RETRACTION
Volume 55, no. 1, p. 334–335, 2017, https://doi.org/10.1128/JCM.01920-16. We hereby retract this article. In our paper we described discrepant results between ceftolozane-tazobactam Etests and broth microdilution. However, when repeat testing on the same isolates was done for a second, larger study recently published by R. M. Humphries, J. A. Hindler, P. Magnano, A. Wong Beringer, R. Tibbetts, and S. A. Miller (J Clin Microbiol 56:e01633-17, 2018, https://doi.org/10.1128/JCM.01633-17) we could not replicate the initial results. Upon review of the procedure used for broth microdilution it was determined that the tazobactam concentrations were not prepared according to CLSI guidelines (Clinical Laboratory Standards Institute, Performance Standards for Antimicrobial Susceptibility Testing, 26th ed, CLSI supplement M100S, 2016). We hypothesized that this may have contributed to the high MICs and very major errors. In order to test this hypothesis, we repeated the broth microdilution in the original laboratory using the same procedure as initially performed; however, we could not replicate the errors. Based on this outcome, a second hypothesis was that perhaps the bacteria possessed a resistance mechanism at the time of initial testing but that upon repeated subculture the mechanism was lost; however, we are unable to test this hypothesis.