BACKGROUND: The aim of this study was to study bacteriology and antibiotic susceptibility in patients with community-acquired perforated appendicitis over a five-year-period. MATERIALS AND METHODS: We conducted a retrospective review of the records of adult patients (age > or =18 years) who were diagnosed as having perforated appendicitis at Ulsan University Hospital between January 2007 and December 2011. Patients who had healthcare-associated or hospital-acquired appendicitis were excluded. Intraoperative specimens submitted to the microbiology laboratory were obtained either by aspiration of pus into a syringe or by use of a swab. Anaerobic bacterial cultures were not performed. RESULTS: Among 216 adult patients with perforated appendicitis, we analyzed 163 culture-positive cases. The overall mortality rate of patients was 0.6% (1/163). Escherichia coli was the most common pathogen (93/163, 57.0%), followed by Streptococcus spp. (45/163, 27.6%), Pseudomonas aeroginosa (13/163, 7.9%), and Enterococcus spp. (17/163, 10.4%). The susceptibility of E. coli to quinolones (ciprofloxacin or levofloxacin) was 74.1%. The susceptibility of E. coli to amoxicillin/clavulanate, cefoxitin, ceftriaxone, piperacillin/tazobactam, and carbapenem reached 75%, 86%, 90%, 98%, and 100%, respectively. Isolated E. coli, including ESBL producing organism and P. aeroginosa, were highly susceptible to piperacillin/tazobactam. Empirical antibiotics used most commonly were a combination of third generation cephalosporin and metronidazole. CONCLUSION: E. coli was the most common pathogen of community-acquired perforated appendicitis, and resistance to quinolone was greater than 25%. We cannot recommend quinolones for use as empirical therapy for treatment of perforated appendicitis.