Pseudomonas infections in the thermally injured patient

Pseudomonas aeruginosa lipopolysaccharide: a major virulence factor, initiator of inflammation and target for effective immunity

Pseudomonas aeruginosa is one of the most important bacterial pathogens encountered by immunocompromised hosts and patients with cystic fibrosis (CF), and the lipopolysaccharide (LPS) elaborated by this organism is a key factor in virulence as well as both innate and acquired host responses to infection. The molecule has a fair degree of heterogeneity in its lipid A and O-antigen structure, and elaborates two different outer-core glycoforms, of which only one is ligated to the O-antigen. A close relatedness between the chemical structures and genes encoding biosynthetic enzymes has been established, with 11 major O-antigen groups identified. The lipid A can be variably penta-, hexa- or hepta-acylated, and these isoforms have differing potencies when activating host innate immunity via binding to Toll-like receptor 4 (TLR4). The O-antigen is a major target for protective immunity as evidenced by numerous animal studies, but attempts, to date, to produce a human vaccine targeting these epitopes have not been successful. Newer strategies employing live attenuated P. aeruginosa, or heterologous attenuated bacteria expressing P. aeruginosa O-antigens are potential means to solve some of the existing problems related to making a P. aeruginosa LPS-specific vaccine. Overall, there is now a large amount of information available about the genes and enzymes needed to produce the P. aeruginosa LPS, detailed chemical structures have been determined for the major O-antigens, and significant biologic and immunologic studies have been conducted to define the role of this molecule in virulence and immunity to P. aeruginosa infection.

Comparative evaluation of surface swab and quantitative full thickness wound biopsy culture in burn patients

A total of 100 cases of burn were examined and screened bacteriologically for evidence of infection by surface swab culture, quantitative full thickness punch biopsy culture and blood culture. Gram negative organisms predominates the gram positive ones. Surface swab was found to correlate well with the biopsy as far the identification of causative organism is concerned. However, the latter technique was found to be more valuable as it also gives the critical load (>10(5)CFU/g of tissue) of the organism beyond which metastatic invasion of the organism takes place (p<0.01), thus obviating the repeated need for blood culture in burn patients.

Differences in pain patterns for infected and noninfected patients with burn injuries

The management of pain is a primary issue in burn care. Patients hospitalized for burn injuries experience severe pain on a daily basis, immediately after the injury and during the healing of the burn wound. Our clinical experience is that the intensity of pain is increased by wound infection. The purpose of this study was to investigate retrospectively whether patients experience increased pain intensity in conjunction with wound infection. A total of 165 patients with burn injuries were included, 60 of whom were diagnosed with infection. The results of this study showed a significant increase in pain intensity in association with infection. An increase in pain is one of the factors to be considered among the many assessments, tests, and treatments for patients with burn injuries.

Bacterial biofilms within the clinical setting: what healthcare professionals should know

Bacterial biofilm formation is the prevailing microbial lifestyle in natural and manmade environments and occurs on all surface types. Biofilm formation develops in several phases and is influenced by various parameters, both environmental and inherent to the attaching cell. Biofilms also serve as protective niches for particular pathogens when outside a host. Although it is accepted that biofilms are ubiquitous in nature, the significance of biofilms in clinical settings, especially with regard to their role in medical-related infections, is often underestimated. It has been found that several aspects of human pathogenesis within a clinical context are directly related to biofilm development. Various types of surfaces in clinical settings are prone to biofilm development and an increased risk of disease may be a direct consequence of their formation. This review describes the process of biofilm formation, highlights the importance of bacterial associations with surfaces in clinical settings and describes various methods for biofilm visualization and control.

