Protamine sulfate reduces the susceptibility of thermally injured mice to Pseudomonas aeruginosa infection

Glycobiology of syndecan-1 in bacterial infections

Syndecan-1 (Sdc1) is a major cell surface heparan sulfate (HS) proteoglycan of epithelial cells, a cell type targeted by many bacterial pathogens early in their pathogenesis. Loss of Sdc1 in mice is a gain-of-function mutation that significantly decreases the susceptibility to several bacterial infections, suggesting that subversion of Sdc1 is an important virulence strategy. HS glycosaminoglycan (GAG) chains of cell surface Sdc1 promote bacterial pathogenesis by facilitating the attachment of bacteria to host cells. Engagement of cell surface Sdc1 HS chains by bacterial adhesins transmits signal through the highly conserved Sdc1 cytoplasmic domain, which can lead to uptake of intracellular bacterial pathogens. On the other hand, several bacteria that do not require Sdc1 for their attachment and invasion stimulate Sdc1 shedding and exploit the capacity of Sdc1 ectodomain HS GAGs to disarm innate defense mechanisms to evade immune clearance. Recent data suggest that select HS sulfate motifs, and not the overall charge of HS, are important in the inhibition of innate immune mechanisms. Here, we discuss several examples of Sdc1 subversion in bacterial infections.

Heparinase Is Essential for Pseudomonas aeruginosa Virulence during Thermal Injury and Infection

The opportunistic pathogen Pseudomonas aeruginosa is a major cause of sepsis in severely burned patients. If it is not eradicated from the wound, it translocates to the bloodstream, causing sepsis, multiorgan failure, and death. We recently described the P. aeruginosa heparinase-encoding gene, hepP, whose expression was significantly enhanced when P. aeruginosa strain UCBPP_PA14 (PA14) was grown in whole blood from severely burned patients. Further analysis demonstrated that hepP contributed to the in vivo virulence of PA14 in the Caenorhabditis elegans model. In this study, we utilized the murine model of thermal injury to examine the contribution of hepP to the pathogenesis of P. aeruginosa during burn wound infection. Mutation of hepP reduced the rate of mortality from 100% for mice infected with PA14 to 7% for mice infected with PA14::hepP While comparable numbers of PA14 and PA14::hepP bacteria were recovered from infected skin, only PA14 was recovered from the livers and spleens of infected mice. Despite its inability to spread systemically, PA14::hepP formed perivascular cuffs around the blood vessels within the skin of the thermally injured/infected mice. Intraperitoneal inoculation of the thermally injured mice, bypassing the need for translocation, produced similar results. The rate of mortality for mice infected with PA14::hepP was 0%, whereas it was 66% for mice infected with PA14. As before, only PA14 was recovered from the livers and spleens of infected mice. These results suggest that hepP plays a crucial role in the pathogenesis of PA14 during burn wound infection, most likely by contributing to PA14 survival in the bloodstream of the thermally injured mouse during sepsis.

Detection of Pseudomonas aeruginosa biomarkers from thermally injured mice in situ using imaging mass spectrometry

Monitoring patients with burn wounds for infection is standard practice because failure to rapidly and specifically identify a pathogen can result in poor clinical outcomes, including death. Therefore, a method that facilitates detection and identification of pathogens in situ within minutes of biopsy would be a significant benefit to clinicians. Mass spectrometry is rapidly becoming a standard tool in clinical settings, capable of identifying specific pathogens from complex samples. Imaging mass spectrometry (IMS) expands the information content by enabling spatial resolution of biomarkers in tissue samples as in histology, without the need for specific stains/antibodies. Herein, a murine model of thermal injury was used to study infection of burn tissue by Pseudomonas aeruginosa. This is the first use of IMS to detect P. aeruginosa infection in situ from thermally injured tissue. Multiple molecular features could be spatially resolved to infected or uninfected tissue. This demonstrates the potential use of IMS in a clinical setting to aid doctors in identifying both presence and species of pathogens in tissue.

