A 36 years old woman recently treated for leukemia is admitted to the hospital with malaise, chills and high fever. Gram stain of blood reveals the presence of gram negative bacilli. The initial diagnosis is bacteremia and parenteral antibiotics are indicated. The record of the patient reveals that she had severe uicarial rash, hypotension and respiratory difficulty after oral pencillin V about 6 months ago. The most appropriate drug should be: