Progressive resistance in a single strain of Acinetobacter calcoaceticus recovered during a nosocomial outbreak

A study was undertaken to investigate a nosocomial outbreak of acinetobacter calcoaceticus. The outbreak occurred in a shock-trauma intensive care unit (ICU) during a 6-month period and involved a total of 37 cases. Although no deaths were attributable to Acinetobacter, 60% of patients from whom Acinetobacter was isolated were determined to have a lower respiratory tract infection. A significant difference (p less than 0.05) was noted between the length to stay in the unit of patients who acquired Acinetobacter (mean 23.9 days) and of those who remained free of the organism (mean 2.58 days). The average day on which cultures first became positive was 14.85. During the last 3 months of the outbreak, the ICU Acinetobacter isolates developed an unusual pattern of aminoglycoside resistance that differed significantly (p less than 0.001) from the sensitivities of non-ICU Acinetobacter isolates. No plasmids could be detected in selected aminoglycoside-resistance Acinetobacter. Pseudomonas aeruginosa isolated concurrently from patients with Acinetobacter showed no abnormal variation in antimicrobial sensitivities. These findings were interpreted to mean that the outbreak was the result of the persistence of a single strain of A. calcoaceticus within the ICU. Termination of the outbreak was attributed to initiation of control measures.