Gallium maltolate treatment eradicates Pseudomonas aeruginosa infection in thermally injured mice

Gallium (Ga) is a semimetallic element that has demonstrated therapeutic and diagnostic-imaging potential in a number of disease settings, including cancer and infectious diseases. Gallium's biological actions stem from its ionic radius being almost the same as that of ferric iron (Fe(3+)), whereby it can replace iron (Fe) in Fe(3+)-dependent biological systems, such as bacterial and mammalian Fe transporters and Fe(3+)-containing enzymes. Unlike Fe(3+), ionic gallium (Ga(3+)) cannot be reduced, and when incorporated, it inactivates Fe(3+)-dependent reduction and oxidation processes that are necessary for bacterial and mammalian cell proliferation. Most pathogenic bacteria require Fe for growth and function, and the availability of Fe in the host or environment can greatly enhance virulence. We examined whether gallium maltolate (GaM), a novel formulation of Ga, had antibacterial activity in a thermally injured acute infection mouse model. Dose-response studies indicated that a GaM dose as low as 25 mg/kg of body weight delivered subcutaneously was sufficient to provide 100% survival in a lethal P. aeruginosa-infected thermally injured mouse model. Mice treated with 100 mg/kg GaM had undetectable levels of Pseudomonas aeruginosa in their wounds, livers, and spleens, while the wounds of untreated mice were colonized with over 10(8) P. aeruginosa CFU/g of tissue and their livers and spleens were colonized with over 10(5) P. aeruginosa CFU/g of tissue. GaM also significantly reduced the colonization of Staphylococcus aureus and Acinetobacter baumannii in the wounds of thermally injured mice. Furthermore, GaM was also therapeutically effective in preventing preestablished P. aeruginosa infections at the site of the injury from spreading systemically. Taken together, our data suggest that GaM is potentially a novel antibacterial agent for the prevention and treatment of wound infections following thermal injury.

Topical p38 MAPK inhibition reduces bacterial growth in an in vivo burn wound model

BACKGROUND

Although the inflammatory response is a prerequisite for wound healing, excessive activation of the innate immune system can induce epithelial cell damage and apoptosis, which may further compromise dermal integrity. In a noninfectious burn wound model, we previously demonstrated that topical inhibition of p38 MAPK, an important inflammatory signaling pathway, attenuated epithelial cell damage and apoptosis. We now question whether attenuating local inflammation would weaken bacterial wound resistance and compromise host defense.

METHODS

Rats received 30% total body surface area burn, and the wound was treated with topical application of a p38 MAPK inhibitor or vehicle. At 24 hours after injury, burn wounds were inoculated with Pseudomonas aeruginosa. At 48 hours postinjury, animals were sacrificed, and the burn wound was analyzed.

RESULTS

Inoculating burn wounds induced significant bacterial growth. Dermal inflammatory changes were markedly accentuated in the inoculated animals. Topical p38 MAPK inhibition reduced the proinflammatory cytokine expression in the burn wounds and neutrophil sequestration with or without bacterial inoculation. Interestingly, the bacterial wound growth was significantly attenuated in animals treated with topical p38 MAPK inhibitor.

CONCLUSIONS

Topical p38 MAPK inhibition attenuated wound inflammation without interfering with bacterial host defense. Attenuation of excessive burn wound inflammatory signaling may prevent secondary damage of the dermal barrier and reduce the growth of opportunistic pathogens.

Syndecan 1 shedding contributes to Pseudomonas aeruginosa sepsis

The innate immune system is comprised of many components that function coordinately to prevent bacterial sepsis. However, thermal injury suppresses many of these factors, and the opportunistic pathogen Pseudomonas aeruginosa takes advantage of this condition, making it one of the leading causes of morbidity and mortality in the setting of thermal injury. P. aeruginosa is extremely efficient at colonizing burn wounds, spreading systemically, and causing sepsis, which often results in a systemic inflammatory response, multiple-organ failure, and death. The pathogenicity of P. aeruginosa is due to the arsenal of virulence factors produced by the pathogen and the immunocompromised state of the host. Syndecan 1 is a major heparan sulfate proteoglycan present on many host cells involved in thermal injury. Syndecan 1 anchored to the cell surface can be cleaved in a process termed ectodomain shedding. Syndecan 1 shedding results in the release of intact, soluble proteoglycan ectodomains that have diverse roles in innate immunity. Here we show for the first time that thermal injury results in shedding of syndecan 1 from host tissue. Our data show that syndecan 1 null mice are significantly less susceptible to P. aeruginosa infection than their wild-type counterparts, as demonstrated by (i) significantly lower mortality; (ii) absence of systemic spread of P. aeruginosa; and (iii) significant reductions in some proinflammatory cytokines. These results suggest that shed syndecan 1 plays an important role in the pathogenesis of P. aeruginosa infection of thermal injury and that syndecan 1-neutralizing agents may be effective supplements to current P. aeruginosa treatments.