Rapid emergence of daptomycin resistance in clinical isolates of Corynebacterium striatum… a cautionary tale

Abstract

The objective of this study was to investigate the observation of daptomycin resistance in Corynebacterium striatum, both in vivo and in vitro. We describe a case of C. striatum bacteremia in a patient with a left ventricular assist device (LVAD); the initial isolate recovered was daptomycin susceptible with a minimum inhibitory concentration (MIC) of 0.125 μg/ml. Two months later, and after daptomycin therapy, the individual became bacteremic with an isolate of C. striatum with a daptomycin MIC of >256 μg/ml. To study the prevalence of daptomycin resistance in C. striatum, clinical isolates of C. striatum were grown in broth culture containing daptomycin to investigate the emergence of resistance to this antimicrobial. Molecular typing was used to evaluate serial isolates from the index patient and the clinical isolates of C. striatum we assayed. In vitro analysis of isolates from the index patient and 7 of 11 additional C. striatum isolates exhibited the emergence of high-level daptomycin resistance, despite initially demonstrating low MICs to this antimicrobial agent. This phenotype was persistent even after serial subculture in the absence of daptomycin. Together, these data demonstrate that caution should be taken when using daptomycin to treat high-inoculum infections and/or infections of indwelling medical devices with C. striatum. To our knowledge, this is the first report characterizing the emergence of daptomycin resistance in C. striatum.

Figure 1. Profile of three C. striatum isolates from our index patient

A. Dendogram illustrating the results of repPCR strain typing analysis of the three C. striatum isolates recovered from the index patient. All isolates have a SI ≥95%, illustrating that the infections were caused by a single strain type. B. Daptomycin MIC results as evaluated by Etest for the index patient's initial C. striatum bloodstream infection (top, left), C. striatum isolated from the blood stream 66 days later (top, center), and C. striatum isolated from tissue 78 days after the initial blood stream infection (top, right). Each isolate was grown overnight in broth culture in the absence (top row) and presence (bottom row) of daptomycin.