Antibiotic Therapy
Advances in bacteriology and the introduction of antibiotics with limited activity against obligate anaerobes (e.g., aminoglycosides and cephalosporins) have refocused the attention on the anaerobic nature of secondary peritonitis during the 1960's and 1970's. This prompted experimental studies that identified E. coli and B. fragilis as the main target organism for therapy. Both are also the most frequent isolates from intra-abdominal infections. Today the practice of early empirical administration of antibiotics targeted against these bacteria is being challenged by a more sophisticated calculated design of antibiotic therapy that is based on concentration dynamics at the site of infection. The choice & timing of drugs, need for operative cultures and the duration of postoperative administration remain controversial and results of clinical trials are mostly inconclusive because the sickest of patients are excluded by study design...

Bacteriology
The microbiology of the gastrointestinal tract has been well defined. The identity and density of microorganisms of intra-abdominal infection depend not only on the site of the gastrointestinal tract perforation but also on the pathogenicity of bacteria, i.e. its ability to survive outside of their natural environment and potential to produce toxins. Bacterial counts in the gastrointestinal tract vary greatly, from nearly sterile in the fasting, normal, low-pH stomach to high concentrations of bacteria approaching 10^12 per mL feces in the distal colon. Of the more than 400 different species of intestinal bacteria, most are symbiotic saprophytes and only a few are capable of survival outside the bowel. In general, gastric, duodenal, and proximal jejunal perforations release small numbers of gram-positive aerobic and gram-negative anaerobic organisms into the peritoneal cavity. These organisms are generally susceptible to beta-lactam antibiotics and are rapidly eradicated by defense mechanisms in intact hosts...