Two patients with uncomplicated enterococcal urinary tract infections were treated with trimethoprim-sulfamethoxazole based on in vitro susceptibilities. Bacteremia developed in both patients and they recovered only after the cessation of trimethoprim-sulfamethoxazole administration and institution of therapy with penicillin G potassium or vancomycin hydrochloride plus streptomycin sulfate. Although the enterococcus may appear susceptible to trimethoprim-sulfamethoxazole in vitro, it escapes the antifolate activity of the drug in vivo by its unique ability to incorporate preformed exogenous folates. The practice by clinical microbiology laboratories of reporting the susceptibilities of the enterococcus to drugs other than the penicillins or vancomycin is misleading and potentially dangerous.
Goodhart GL. In Vivo v In Vitro Susceptibility of Enterococcus to Trimethoprim-Sulfamethoxazole: A Pitfall. JAMA. 1984;252(19):2748–2749. doi:10.1001/jama.1984.03350190050019
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