Bacteria identification on NPWT foams: clinical relevance or contamination?

To compare and interpret the microbiological findings of tissue samples prior to and during negative pressure wound therapy (NPWT), as well as those of NPWT foams.

A retrospective evaluation of 101 NPWT dressings (29 polyurethane, 72 polyvinylalcohol; 43 deeply inserted, 56 superficially, two combined deeply and superficially; 67 hardware present and 34 no hardware present) in 64 patients was conducted. All foam and tissue samples were cultured over a period of 7 days. Tissue and foam samples were incubated on blood agar plates and in tryptic soya broth. Positive results indicated a microbial growth with > 10(5) colony forming units (CFU)/ml.

Total mean implantation period was 6 ± 2 days (2-14 days). On 39 foams (39%), at least one organism could be identified. While S. aureus and S. epidermidis were the most common organisms prior to NPWT, during therapy S. aureus and S. epidermidis were most frequently identified in the tissue samples, with E. faecalis and S. epidermidis on the foams. In 12 cases (31%), the organisms in the tissue samples during NPWT and on the foams were identical, whereas in two cases a different organism was evident on the foam. In 6 cases (15%) the microbiological examination of the tissue samples was negative while positive on the foam. In the remaining 19 cases (54%), an organism could be identified on the foams while no tissue samples were microbiologically examined.

This study shows that, in at least 20% of the cases, a bacterium might be solely identified on the NPWT foam or differ to those isolated in the tissue; whether this is a contamination, bacterial switch or the results of insufficient microbiological diagnostics prior to NPWT cannot be surely stated.