The use of ampicillin and amoxicillin remains a mainstay of urologic prophylaxis and therapy, however ampicillin's effectiveness against aerobic gram-negative bacilli, especially Escherichia coli has steadily decreased over time due to increasing beta-lactamase-induced resistance. Presently, up to 40-60 percent of E. coli may be ampicillin resistant. Since there have been no major resistance problems with enterococci, ampicillin remains the preferred anti-enterococcal antibiotic in penicillin-tolerant patients. Ampicillin/beta-lactamase inhibitor combinations are preferred when penicillins are used to treat ampicillin resistant aerobic gram-negative bacillary urinary tract infections (UTIs). One response to the problem of resistance has been short-course or single-dose therapy for uncomplicated cystitis in immunocompetent adults, which is less likely to cause side effects, bacterial resistance, or alterations in bowel flora. Another response has been to combine the aminopenicillins with beta-lactamase inhibitors such as clavulanate or sulbactam.