A comparison of methods for the quantitation of bacteria in burn wounds. I. Experimental evaluation

Sample procurement for cultures of infected wounds: a systematic review

OBJECTIVE

Identifying the responsible pathogen is required in order to select optimum antimicrobial therapy for infected wounds, but the best techniques to use remain the subject of debate. Our aim was to assess the evidence on the value of wound swabs compared with biopsies.

METHOD

We conducted a systematic literature review with the terms 'wound infection', 'wound swab', 'wound swab technique', 'wound biopsy', 'wound culture', 'wound swab comparison', 'Levine swab technique', 'microbiological technique', 'specimen handling', 'bacterial load', 'perioperative care', 'swab', and 'culture'. We examined yields in identifying relevant pathogens, summarised salient features of qualifying studies, and defined knowledge gaps and endpoints that future studies should address.

RESULTS

Studies have been inconsistent, lacking specificity regarding wound types, clinical features, and sampling methods. We found moderate quality evidence that punch biopsies provide qualitative and quantitative information about the bacterial load and tissue reaction with nearly 100% sensitivity, 90% specificity and 95% accuracy for predicting wound closure. Biopsies are relatively invasive, costly, require skilled operators, and potentially exacerbate infection. Needle aspiration samples a limited portion and may enter uninfected tissue and extend infection. Wound swabs are minimally invasive, easier to perform and widely employed in clinical practice, but techniques vary. In comparative studies, the Levine technique was superior to the Z-swab techniques, and biopsies were more sensitive for antibiotic-resistant wounds than Levine or Levine-like swabs, suggesting that swabs may be useful for initial wound monitoring, but biopsies are preferred when antibiotic resistance is suspected.

CONCLUSION

The Levine swab is superior to the Z-swab technique and may be useful for initial wound monitoring, but quantitative biopsies are preferred for evaluation of antibiotic-resistant wounds and to monitor the response to treatment. There is limited evidence on the role of wound swabs for detecting wound colonisation versus infection and the impact of culture-guided therapy on such clinical outcomes as eradication of infection and accelerated healing. Future studies should specify patient populations, wound types, sampling protocols, and outcomes based on culture yield and treatment results, using rigorous statistical methodology.

Infection in patients with severe burns: causes and prevention thereof

The better understanding of burn pathophysiology has resulted in effective fluid resuscitation in the acute stage, but the morbidity and mortality of burn patients are mostly linked to the burn wound consequences. Once the initial acute phase is over, the burn wound becomes the source of virtually all ill effects, local and systemic. The dysfunction of the immune system, a large cutaneous bacterial load, the possibility of gastrointestinal bacterial translocation, prolonged hospitalization, and invasive diagnostic and therapeutic procedures all contribute to infectious complications. Wound infection may lead to septicemia that may not only consume additional resources but is associated with significant morbidity and mortality despite the advances in burn care.

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.

What's new in burn microbiology? James Laing Memorial Prize Essay 2000

A variety of factors contribute to the development of infection in burned patients. The role of wound management procedures, risk factors associated with infection, typical bacterial pathogens and associated exotoxins, current problems with antibiotic resistance, wound sampling and rare complications of infection are described. The use of new novel treatments that are currently being developed, such as cell signalling molecules and the increasing use of natural antimicrobial agents, for example honey, papaya fruit and tea-tree oil are discussed. The impact of new methods for earlier detection of infectious agents that could change future practices in burn care is also described.

Wound fluid bacterial levels exceed tissue bacterial counts in controlled porcine partial-thickness burn infections

In the present study, an established controlled burn wound model was used to test the hypothesis that controlled surface contamination with is capable of generating a noninvasive method for the creation of a reproducible deep tissue burn wound infection. Using a liquid tight-wound chamber in Yorkshire pigs, partial-thickness burns were inoculated with saline-immersed for 24 hours. Noninoculated burns and unwounded skin immersed in normal saline served as controls. Bacterial cultures of wound fluid were performed daily, and tissue biopsies for bacteriological and histological evaluations were performed on days 1, 3, and 5. was only recovered from -inoculated wounds (tissue and fluid), whereas all controls contained endogenous only. The number of colony-forming units per gram of wound tissue did not correlate with the bacterial counts found in the overlying wound fluid for any wounds. Fluid counts were consistently higher than tissue counts by two logs. -inoculated wounds showed three times deeper tissue destruction than control wounds. Obtaining consistently deep tissue colonization without cross-contamination among wounds, this study introduces a noninvasive model for controlled burn wound infection suitable for future investigations regarding the efficacy of topical antibiotic wound treatment in experimental burns.

Quantitative microbiology in the management of burn patients. I. Correlation between quantitative and qualitative burn wound biopsy culture and surface alginate swab culture

The diagnosis and treatment of burn wound infection is commonly determined by clinical impression and the qualitative results of surface swabs. It has been suggested that quantitative bacteriology from burn wound biopsies confirms burn wound infection and improves patient management. Methods for quantitating surface flora have been described, but comparisons with biopsy specimens have been contradictory. The quantitative and qualitative results of 141 pairs of biopsies and surface swabs, from 74 burn patients, were compared. Staph. aureus was the commonest organism isolated (29 per cent of biopsies and 35 per cent of swabs). Recovery of the same set of species from biopsy and swab occurred in 54 per cent of pairs. There was a significant correlation between the bacterial count obtained by biopsy and by surface swab (P < 0.001), but using various threshold values, the predictive value of the counts obtained by one method to predict the counts obtained by the other was poor. Parallel cultures taken on 18 occasions, showed a significant correlation between bacterial counts obtained from two biopsies or two swabs taken simultaneously (P < 0.002), but there was wide variation in bacterial densities from the same burn wound at the same time. Recovery of the same set of species from both biopsies occurred in 56 per cent of pairs, and from both swabs in 50 per cent of pairs. The use of quantitative microbiology in burns is limited by the unreliability of a single surface swab or biopsy to represent the whole burn wound.

