Background: Negative pressure wound treatment is increasingly used through a Vacuum-Assisted Closure (VAC) device in complex wound situations. For this purpose, sterile polyurethane (PU) and polyvinyl alcohol (PVA) foam dressings are fitted to the wound size and covered with an adhesive drape to create an airtight seal. Little information exists about the type and quantity of microorganisms within the foams. Therefore, we investigated VAC foams after removal from the wound using a validated method (sonication) to detect the bacterial bioburden in the foam consisting as microbial biofilms.Methods: We prospectively included VAC foams (PU and PVA, KCI, Rümlamg, Switzerland) without antibacterial additions (e.g. silver), which were removed from wounds in patients with chronic ulcers from January 2007 through December 2008. Excluded were patients with acute wound infection, necrotizing fasciitis, underlying osteomyelitis or implant. Removed foams from regular changes of dressing were aseptically placed in a container with 100 ml sterile Ringer's solution. Within 4 hours after removal, foams were sonicated for 5 min at 40 kHz (as described in NEJM 2007;357:654). The resulting sonication fluid was cultured at 37°C on aerobic blood agar plates for 5 days. Microbes were quantified as No. of colony-forming units (CFU)/ml sonication fluid and identified to the species level.Results: A total of 68 foams (38 PU and 30 PVA) from 55 patients were included in the study (median age 71 years; range 33-88 years, 57% were man). Foams were removed from the following anatomic sites: sacrum (n=29), ischium (n=18), heel (n=13), calves (n=6) and ankle (n=2). The median duration of being in place was 3 days (range, 1-8 days). In all 68 foams, bacteria were found in large quantities (median 105 CFU/ml, range 102-7 CFU/ml sonication fluid. No differences were found between PU and PVA foams. One type of organisms was found in 11 (16%), two in 17 (24%) and 3 or more in 40 (60%) foams. Gram-negative rods (Escherichia coli, Proteus mirabilis, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa) were isolated in 70%, followed by Staphylococcus aureus (20%), koagulase-negative staphylococci, streptococci (8%), and enterococci (2%).Conclusion: With sonication, a high density of bacteria present in VAC foams was demonstrated after a median of 3 days. Future studies are needed to investigate whether antimicrobial-impregnated foams can reduce the bacterial load in foams and potentially improve wound healing.