Background: To prevent surgical site infection (SSI) after appendectomy, antibiotic prophylaxis has been recommended for all patients, but this approach is based largely on bacteriologic findings that are decades old. The objective of this study was to reevaluate the bacteriology of acute appendicitis in order to assess the usefulness of current antibiotic prophylaxis. Methods: Between January 1 and December 31, 2010, 117 patients with pathology-proved acute appendicitis were recruited. Antibiotic prophylaxis was given according to national guidelines. Immediately after operation, the luminal contents of the appendices were swabbed for bacterial culture. The charts of the patients were surveyed retrospectively for postoperative complications until June 30, 2011. Results: Bacteria were isolated from 115 of 117 specimens sent for culture (98%). Of the 115 samples that yielded bacteria, all gave rise to aerobic isolates and five yielded mixed aerobic and anaerobic isolates. The most common aerobic organism was Escherichia coli, which was present in 100 of 117 patients who had pathology-proved acute appendicitis (85%). Less frequent organisms were Klebsiella pneumoniae (30 cases; 26%), Streptococcus spp. (29 cases; 25%), Enterococcus spp. (21 cases; 18%), and Pseudomonas aeruginosa (18 cases; 15%). All P. aeruginosa isolates were sensitive to amikacin, ceftazidime, and cefepime; but seven of the eight were resistant to cefuroxime. Eight patients were identified as having had a postoperative SSI, and P. aeruginosa was isolated from five of these cases. The isolation of P. aeruginosa correlated significantly with SSI (p=0.002). Conclusions: The most commonly identified aerobic bacteria associated with acute appendicitis were E. coli, followed by K. pneumoniae, Streptococcus, Enterococcus, and P. aeruginosa. Pseudomonas aeruginosa frequently was not covered by the prophylactic antibiotics chosen and might be associated with SSI.