Snapshot A 28-year-old woman presents to her primary care physician due to dysuria. Her symptoms began approximately 2-3 days ago and have progressively worsened. She has a past medical history of mild asthma. She is allergic to penicillin, which causes her to have a severe skin rash and shortness of breath. Her symptoms are associated with urinary urgency and frequency. Physical examination demonstrated tenderness to palpation of the suprapubic region. A urinalysis and urine culture are ordered, which demonstrates Proteus mirabilis infection. She is treated with aztreonam. Introduction Mechanism of action a monobactam that binds to penicillin-binding proteins 3 (PBP-3), which inhibits transpeptidation impairs peptidoglycan synthesis, a needed component of cell wall biosynthesis no cross-allergy with penicillins Mechanism of resistance altered PBP-3 Clinical use gram-negative rods only especially in patients who are allergic to penicillins or intolerable to aminoglycosides Adverse effects gastrointestinal upset (occasional)