Amplification of Aminoglycoside Resistance Gene aphA1 in Acinetobacter baumannii Results in Tobramycin Therapy Failure
All isolates () were cultured from a 20-year-old male polytrauma victim sustained in combat operations in Afghanistan (day 1). Cultures obtained from day 3 grew A. baumannii (isolates MRSN 3361 and MRSN 3363) susceptible to tobramycin (MIC = 0.5 µg/ml). Tobramycin-susceptible MRSN 3364 was cultured on day 8 from a tissue sample collected from the left hip injury. Upon stabilization of the patient, he was transferred to the United States for definitive care, arriving on day 9, where antibiotics were changed to intravenous tobramycin, meropenem, and vancomycin based on culture results. A. baumannii MRSN 56 was cultured from the left hip wound on day 11 and was susceptible to tobramycin (MIC = 0.5 µg/ml). Throughout this escalation of care, tobramycin was continuously administered. On day 12, MRSN 57 was cultured from a tissue sample collected from the right hip but had an MIC of tobramycin of 8 µg/ml. The following day, MRSN 58 was cultured from a tissue sample collected from the left hip. MRSN 58 had a tobramycin MIC of = 16 µg/ml, which denotes resistance by CLSI guidelines (). Consequently, tobramycin therapy was discontinued, and the patient was prescribed colistin while continuing meropenem.