Table 2. Suggested ABA consensus

At least 3 of the following parameters: r > 38.5 or < 36.5 ºC

r Progressive tachycardia > 90 bpm in adults or > 2 SD above age-specific norms in children

r Progressive tachypnea > 30 bpm in adults or > 2 SD above age-specific norms in children

r WBC > 12000 or < 4000 in adults or > 2 SD above age-specific norms in children

r Refractory hypotension: SBP < 90 mmHg, MAP < 70, or a SBP decrease > 40 mmHg in adult or < 2SD below normal for age in children

r Thrombocytopenia: platelet count < 100,000/μl in adults, < 2 SD below norms in children

r Hyperglycemia: plasma glucose > 110 mg/dl or 7.7 mM/l in the absence of diabetes

r Enteral feeding intolerance (residual > 150 ml/hr in children or 2 times feeding rate in adults; diarrhea

>2500 ml/day for adults or > 400 ml/day in children) 10

AND

r Pathologic tissue source identified: > 105 bacteria on

quantitative wound tissue biopsy or microbial invasion on biopsy.

Bacteremia or fungemia, or documented infection as defined by CDC

or black lesions in unburned tissue (ecthyma gangrenosum). When a patient exhibits signs and symptoms of sepsis, immediate institution of antibiotics is obligatory while awaiting confirmatory cultures. If a patient is already on antibiotics then the coverage should be broadened to include MRSA and Pseudomonas (i. e. Pip/Tazo plus Vancomycin in our institution). A delay of treatment for 3–4 days in a patient with a major burn results in an inordinately high morbidity. Specific antibiotic treatment should be administered if routine surveillance has identified the predominant organism. In addition to institution of antibiotic treatment, there should be a rapid aggressive attempt at source control.

Tight glycemic control is showing promise in decreasing infection rates in the intensive care unit. In a recent study in pediatric burn patients this finding has been upheld, is now being recommended and is an area of ongoing active research [19].

The microbiology of burn wound infection

Sources of organisms

Sources of organisms are found in the patient’s own endogenous (normal) flora, from exogenous sources in the environment, and from healthcare personnel [20, 21]. Exogenous organisms from the hospital environment are generally more resistant to antimicrobial agents than endogenous organisms. Organisms associated with infection in burn patients include gram-positive, gram-negative, and viral and yeast/ fungal organisms. The distribution of organisms changes over time in the individual patient and such changes can be ameliorated with appropriate management of the burn wound and patient.

The typical burn wound is initially colonized predominantly with gram-positive organisms, which are fairly quickly replaced by antibiotic-susceptible gram-negative organisms, usually within a week after the burn trauma. This wound flora may be replaced by yeasts, fungi and antibiotic-resistant bacteria, due to treatment with broad-spectrum antibiotics.

Gram-positive organisms

Gram-positive organisms of particular concern include methicillin-resistant Staphylococcus aureus (MRSA), enterococci, group A b-hemolytic Streptococcus and coagulase negative Staphylococcus.

Staphylococci

Staphylococcus aureus is the most frequently isolated pathogen from infected burn wounds and a leading cause for morbidity and mortality post burn [20, 21] Strains of S. aureus as well as other Staphylococcus species produce a wide variety of metabolites. Some are pathognomonic and also toxigenic, while those with minimal toxicity or no toxic effects at all, are of some diagnostic significance. An array of biproducts such as proteinases, collagenases and hyaluronidase digest the extra-cellular matrix, which serves as the structural integrity essential in wound healing [20]. Most of the human pathogens produce

a- and ß-lysins. Some exotoxins, which are produced by the pathogenic strains of staphylococci, include a pyrogenic toxin, a dermo-necrotizing toxin, a lethal toxin, and leukocidin. These organisms can also produce an exotoxin, TSST-1 and enterotoxins A, B and C, which are risk factors for toxic shock syndrome in susceptible patients [20]. Toxic shock syndrome (TSS) was first described in 1978. The disease is characterized by sudden onset of fever, vomiting, diarrhea, shock, and a diffuse macular erythematous rash, followed by desquamation of the skin on the hand and feet as well as hyperemia of various mucous membranes. However, the role of TSS has not been completely elucidated in the burn patient. It has been our experience that while burn patients may be infected with a TSS potential S. aureus, no other serious or untoward complications have been observed. In fact treatment of their burn wounds was not any different than those patients infected with a nontoxic TSS S. aureus.

Another member of the Staphylococci genus, Staphylococcus epidermidis, is a resident of human skin and mucous membranes therefore also frequently associated with burn wound infection. S. epidermidis resembles S. aureus microscopically, and is tolerant to high NaCl concentrations as is S. aureus. Unlike S. aureus it is mannitol-, coagulaseand thermonuclease-negative. It is as pathogenic as S. aureus, and is the main pathogen for injuries such as subacute bacterial endocarditis or infected surgical prosthesis.