Burn wound infections

Burns are one of the most common and devastating forms of trauma. Patients with serious thermal injury require immediate specialized care in order to minimize morbidity and mortality. Significant thermal injuries induce a state of immunosuppression that predisposes burn patients to infectious complications. A current summary of the classifications of burn wound infections, including their diagnosis, treatment, and prevention, is given. Early excision of the eschar has substantially decreased the incidence of invasive burn wound infection and secondary sepsis, but most deaths in severely burn-injured patients are still due to burn wound sepsis or complications due to inhalation injury. Burn patients are also at risk for developing sepsis secondary to pneumonia, catheter-related infections, and suppurative thrombophlebitis. The introduction of silver-impregnated devices (e.g., central lines and Foley urinary catheters) may reduce the incidence of nosocomial infections due to prolonged placement of these devices. Improved outcomes for severely burned patients have been attributed to medical advances in fluid resuscitation, nutritional support, pulmonary and burn wound care, and infection control practices.

Burn wound infections

Burns are one of the most common and devastating forms of trauma. Patients with serious thermal injury require immediate specialized care in order to minimize morbidity and mortality. Significant thermal injuries induce a state of immunosuppression that predisposes burn patients to infectious complications. A current summary of the classifications of burn wound infections, including their diagnosis, treatment, and prevention, is given. Early excision of the eschar has substantially decreased the incidence of invasive burn wound infection and secondary sepsis, but most deaths in severely burn-injured patients are still due to burn wound sepsis or complications due to inhalation injury. Burn patients are also at risk for developing sepsis secondary to pneumonia, catheter-related infections, and suppurative thrombophlebitis. The introduction of silver-impregnated devices (e.g., central lines and Foley urinary catheters) may reduce the incidence of nosocomial infections due to prolonged placement of these devices. Improved outcomes for severely burned patients have been attributed to medical advances in fluid resuscitation, nutritional support, pulmonary and burn wound care, and infection control practices.

Deficiency of mannose-binding lectin greatly increases susceptibility to postburn infection with Pseudomonas aeruginosa

Burn injury disrupts the mechanical and biological barrier that the skin presents against infection by symbionts like the Pseudomonas aeruginosa, a Gram-negative bacteria. A combination of local factors, antimicrobial peptides, and resident effector cells form the initial response to mechanical injury of the skin. This activity is followed by an inflammatory response that includes influx of phagocytes and serum factors, such as complement and mannose-binding lectin (MBL), which is a broad-spectrum pattern recognition molecule that plays a key role in innate immunity. A growing consensus from studies in humans and mice suggests that lack of MBL together with other comorbid factors predisposes the host to infection. In this study we examined whether MBL deficiency increases the risk of P. aeruginosa infection in a burned host. We found that both wild-type and MBL null mice were resistant to a 5% total body surface area burn alone or s.c. infection with P. aeruginosa alone. However, when mice were burned then inoculated s.c. with P. aeruginosa at the burn site, all MBL null mice died by 42 h from septicemia, whereas only one-third of wild-type mice succumbed (p = 0.0005). This result indicates that MBL plays a key role in containing and preventing a systemic spread of P. aeruginosa infection following burn injury and suggests that MBL deficiency in humans maybe a premorbid variable in the predisposition to infection in burn victims.

CARDIOMYOCYTE FUNCTION AFTER BURN INJURY AND LIPOPOLYSACCHARIDE EXPOSURE: SINGLE-CELL CONTRACTION ANALYSIS AND CYTOKINE SECRETION PROFILE

A component of multiorgan dysfunction in burned patients is heart failure. Burn trauma induces cytokine synthesis of interleukin (IL) 1beta, IL-6, and tumor necrosis factor alpha (TNF-alpha) which can negatively impact cardiac function. Infectious complications are common following severe burn injury. We hypothesized that burn injury and lipopolysaccharide (LPS) exposure independently influence peak cardiomyocyte contraction and cytokine secretion. Rats underwent a full-thickness 30% total body surface area scald or sham burn. At 1, 6, 12, and 24 h after burn, cardiomyocytes were isolated and incubated with increasing LPS doses. Peak sarcomere shortening and contractile velocity parameters were recorded using a variable-rate video camera with sarcomere length detection software. Supernatants were assayed for IL-1beta, IL-6, and TNF-alpha by ELISA. Peak sarcomere shortening was decreased in the burn group at 1, 6, 12, and 24 h after burn. IL-1beta, IL-6, and TNF-alpha levels were increased in cardiomyocytes isolated 1 h after burn compared with sham controls, but returned to sham levels at 6, 12, and 24 h after burn. LPS exposure caused dose-dependent decreases in sarcomere shortening in sham and burn animals. LPS exposure did not produce increased cardiomyocyte cytokine expression. Burn injury diminished peak sarcomere shortening. Whereas exposure to LPS did not have an effect on cardiomyocyte cytokine expression, LPS significantly inhibited sarcomere shortening in a dose-dependent fashion. Combined burn and LPS exposure inhibited sarcomere shortening more than each alone. These results demonstrate that LPS exposure and burn injury independently decrease peak cardiac shortening. These decreases did not directly correlate with the levels of cytokines released in response to each stressor.

