Mechanisms of Resistance, Clonal Expansion, and Increasing Prevalence of Acinetobacter baumannii Strains Displaying Elevated Tigecycline MIC Values in Latin America.

The aim of the study was to characterize forty-eight Acinetobacter baumannii (ACB) isolates with confirmed tigecycline MIC values >2 mg/L observed in six Latin American (LATAM) hospitals (four countries) in 2011. During 2005-2011, 6,923 ACB isolates were collected as part of the SENTRY Program, and tigecycline susceptibility was quantified using the reference broth microdilution method. A total of 102/1881 ACB from LATAM hospitals displayed tigecycline minimum inhibitory concentration (MIC) values >2 mg/L, showing an increase from 4.3% in 2010 to 10.5% in 2011, which is considerably high when compared to other geographical regions. Forty-eight ACB from 2011 displaying elevated tigecycline MICs were typed by pulsed-field gel electrophoresis, which showed multiple clusters in Sao Paulo, Brazil, and a major clone in Guadalajara, Mexico. Eighteen unique isolates had the expression of adeA and adeF determined and results compared to a group of tigecycline-susceptible strains, which demonstrated that 18/18 strains had significantly increased expression of AdeABC and three isolates overexpressed AdeFGH. Sequencing of adeS and adeR revealed that 11 isolates displayed adeS mutations, and 5 isolates had mutations in adeR. Sequencing of trm showed frameshift mutations in eight isolates and insertion sequences leading to nonfunctional proteins in three isolates. TetX-encoding genes were not detected. We documented the recent increase of ACB displaying elevated tigecycline MICs in LATAM hospitals, dominantly due to the clonal expansion of isolates in Brazil and Mexico. Control of tigecycline usage in those countries and more strict infection control practices in the involved hospitals should be considered to reduce such outbreaks.