Streptococci

Streptococci are catalase-negative and produce a variety of hemolytic activity in the presence of blood. Types of hemolysis produced on SBA are frequently used as an initial method to identify streptococci. ß- hemolytic streptococci, such as streptococcus pyogenes, ß-hemolytic streptococci group A and S. agalactiae ß-hemolytic streptococci group B are very virulent even in low concentrations and may lead to wound infection, failure of a primary closure, and loss of a skin graft. Enterococcus faecalis and Enterococcus faecium were first discovered in 1984 [22]. Since then 12 species have been classified within the Enterococcus group [22]. Due to their resistance third-generation cephalosporins enterococci present

today one of the most common pathogens responsible for burn wound infection. Enterococci are divided into three main groups based on specific biochemical reactions. Group I does not hydrolyze arginine but forms acid in a manitol, sorbitol and sorbose broth. The most common pathogen within this group is Enterococcus avium. E. faecalis and E. faecium, agents of Group II, hydrolyzes arginine and forms acid in mannitol and sorbitol broth only. Group III, including Enterococcus durans, is negative for all tests described above [22].

Gram-negative organisms

The family of gram-negative organisms contains a distinctive group of etiologic pathogens commonly associated with burn wound infections. Pseudomonas aeruginosa was discovered 1960 and is the second most common pathogen within this group frequently associated with burn wound infections [21]. Because of its ability to survive in aqueous environments, these organisms have been become problematic in the hospital environment. Infections caused by this agent range from superficial skin infections to fulminant sepsis. Pseudomonas aeruginosa is the leading cause of nosocomial respiratory tract infection. Patients receiving ventilator assistance in the ICU have a 20-fold higher likelihood of developing nosocomial pneumonia caused by Pseudomonas aeruginosa. Wound infections due to Pseudomonas aeruginosa are particularly troublesome in burn patients. Even though the incidence of such infections has declined within this particular patient population, the high rate of sepsis following these wound infections is still strongly associated with the incidence of mortality in severely burned patients [22].

Acinetobacter sp. is part of the resident flora of the respiratory, skin, as well as the gastrointestinal and genitourinary tracts. These pathogens are frequently isolated from a diversity of clinical sources, including upper and lower respiratory tracts, urinary tract, surgical and burn wounds, and in bacteremia secondary to IV catheterization. Also occasional cases of septicemia or pneumonia have occurred in attenuated patients. Many of these patients who develop infections with this microorganism have had various manipulations including the use of respira-

tory therapy equipment, tracheal intubation, bladder or central venous line catheterization.

The most common and largest group of microorganisms, populating the burn wound environment along with the Staphylococcus sp. are Enterobacteriaceae. 12 sub-species have been identified within this family so far, including Klebsiella, Serratia marcescens, Providencia species and Erwinia species [8, 11, 23].

Eschericia coli, probably the most well known enteric pathogen, has been responsible for a wide diversity of infectious processes, such as appendicitis, cystitis, peritonitis, septicemia, and cholecystitis as well as surgical and burn wound sepsis and epidemic diarrhea. Eschericia coli can be differentiated from other members of Enterobacteriacae by its response to the classical IMVIC (indole, methyl red, VogesProskauer and citrate) reaction.

The Klebsiella-Enterobacter group (K-E) consists of gram-negative organisms that are either motile or non-motile. Fifty-one percent of moderately ill hospitalized patients were infected with either K. pneumoniae, Enterobacter aerogenes or Enterobacter cloacae isolated from their oropharyngeal space [1]. K. pneumoniae and are now considered a common pathogen causing nosocomial infections [22].

The remaining groups of enterics consist of three additional species: Proteus, Providencia, and Morganella, the latter frequently associated with wound infections or urinary tract infections. Proteus mirabilis and Proteus vulgaris are both found in massive concentrations in feces of patients receiving oral antibiotic therapy. Both pathogens are commonly leading to surgical and burn wound infections, intraabdominal infections, as well as bacteremia and urinary tract infections.

Anaerobes

The most repeatedly confronted organisms in this group, which may play a fearful role in surgical and burn wound infections are Bacteroides spp. and Fusobacterium spp. They are considered normal flora of the human body, beginning at the oropharyngeal cavity and ending at the gastrointestinal (GI) and urogenital tracts. Numerically they account for the major population in the oropharyngeal region in a 5:1 ratio over the aerobes and facultative anaerobes,

while in the urogenital and GI tract the ratio is more dynamic at a 1000:1 [22, 24]. Examining the current statistics of anaerobic infections related to locality, all but 2–5% of surgical wound infections in the oropharyngeal area are caused by the anaerobic flora [25, 26]. Those occurring in the GI and urogenital tract are only responsible for about 10–15% of the wound infections [27, 28]. Prior surgery, malignant neoplasms, arteriosclerosis, diabetes mellitus, prior antibiotic therapy, alcoholism, improper debridement, and steroid and immunosuppressive therapy are commonly major contributors associated in these types of wound infections [26, 27]. Specimens collected for anaerobic organisms should be placed in appropriate transport tubes void of atmospheric O2 or remain in syringes with attached needles containing suspect aspirates sealed off with a rubber stopper. However, with the advent of the early excision and grafting, the incidence of anaerobic infections in the thermal injury has been significantly reduced; therefore anaerobic cultures are not economical. Culturing for these anaerobes would be futile, as they require vascular tissue for survival. Their presence would only be observed if such tissue were to remain.

