Oral antibiotic prophylaxis induces changes in the microbiology of surgical site infection after colorectal surgery. A matched comparative study.

AIM: Oral antibiotic prophylaxis (OAP) lowers rates of surgical site infection (SSI) and may aid anastomotic healing in colorectal surgery. The aim of this study was to analyse the understudied impact of OAP on SSI microbiology after colorectal surgery.

METHOD: A post hoc analysis was performed on a previous prospective, multicentre study of elective colorectal surgery. For 1000 patients with SSI, this study compared the microbiology of SSIs in procedures without OAP (SSI/OAP-) and with OAP (SSI/OAP+).

RESULTS

There were 340 patients in the SSI/OAP- group and 660 in the SSI/OAP+ group. The use of OAP increased the presence of Gram-positive cocci (GPC) (OR 1.542, 95% CI 1.153-2.062) and fungi (OR 2.037, 95% CI 1.206-3.440), but reduced rates of Gram-negative bacteria (GNB) (OR 1.461, 95% CI 1.022-2.088) and anaerobe isolation (OR 0.331, 95% CI 0.158-0.696). Specifically, it led to increases in the isolation of Enterococcus faecium (OR 1.450, 95% CI 0.812-2.591), methicillin-resistant Staphylococcus aureus (OR 2.000, 95% CI 1.043-3.834) and Candida spp. (OR 2.037, 95% CI 1.206-3.440). In colon surgery with OAP, GPC infections were more likely (OR 1.461, 95% CI 1.022-2.088). In rectal surgery, organ/space SSIs had a higher risk of harbouring GPC (OR 1.860, 95% CI 1.153-2.999) and a lower risk of GNB (OR 0.321, 95% CI 0.200-0.515).

CONCLUSION

OAP reduced the presence of anaerobes and GNB in SSIs, but increased the isolation of GPCs and fungi, with E. faecium and Candida being of particular concern. This information should guide empirical antibiotic therapy for postoperative colorectal SSIs in patients who have received preoperative OAP.