(197c) Biofilm Reduction Via Surface Vs. Immersion Heating

Despite tremendous breakthroughs in the development of medical implants, biofilm infections on these devices remain a perennial problem. One approach to biofilm destruction is localized thermal shock, which triggers both dispersion of bacteria from the biofilm and death of bacteria remaining in the biofilm. The relative magnitudes of these phenomena may be difficult to compare, however. Most reports on biofilm thermal susceptibility are from immersion in thermostatted media as this enables prompt and uniform delivery of thermal shocks for biofilm destruction. However, this approach also kills much of the dispersed bacteria, which cannot then be readily quantified, making the magnitude of dispersed bacteria unknown. Although dead bacteria in the biofilm can be quantified arithmetically via confocal microscopy, this approach is inaccurate and cannot be used for quantification of dead bacteria in the media. Moreover, delivering thermal shocks via immersion is only possible in limited circumstances clinically (e.g., within the lumen of a catheter). Therefore, this study investigates the efficacy of surface heating for biofilm reduction.

Mature Pseudomonas aeruginosa biofilms cultured on thermoelectric devices were thermally shocked at 37, 60, 70, or 80 °C for 1, 5, or 30 min in the presence of 5 ml 3 g/l tryptic soy broth. Quantifying bacterial population of the media immediately after the thermal shocks showed that there is a roughly constant ratio between the population density on the surface and in the media throughout all the trials, including controls with no thermal shock. This suggests that dispersion is an equilibrium process and thermal shock does not alter this equilibrium significantly. This shows that bacterial reduction by thermal shocks can be achieved by bacterial death in either the biofilm or the surrounding media. Furthermore, the efficacy of surface heating in bacterial reduction was higher at shortest (1 min) thermal shocks compared to immersion heating. However, surface heating was less effective than immersion heating when shock exposure increased to 5 or 30 min. The decrease in efficacy at longer times may be due to bacteria partitioning into the cooler media, where they experience less thermal damage, before returning to the biofilm to maintain the population equilibrium between the phases. This hypothesis was tested by replacing the liquid media above the biofilm with hydrogel tissue mimic to inhibit dispersion from the biofilm.