Validation of the Zürich burn-biofilm model.

Background: Despite advances in the use of topical and parenteral antimicrobial therapy and the practice of early tangential burn-wound excision, bacterial infection remains a major problem in the management of burn victims today. The purpose of this study was to design and evaluate a polyspecies biofilm model with bacteria known to cause severe infections in burn patients. The model is simple to prepare, maintain and analyse, and allows for short-term exposure to antimicrobials.

Results: Initial experiments showed that it was impossible to establish balanced polyspecies biofilms with an inoculum of Gram-positive and -negative bacteria. After 64.5 h of incubation, the Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) had suppressed the Gram-positives (Enterococcus faecalis, Staphylococcus aureus and Streptococcus intermedius). However, adding the Gram-negative bacteria after 41.5 h to an established biofilm of Gram-positives resulted in a balanced microbial consortium. After 64.5 h, all species were present in high numbers (10(7) to 10(8) colony forming units (CFU) per biofilm). Multiple repetitions showed high reproducibility of biofilm formation without significant differences between and within experiments. Combined fluorescence in situ hybridisation/confocal laser scanning microscopy (FISH/CLSM) analyses, for which biofilms had to be grown on a different non-flexible substrate (hydroxy apatite), revealed that, by 41.5 h, the biofilm consisted of an almost confluent layer of bacteria firmly adherent to the substratum. After 64.5 h (22 h after the addition of the Gram negatives), the biofilm consisted of a confluent mixture of single cells, an abundance of galaxies of bacteria with small lacunae and large amounts of extracellular matrix polysaccharides.

Conclusions: The polyspecies biofilm model contains the most prevalent burn-associated Gram-positive and Gram-negative bacterial pathogens and mimics the Gram-negative shift observed in vivo. It shows excellent reproducibility. It should allow adaptation to the bacterial spectrum prevalent in different burn centres and lead to a much more reliable investigation of the efficiency of topical antimicrobial agents than models operating with planktonic bacteria. The experiments further open up the perspective to create an in vivo model using these biofilms as infectious agents.