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.

Endotoxin priming improves clearance of Pseudomonas aeruginosa in wild-type and interleukin-10 knockout mice

Endotoxin (lipopolysaccharide [LPS]) tolerance is an altered state of immunity caused by prior exposure to LPS, in which production of many cytokines, including gamma interferon (IFN-gamma) and interleukin-12 (IL-12), are reduced but secretion of the anti-inflammatory cytokine IL-10 is increased in response to a subsequent LPS challenge. This pattern of cytokine production is also characteristic of postinflammatory immunosuppression. Therefore, we hypothesized that LPS-primed mice would exhibit an impaired ability to respond to systemic infection with the opportunistic pathogen Pseudomonas aeruginosa. We further hypothesized that depletion of IL-10 would reverse the endotoxin-tolerant state. To test this hypothesis, systemic clearance of Pseudomonas aeruginosa was measured for LPS-primed wild-type and IL-10-deficient mice. LPS-primed wild-type mice exhibited significant suppression of LPS-induced IFN-gamma and IL-12 but increased IL-10 production in blood and spleen compared to levels exhibited by saline-primed wild-type mice. The suppressed production of IFN-gamma and IL-12 caused by LPS priming was ablated in the spleens, but not blood, of IL-10 knockout mice. LPS-primed wild-type mice cleared Pseudomonas aeruginosa from lungs and blood more effectively than saline-primed mice. LPS-primed IL-10-deficient mice were particularly efficient in clearing Pseudomonas aeruginosa after systemic challenge. These studies show that induction of LPS tolerance enhanced systemic clearance of Pseudomonas aeruginosa and that this effect was augmented by neutralization of IL-10.

INFLAMMATORY/CARDIOVASCULAR-METABOLIC RESPONSES IN A RAT MODEL OF BURN INJURY WITH SUPERIMPOSED INFECTION

Infection remains the major cause of morbidity and mortality in burn patients. Furthermore, the use of antibiotics in such patients has led to the prevalence of antibiotic-resistant microbial infections; one such infection in intensive care unit turns out to be caused by the enterococcal organisms. Our laboratory studies have used a rat model of bum injury and Enterococcus faecalis infection. Sprague-Dawley male rats ( approximately 250 g) were initially given an intragastric gavage of the antibiotic ciprofloxacin for 3 days. This procedure allowed for decontamination of intestine of gram-negative and some gram-positive organisms. The remainders of the gram-positive organisms were, to a large extent, Enterococci. After the decontamination procedure, rats were intra-abdominally inoculated with E. faecalis; inoculation involved preparation of sterilized rat fecal pellets impregnated with E. faecalis (10 colony-forming units) and their implants through a midline abdominal wall incision. Some of the rats that were implanted with the fecal pellets were subject to full-thickness skin bums ( approximately 30% total body surface area; 95 degrees C water for 7 s). Sham abdominal infection rats received a sterile pellet only; sham bum procedure consisted of exposing the skin to room temperature water. All sham and bum and/or E. faecalis infection procedures were carried out on rats under pentobarbital anesthesia. Inflammation and innate host defense-related responses were assessed via measurements of neutrophil effector responses, i.e., oxygen anion free radical (O2)/eIastase production, CD11b/CD18 expression, apoptosis, and tissue infiltration. Determining epithelial lactulose permeability, microvascular albumin leakage, and epithelial tight junction integrity assessed the status of intestinal function/structural derangements. The animals' metabolic and cardiovascular integrity was evaluated determining blood pH, p02, pC02, heart rate, respiratory rate, blood pressure, and cardiac output. Whereas the aforementioned measurements were carried out at 24 to 48 h postbum injury with and without the Enterococcal infection, animal mortality was determined for up to 5 days after the experimental injuries. The results of the studies indicated that whereas bum or E. faecalis infection alone did not produce significant mortality, the dual insult with bum and E. faecalis infection resulted in significant animal death accompanied by relatively more profound metabolic and cardiovascular derangements. Inappropriately heightened neutrophil effector responses were present with bum alone as well as with the dual bum and infection complications. These studies suggest that animal models of bum injury with Enterococcal infection complications simulate the adverse outcomes bum patients infected with Enterococcal organisms.

