False non-susceptible results of tigecycline susceptibility testing against Enterobacteriaceae by an automated system: a multicentre study

It has been previously shown that different antimicrobial susceptibility testing (AST) methodologies can influence susceptibility results for tigecycline (Hope et al., 2010; Cohen Stuart et al., 2010; Marchaim et al., 2014; Torrico et al., 2010). Even within a single AST method, several testing conditions including medium age and ion content may cause discrepant results (Bradford et al., 2005; FernandezMazarrasa et al., 2009). For tigecycline, Leal Castro et al. (2010) and Lat et al. (2011) provided early data demonstrating overestimation of MICs and resistance rates by Vitek 2 system. Our study, therefore, aimed to systematically determine the error rates of tigecycline AST for Enterobacteriaceae by Vitek 2 (focusing on false non-susceptible results) as well as other automated systems in comparison to a reference method. Additionally, we compared several other AST methods and testing conditions.