ampicillin + gentamicin + metronidazole

This is a combination of three antibiotics—ampicillin, gentamicin, and metronidazole—used together for the treatment of severe polymicrobial infections, particularly those involving both aerobic and anaerobic bacteria. The regimen is commonly used in complicated intra-abdominal infections (such as peritonitis), pelvic inflammatory disease, postpartum or postabortion uterine infections, and other serious mixed bacterial infections.

• Ampicillin is a beta-lactam antibiotic (aminopenicillin) that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins; it is effective against many Gram-positive organisms and some Gram-negative bacteria.
• Gentamicin is an aminoglycoside antibiotic that binds to the 30S ribosomal subunit, inhibiting protein synthesis; it has potent activity against many aerobic Gram-negative bacilli and provides synergistic effects with beta-lactams against certain Gram-positive organisms such as Enterococcus species.
• Metronidazole is a nitroimidazole antibiotic that disrupts DNA synthesis in anaerobic bacteria and protozoa; it provides coverage for obligate anaerobes.

This triple therapy offers broad-spectrum coverage suitable for severe or high-risk polymicrobial infections where both aerobic (including enteric Gram-negatives) and anaerobic pathogens are likely involved. It may be used empirically before culture results are available or when resistant organisms are suspected.