Effects of intratracheal administration of novispirin G10 on a rat model of mucoid Pseudomonas aeruginosa lung infection

Chronic Pseudomonas aeruginosa lung infection is a major problem for patients with cystic fibrosis (CF). The biofilm mode of growth of the pathogen makes it highly resistant to antibiotic treatment, and this is especially pronounced with mucoid strains. In this study, novispirin G10, a synthetic antimicrobial peptide patterned loosely on sheep myeloid antimicrobial peptide 29, was tested in a rat model of mucoid P. aeruginosa lung infection. P. aeruginosa NH57388A, a mucoid strain isolated from a CF patient, was mixed with the alginate produced by the bacterium itself and adjusted to a concentration of 10(10) CFU/ml. Each rat received 10(9) CFU of bacteria intratracheally in the left lung to establish lung infection. At 0 and 3 h post P. aeruginosa infection, the treated group of rats received novispirin G10 (0.1 mg/ml, 0.1 ml/rat) intratracheally, whereas the control group received vehicle treatment only. The animals were sacrificed on days 3, 5, 7, and 10 after challenge for evaluation of various parameters. On day 5, 50% of the rats in the treated group had cleared the bacteria from the lungs, whereas in the control group, none of the rats cleared the pathogen (P < 0.03). The average bacterial loads remaining in the lungs of treated rats on days 3 and 5 were more than 170- and 330-fold lower than in the control groups (P < 0.0005 and P < 0.0003). In accordance, the macroscopic and microscopic lung pathology was also significantly milder in the treated group compared to the control group (P < 0.0002). Lung cytokine responses in the treated group were significantly lower than in the control group. The results suggest that novispirin G10 might be useful in treating antibiotic-resistant P. aeruginosa lung infections.

The Bali burn disaster: implications and lessons learned

In October 2002, a terrorist attack on a nightclub in Bali resulted in an explosion and fire, causing the deaths of more than 200 people, including 88 Australian citizens. After first aid and primary care, the injured were repatriated to Darwin for triage and continued treatment and were then disseminated to various burn units throughout Australia. At the Repatriation General Hospital Concord Sydney, we received 12 patients with burns and a variety of blast injuries. Their treatment was complicated by infection with multiresistant organisms that were previously unseen in our unit and the presence of complex shrapnel wounds. There were no deaths and, with two exceptions, all patients were discharged within 6 weeks. This incident had profound effects on our unit, particularly related to the management of high-velocity shrapnel injuries, serious ongoing septic complications, and the psychological effects on both patients and staff, all of which are detailed and discussed.

Burn care standards in Israel: lack of consensus

In recent years, the need for a national burn center based on ABA guidelines has emerged in Israel. The formation of such a center is now underway in the Chaim Sheba Medical Center. As a first step in the standardization of burn care in Israel, we have conducted a nation-wide survey among burn care personnel (physicians, nurses and other burn team members), regarding different aspects of the treatment of burn patients.

METHODS

A questionnaire comprised of 30 questions regarding the severity of burns admitted, the site of initial management, wound care (both burn/skin-graft sites and donor sites), dressing changes protocols, sterility precautions, hydrotherapy, and pressure dressings was presented to 70 health-care professionals involved in the treatment of burns.

