Introduction: Contaminated surgical wounds may be treated with primary closure and systemic antibiotics, with an expected infection rate of 20% (range 7-50%). The change in bacteriology and the counts within these wounds have never been studied as a function of time.

Methods: Patients with perforated appendicitis underwent open appendectomy and qualitative intra-abdominal cultures. All patients were given antibiotics and randomized to primary wound closure vs wound closure plus daily wound probing using a sterile cotton tip applicator. Quantitative cultures of the wound were performed on post-operative days (PODs) 1, 3, and 5, or until the wound had sealed and was impenetrable.

Results: From 2007-2009, 77 patients were randomized. Patients in the wound-probing arm (N=38) had significantly less wound infection compared to the non-wound-probing arm (2.6% vs 18.9%, p=0.028).

Quantitative wound cultures on POD 1 showed aerobic growth in 30/38 (78.9%, range bacterial counts:<102, 7x105). The most common aerobic bacteria were Coag(-) Staph. (60.5%), Alpha-Strep. (26.3%), and Gram(-) lactose fermenting rods (13.2%). In comparison, the abdominal cultures of aerobes were E. coli (44.7%), Strep. (36.8%) and P. aeruginosa (21.1%). Anaerobic growth was found in 13/38 (34.2%, range bacterial counts:<102, 3.2x105). The most common anaerobic bacteria were Gram(+) bacilli (36.8%) and Gram(+) cocci (7.9%). In comparison, the abdominal cultures of anaerobes were Bacteroides (68.4%) and Prevotella (10.5%) species.

Most wounds were impenetrable within 3 to 4 days (See Figures). All bacterial counts prior to wound healing were less than 104 for aerobes and less than 103 for anaerobes. We found no correlation between bacterial counts or flora and patient demographics, diabetes, body mass index, and abdominal girth.

Conclusion: The microbiology of contaminated wounds are mixed flora of aerobic bacteria, with one-third also having anaerobic bacteria. This is independent of diabetes and body habitus. There is poor correlation between abdominal cultures and wound cultures, with a predominance of Staph. and other Gram(+) bacteria found in the wound even on POD 1.

The bacterial counts decrease sharply, with less than 104 aerobes and 103 anaerobes by POD 3.

While direct comparison of wound bacterial counts cannot be made between the probed and the non-probed wounds, the six-fold reduction in wound infections in the probed wounds cannot be ignored. We surmise that wound probing appears to reduce the rich substrate necessary for bacterial proliferation and may be the reason for the drops in bacterial counts. This minimally invasive procedure certainly warrants additional study as an effective tool in dramatically reducing wound infections in contaminated settings.