal models that are used in preclinical studies are discussed. Various silver preparations (nanocrystalline and slow release) are the mainstay of many approaches but antimicrobial peptides, topical photodynamic therapy, chitosan preparations, new iodine delivery formulations, phage therapy and natural products such as honey and essential oils have all been tested. This active area of research will continue to provide new topical antimicrobials for burns that will battle against growing multidrug resistance.

Fig. 1. Degrees of burn

Relative depths of 1^st, 2^nd, 3^rd, and 4^th degree burns illustrated on a foot.

Fig. 2. Lund & Browder charts

These are used to calculate the % of body surface area affected by burns and determine the prognosis and susceptibility to infection.

Fig. 3. Inflammatory signaling after burn that causes immunosuppression

Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNFα) are produced by a wide variety of cells. The production of prostaglandin E2 (PGE2) and IL6 are upregulated by endothelial cells and macrophages, while the latter secrete decreased amounts of IL-12. T helper cells begin to preferentially differentiate into Th-2 cells, which produce the anti-inflammatory cytokines IL-4 and IL-10. Neutrophil dysfunction occurs despite higher numbers after significant thermal injuries, while macrophage phagocytosis is lower.

Fig. 4. Cell wall structure

Gram-positive bacteria, Gram-negative bacteria and fungi have different cell wall structures that govern their susceptibility to different antimicrobial agents.

Fig. 5. Mouse model of third degree burn infection

Two pre-heated brass blocks are pressed to the opposing sides of an elevated skin fold on the backs of shaved mice, followed by topical application of a suspension of bacteria in saline.

Fig. 6. Use of bioluminescent imaging of genetically engineered bacteria and fungi in burn infections

(A) Picture of the low-light camera suitable for in vivo bioluminescence imaging of small animals. (B) Example of 3^rd degree burn on mouse back infected with bioluminescent P. aeruginosa showing color look-up-table for photon intensity. Successive bioluminescence images of representative mouse burns infected with bioluminescent strains of (C) P. aeruginosa, (D) Acinetobacter baumannii, (E) Staphylococcus aureus, and (F) Candida albicans, respectively.

Fig. 7. Chemical structures of antibiotics used in topical antimicrobial therapy of burns

Mafenide acetate, bacitracin, mupirocin, neomycin, polymyxin B, nitrofurazone, nystatin.

Fig. 8. Use of silver as a topical antimicrobial agent in burns

Silver has multiple mechanisms of action as an antimicrobial agent including permeabilization of bacterial cell walls and generation of intracellular reactive oxygen species.

Fig. 9. Concept of antimicrobial photodynamic therapy

The photosensitizer is excited to its triplet state by light of the correct wavelength and then transfers its energy to molecular oxygen forming reactive oxygen species that are able to kill all classes of pathogenic micro-organisms.

Fig. 10. Chemical structures of photosensitizers used in antimicrobial applications

These molecules possess either constitutive cationic charges or basic amino groups. Toluidine blue O (TBO), cationic fullerene (BF6), cationic porphyrin (XF73), meso-mono-phenyl-tri(N-methyl-4-pyridyl)-porphyrin (Sylsens B), polyethylenimine chlorin(e6) conjugate (PEI-ce6), cationic phthalocyanine (RLP068), selenium benzo(A)phenoxazinium chloride (EtNBSe).

Fig. 11. Use of PDT to treat mouse burn infection caused by bioluminescent A. baumannii

Bacterial luminescence of a representative mouse burn infected with A. baumannii and treated with PDT at 30 minutes after infection (top row), a mouse burn treated with PS only (dark control, middle row), and a mouse burn treated with light only (light control, bottom row). After 240 J/cm² red light had been delivered (40 minutes irradiation time at an irradiance of 100 mW/cm²), PDT induced an approximately 3-log-unit reduction in bacterial luminescence from the mouse burn, while during the same period of time, only modest reduction of bacterial luminescence was observed in the dark control and the light control.

Fig. 12. Chemical structure of chitin and chitosan

Treatment of chitin (poly-N-acetyl glucosamine) with hot sodium hydroxide partially hydrolyzes the amide bonds to form chitosan (poly-glucosamine).

Fig. 13. Use of a chitosan acetate bandage to treat mouse burn infection

The successive bioluminescence images from day 0 to day 3 after P. aeruginosa infection of an untreated burn (top row), a silver dressing-treated burn (middle row), and a chitosan acetate-treated burn (bottom row).

Fig. 14. Antimicrobial action of AMPs

Schematic illustration of the selectivity exhibited by antimicrobial peptides that allow them to lyse microbial cell membranes but not those of host cells.