RESULTS AND DISCUSSION

Seventy-seven percent of interviewed personnel participated in the survey. Consensus was found regarding most local (topical) wound care, (SSD for clean non-facial burns, Sulfamylon (mafenide-acetate) for contaminated non-facial burns, Threolone (chloramphenicol 3% and prednisolone 0.5%) or Bacitracin for facial burns, Paraffin gauzes with or without Sulfamylon for donor and graft sites). Dressing changes regimes were also agreed upon generally. However, there was no consensus regarding the ideal time for the removal of donor site dressings and this issue will need to be resolved. Other important findings are that both Edinborough University Solution of Lime (EUSOL), which has been deemed unsuitable for burn treatment due to toxic effects, and hydrotherapy, which has been proposed as a source of infection and contamination, are still widely used. We anticipate that these issues will be settled in our unified national burn care protocols (which are currently under development and revision).

Passive anti-PcrV treatment protects burned mice against Pseudomonas aeruginosa challenge

The type III secretion system consists of secreted exoproducts and structural components, such as PcrV, and this system plays an important role in the virulence of Pseudomonas aeruginosa in burned hosts. The purpose of this study was to determine if passive anti-PcrV treatment would protect burned mice from fatal P. aeruginosa challenge, and to determine the type III exoproduct phenotype of the P. aeruginosa used as challenge strains. Antiserum was raised in rabbits. Mice were given a third degree burn, challenged with a lethal dose of P. aeruginosa, and treated with either anti-PcrV or control immunoglobulin intraperitoneally. Protection against three different pseudomonads was tested. Genotyping by PCR and phenotyping by immunoblots showed the P. aeruginosa strains to all be of the invasive type III phenotype: ExoS+ and/or ExoT+, ExoU-, ExoY+. Against all strains, the anti-PcrV treatment yielded significantly better survival (p<0.05) than the control immunoglobulin treatment. Duration of significant protection was improved by giving a second injection of PcrV antisera at 24h postburn. Hence, passive anti-PcrV immunization could protect burned mice against fatal challenge with P. aeruginosa of an invasive type III phenotype. This immunotherapy might be explored further as possible treatment for highly antibiotic resistant P. aeruginosa infections in burned hosts.

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

BACKGROUND

In this study, we investigated the ability of protamine sulfate, at sub-bactericidal dosing, to interfere with the in vivo virulence of Pseudomonas aeruginosa (PAO1) during burn wound infection.

MATERIALS AND METHODS

The study was conducted using the murine model of thermal injury. Preliminary experiments determined a protocol for administration of protamine sulfate that had no in vivo bactericidal effects. Based on this, the effect of local injection of protamine sulfate on the in vivo virulence of PAO1 was assessed using these parameters: (1) the percent mortality among PAO1-infected, thermally injured mice; (2) the local proliferation and spread of PAO1 within the infected burned tissue; (3) the systemic spread of PAO1 within thermally injured/infected mice; and (4) the local cytokine response elicited by PAO1 thermally injured/infected mice.

RESULTS

Injection of protamine sulfate into the thermally injured tissue of PAO1-infected/thermally injured mice significantly decreased the percent mortality and inhibited the systemic dissemination of PAO1 microorganisms to the liver and spleen. It had no effect, however, on the ability of the bacteria to proliferate and spread within the thermally injured tissue. It also was determined that protamine sulfate was ineffective at preventing mouse death at the dose administered if injected intramuscularly instead of directly into burned tissue. Protamine sulfate reduced the expression of the proinflammatory cytokines IL-6 and LIF in the injured/infected tissue. Heparan sulfate given in conjunction with protamine sulfate returned mortality levels to those of untreated mice.

CONCLUSIONS

Our results suggest that: (1) local injection of sub-bactericidal doses of protamine sulfate reduces the virulence of P. aeruginosa; (2) this effect is due to interference with the systemic rather than local spread of P. aeruginosa; and (3) local application of protamine sulfate may have potential as supportive therapy for prevention of systemic P. aeruginosa infection in severely burned patients.