Emergence of isolates resistant to ampicillin

Clinical isolates of Streptococcus faecium demonstrating ampicillin resistance were recovered from eight pediatric patients. Sites of isolation included blood, surgical wound, bile drainage, urine, burns, and peritoneal fluid. Seven patients had prolonged hospitalization, and all had been treated with broad-spectrum antibiotics prior to isolation of the resistant enterococcus. One isolate was from an ill, bacteremic patient; the others were in mixed culture and were not considered causes of disease. The isolates were not epidemiologically related. Minimal inhibitory concentrations for various antibiotics included ampicillin (16 to 32 mg/L), penicillin (128 mg/L), gentamicin (16 mg/L), and vancomycin (2 mg/L). Three isolates demonstrated high-level resistance (greater than 2000 mg/L) to streptomycin; none did so to gentamicin. In vitro synergy testing performed on seven available isolates for ampicillin and gentamicin demonstrated no synergy to this combination. None produced beta-lactamase. Combined antibiogram and plasmid data showed at least five distinct patterns. These strains present a new clinical problem in their high level of resistance to ampicillin and to the combination of ampicillin and gentamicin.