Bacterial growth kinetic without the influence of the immune system using vacuum-assisted closure dressing with and without negative pressure in an in vitro wound model

The physical capacity of negative pressure wound therapy (NPWT) dressing on the bacterial growth in an in vitro wound model was investigated. Standardised wounds were contaminated with a clinical Staphylococcus aureus strain and incubated at 35 degrees C for 6 hours. Four wounds were treated with continuous negative pressure (125 mmHg) and four controls without. Bacterial load per gram tissue and per gram polyurethane sponge were measured after 24, 36 and 72 hours. Without negative pressure, the initial mean S. aureus load per gram tissue was 1.42 x 10(4), with negative pressure 1.84 x 10(4), P = 0.294. After 24, 36 and 72 hours, both models yielded comparable numbers of organisms (24 hours: P = 0.081; 48 hours: P = 0.455; 72 hours: P = 0.825, respectively). Bacterial load of sponges with or without negative pressure also did not differ. Over a period of 72 hours, sponges with negative pressure yielded 1.60 x 108, those without negative pressure yielded 1.74 x 10(8) CFU/g sponge (P = 0.876). In non vital tissue without the influence of the immune system, the bacterial load did not decrease in our in vitro model using an NPWT dressing. This observation was independent of the physical effect of continuous negative pressure at 125 mmHg. The reduction in bacteria demonstrated in previous studies appears to be caused by other effects than physical suction alone. However, the results obtained are limited as non viable tissue was used and the effect of suction on dead tissue might be very different from that occurring on perfused tissue, for example, in an animal model or in patients.