Antibiotics covering aerobic and aerobic bacteria should be delivered orally and intravenously prior to colorectal surgery.  Antibiotics delivered within this framework will reduce the risk of postoperative SWI by at least 75%.  Further research is required to establish the optimal timing and duration of dosing, and frequency of longer-term adverse effects such as Clostridium difficile pseudomembranous colitis.
Single-dose intravenous metronidazole v. doxycycline prophylaxis in colorectal surgery. An open prospective, randomized trial.
In an open prospective, randomized study of antimicrobial prophylaxis in colorectal surgery, using a single intravenous dose, metronidazole (1 g) was more effective than doxycycline (0.2 g). The difference in infection rates (2/41 = 5% v. 7/33 = 21%) was statistically significant. Prolonged administration of either agent (2-7 days) yielded results comparable to those with single doses. Five emergency cases were admitted to the study. In two of them, a metronidazole-based therapeutic regimen prevented postoperative infection, whereas infection occurred in all 3 patients given doxycycline therapy. The total infection rate after elective surgery was 4% with metronidazole and 25% with doxycycline prophylaxis. The infections in the metronidazole group were superficial and caused by Escherichia coli, whereas half of the infections in the doxycycline group were intra-abdominal and due to intestinal aerobic and/or anaerobic bacteria. In contrast to doxycycline, therefore, metronidazole prevented postoperative anaerobic infection and was associated with low incidence of aerobic infection.