Most frequent isolates are Aerobic gram-negative bacilli (eg, Escherichia coli and Klebsiella) Anaerobes (especially Bacteroides fragilis) Table Intra-Abdominal Abscesses Location Etiology Organisms Intraperitoneal Subphrenic Right or left lower quadrant Interloop Paracolic Pelvic Postoperative; perforation of hollow viscus, appendicitis, diverticulitis, or tumor; Crohn disease; pelvic inflammatory disease; generalized peritonitis of any etiology Bowel flora, often polymicrobial Retroperitoneal Pancreatic Trauma, pancreatitis Bowel flora, often polymicrobial Perinephric Spread of renal parenchymal abscess (complication of pyelonephritis or rarely hematogenous from a remote source) Aerobic gram-negative bacilli Visceral Hepatic Trauma, ascending cholangitis, portal bacteremia Aerobic gram-negative bacilli if origin is biliary Polymicrobial bowel flora Amebic infection via the portal venous system Splenic Trauma, hematogenous, infarction (as in sickle cell disease and malaria) Staphylococci, streptococci, anaerobes, aerobic gram-negative bacilli including Salmonella Candida in patients with immunocompromise Symptoms and Signs of Intra-Abdominal Abscesses Abscesses may form within 1 week of perforation or significant peritonitis, whereas postoperative abscesses may not occur until 2 to 3 weeks after operation and, rarely, not for several months.