For patients with confirmed allergy or adverse reaction to beta-lactams, one of the following regimens is recommended: Clindamycin plus gentamicin, aztreonam, or ciprofloxacin Metronidazole plus gentamicin or ciprofloxacin Key Points Mixed anaerobic infections occur when the normal commensal relationship among the normal flora of mucosal surfaces (eg, skin, mouth, gastrointestinal tract, vagina) is disrupted (eg, by surgery, injury, ischemia, or tissue necrosis).