Correlation of quantitative burn wound biopsy culture and surface swab culture to burn wound sepsis

The present study includes seventeen patients with second and third degree fresh burns involving 15-50 per cent total body surface area (TBSA). Surface swabs and quantitative burn wound biopsy cultures were obtained during postburn weeks 1, 2 and 3 and correlation was studied. To obtain bacterial counts the technique described by Loebel et al. (1974) was used. The patients were divided in two groups depending upon burn body surface area involved. The first group includes five patients with burns between 15-29 per cent body surface area and the second group includes the rest of the twelve patients with burns between 30-50 per cent body surface area. No patient from group I showed any sign or symptom of sepsis whereas seven patients from group II developed sepsis and three died. These three patients showed positive blood culture at the time of death. Of the 48 cultures obtained in all the patients over 3 weeks, 7 cultures showed differences between swab and biopsy cultures. Genticyn was the most effective drug against Gram-negative organisms.

Methods of quantitative microbiological analyses that support the diagnosis, treatment, and prognosis of human infection

The microorganisms responsible for the production of an infection may be considered to be in two classes: classical microbes and host-defined microbes. Classical microbes are those pathogens which fulfill the Koch-Henle postulates, and their isolation from a host indicates infection. They are not normally part of the body's normal flora, although they may be acquired by the host and enter into a passive relationship known as the carrier state. Examples of this type of microbe are Bacillus anthracis (anthrax), Yersinia pestis (plague), and Bordetella pertussis (whooping cough). Pathogens that require specific hosts have largely replaced the classical pathogen as a cause of infection in hospitalized patients. Especially in recent years, with the advent of new modes of anticancer treatment and the general ability of the medical community to extent a patient's life span by chemotherapy and innovative surgery, the contribution to morbidity and mortality by microbes has substantially increased. These host-specific pathogens are largely part of the body's normal flora. It is incumbent upon the clinical microbiologist to be able to distinguish the patient's normal microbial load, an increased load due to physiological factors, but not representing infection, and a significant change from normal which should be considered infection. The ability to distinguish infection from noninfection is one of the prime responsibilities of the clinical microbiology laboratory and has contributed to the development of the infectious disease subspecialty of internal medicine. This article will examine a critical question: Is there a relationship between the numbers of microorganisms isolated from a specimen and the production of infection, and, if so, does this relationship vary for the different anatomical sites of the body?

Structure of chelocardin, a novel tetracycline antibiotic

Reproducible experimental surgical-wound infections in mice for use in the evaluation of topical antibacterial agents are described. The experimental would was created on the backs of mice by means of a midline incision and was infected by means of cotton sutures monocontaminated with Staphylococcus aureus or Pseudomonas aeruginosa. The course of these wound infections was followed by quantitation of surface bacteria through use of a surface rinse technique. Surface wound counts of the infecting organisms thus obtained appeared to reflect the dynamics of the total wound count, as determined by homogenization of biopsied tissue. Treatment of infected wounds with a placebo cream had only a slight effect on surface wound counts and on mortality in the case of the S. aureus infection but enhanced markedly the lethality of the P. aeruginosa infection.

New technique for quantitative bacteriological sampling of wounds by velvet pads: clinical sampling trial

A new surface sampling method, the velvet pad rinse technique, was compared with the velvet pad imprint method, in a randomized bacteriological sampling trial, using surgical wounds to evaluate quantitative results. The efficiency of bacterial recovery was increased 20-fold by the new technique, and the improvement was ascribed in particular to the introduction of a procedure of rinsing the velvet pad after sampling.

Modified collagen membrane as a skin substitute: preliminary studies

Modified collagen membrane when compared to autograft, homograft, heterograft and silicone polymer membrane, demonstrated a superior adherence on split and full-thickness surfaces, but proved to be an inferior covering for granulating surfaces. Water vapor transport was found to be adequate and adaptable to modification. It was found to be permeable to most commonly used topical antibiotics, and no significant antigenicity could be demonstrated. In a controlled animal burn study, modified collagen membrane proved superior to homograft and heterograft in regard to mortality, time to grafting, bacteriology and autograft take. These studies provide a background for further laboratory and clinical studies now in progress.

Infected surface wound: an experimental model and a method for the quantitation of bacteria in infected tissues

Methods for the quantitation of bacteria in infected tissues must be rigidly standardized to insure uniformity of results. In this communication we report on a laboratory animal model for the study of surface wound infection and the development of a standardized method for the quantitative estimation of bacteria in infected surface wound tissue by mechanical tissue homogenization and serial dilution. Parallel comparative studies demonstrated that a moist-swab sampling procedure detected only 10% of the bacteria recoverable by a surface-wash procedure. Either tissue homogenization or surface-wash procedures recovered significantly more bacteria from contaminated surface wounds than were obtained by surface-swab sampling techniques.