The most appropriate treatment regimen for this patient is oral therapy with ciprofloxacin and metronidazole. Aerobic gram-negative and anaerobic organisms should be considered for empiric coverage in this patient with a community-acquired intra-abdominal abscess pending drainage and fluid Gram stain and culture results. Admission to an ICU or step-down unit does not require that the patient receive intravenous antimicrobial therapy. Oral antimicrobials can be used in critically ill patients who have an intact and functioning gastrointestinal tract (no vomiting, no ileus), if the type of infection being treated does not require intravenous antibiotics (infection involving the central nervous system, Staphylococcus aureus bacteremia, endocarditis, osteomyelitis), and if the oral agent(s) with the appropriate spectrum of activity and acceptable bioavailability for the infection being treated is available. This patient has an intact and functioning gastrointestinal tract and is a candidate for oral antimicrobial therapy. Ciprofloxacin and metronidazole have excellent bioavailability and tissue penetration (including abscesses) and provide the necessary spectrum of antimicrobial coverage for aerobic gram-negative and anaerobic bacteria.

Aztreonam provides aerobic gram-negative coverage, and vancomycin adds gram-positive coverage, but the combination does not provide the anaerobic coverage for this patient's empiric therapy.

The combination of vancomycin and clindamycin provides gram-positive and anaerobic coverage but no aerobic gram-negative coverage. Clindamycin has excellent bioavailability and should be considered for oral administration when it is the indicated antibiotic.

Although ampicillin and metronidazole are offered as an oral regimen, they do not provide adequate aerobic gram-negative coverage, including commonly resistant Escherichia coli.