Anaerobic infections in burned patients are usually associated with avascular muscle found in electrical injuries, frostbite, or cutaneous flame burns with concomitant crush-type injuries.

Fungi

Until the advent of topical antimicrobial agents, fungal infections were not common in burned patients. However, the incidence of mycotic invasion has doubled since the implementation of topical antimicrobial agents to control bacterial colonization. The burn wound is the most commonly infected site, although local or disseminated fungal infections of the respiratory tract, urinary tract, GI tract, and vagina are increasingly common [29].

Fungi are generally isolated through the inoculation of both non-selective and selective culture media. The most commonly used non-selective media includes inhibitory mold agar and SABHI agar, which permit the enhanced growth of almost all fungi, including some of the more fastidious and slowly growing fungi. The most commonly utilized selective

media include those containing antimicrobial agents such as penicillin (20 U/ml) plus streptomycin (40 U/ml) or gentamicin (5 μg/ml) plus chloramphenicol (16 μg/ml) to inhibit growth of bacteria. Media containing cyclohexamide (0.5 μg/ml) may also be utilized to inhibit growth of rapidly growing moulds that often overgrow slower growing dimorphic fungi; however, a medium without cyclohexamide should also be used, because this agent can inhibit the growth of some medically important fungi (e. g.

Cryptococcus neoformans and Aspergillus fumigatus). Additionally, Sabouraud’s dextrose or potato dextrose agar is used to allow for identification of fungi through their sporulation characteristics. Yeasts are identified on the basis of specific biochemical tests, whereas identification of moulds is based on growth rate, colony structure, microscopic appearance, dimorphism at different incubation temperatures, inhibition of growth by cyclohexamide, and a few biochemical tests [26].

Candida sp. are the most common nonbacterial colonizers of the burn wound, although true fungi such as Aspergillus, Penicillium, Rhizopus, Mucor, Rhizomucor, Fusarium, and Curvularia are not uncommon and have a much greater invasive potential than the yeasts [25].

Early diagnosis of fungal infection is difficult as clinical symptoms frequently mimic low-grade bacterial infections. Routine culture techniques may require 7 to 14 days for identifying fungal contaminants, with resultant delay in the initiation of treatment as these pathogens are frequently not recovered in culture [25]. In contrast to bacterial sepsis, venous blood cultures may not reflect the causative organism. Arterial blood cultures and retinal examination for characteristic candida lesions can be useful.

Unlike candida infections, true fungal infections occur early in the hospital course of patients with specific predisposing characteristics [30]. Most frequently, burned patients infected with fungi are exposed to spores in the environment by either rolling on the ground or jumping into surface water at the time of injury. Other environmental foci have been cited as the source of nosocomial fungal infection, including bandaging supplies left open to air, heating, and air conditioning ducts and floor drains [25]. Once colonized, broad non-branching hyphae ex-

tend into subcutaneous tissue, stimulating an inflammatory response. This phenomenon is diagnostic of fungal wound infection. Vascular invasion is common and often accompanied by thrombosis and avascular necrosis, clinically observed as rapidly advancing dark discolorations of the wound margins or well-described lesions. Systemic dissemination of the infection occurs with invasion of the vasculature.

Infection control

The microorganisms initially populating the burned wound represent a mixture of endogenous resident flora and airborne contaminants seeded by contact with the environment and attending personnel. Burn patients are immunosuppressed and should be protected from exposure to environmental contaminants. The most effective means of decreasing exposure of burned patients to exogenous bacteria is strict observation of appropriate hand washing among the health care providers. Face masks waterproof gowns and gloves should be worn whenever direct contact with body fluids and wound exudates are unavoidable, thus protecting both the patient and the health care provider from inadvertent contamination. All dressing materials should be maintained as patient specific. IV pumps and poles, blood pressure devices, monitoring equipment, bedside tables and beds should be cleaned on at least a daily basis with antibacterial solutions. Many items such as blood pressure cuffs, stethoscopes, bedpans, if used on areas without dry, occlusive dressings, may need high-level disinfection as a semi-critical item or may need to be restricted to an individual patient.

Plants and flowers should not be allowed in units with burn patients because they harbor gram-nega- tive organisms, such as Pseudomonas species, other enteric gram-negative organisms, and fungi. Many of these organisms are intrinsically resistant to multiple antibiotics, which may serve as reservoirs to colonize the burn wound [31]. Pediatric burn patients should also have policies restricting the presence of non-washable toys such as stuffed animals and cloth objects. These can harbor large numbers of bacteria and are difficult to disinfect. Toys should be nonporous and washable, designated for individual patient use, and thoroughly disinfected after use and before being given to another child to use. Paper