Evaluating the Rapid Emergence of Daptomycin Resistance in Corynebacterium: a Multicenter Study

Abstract

Members of the genus Corynebacterium are increasingly recognized as pathobionts and can be very resistant to antimicrobial agents. Previous studies have demonstrated that Corynebacterium striatum can rapidly develop high-level daptomycin resistance (HLDR) (MIC, ≥256 μg/ml). Here, we conducted a multicenter study to assay for this in vitro phenotype in diverse Corynebacterium species. Corynebacterium clinical isolates (n = 157) from four medical centers were evaluated. MIC values to daptomycin, vancomycin, and telavancin were determined before and after overnight exposure to daptomycin to identify isolates able to rapidly develop daptomycin nonsusceptibility. To investigate assay reproducibility, 18 isolates were evaluated at three study sites. In addition, the stability of daptomycin nonsusceptibility was tested using repeated subculture without selective pressure. The impact of different medium brands was also investigated. Daptomycin nonsusceptibility emerged in 12 of 23 species evaluated in this study (C. afermentans, C. amycolatum, C. aurimucosum, C. bovis, C. jeikeium, C. macginleyi, C. pseudodiphtheriticum, C. resistens, C. simulans, C. striatum, C. tuberculostearicum, and C. ulcerans) and was detected in 50 of 157 (31.8%) isolates tested. All isolates displayed low (susceptible) MIC values to vancomycin and telavancin before and after daptomycin exposure. Repeated subculture demonstrated that 2 of 9 isolates (22.2%) exhibiting HLDR reverted to a susceptible phenotype. Of 30 isolates tested on three medium brands, 13 (43.3%) had differences in daptomycin MIC values between brands. Multiple Corynebacterium species can rapidly develop daptomycin nonsusceptibility, including HLDR, after a short daptomycin exposure period.

Keywords: Corynebacterium; daptomycin; nonsusceptible; resistance; telavancin; vancomycin.

FIG 1

Examples of isolates incubated in broth overnight with and without exposure to daptomycin. Isolate 1 was rated as not visibly turbid following overnight incubation and therefore was deemed susceptible to daptomycin. Isolate 2 was rated as turbid, prompting susceptibility testing of the isolate for daptomycin, vancomycin, and telavancin.

FIG 2

Clinical Corynebacterium isolates rapidly developed resistance to daptomycin following overnight exposure. (A) Summary table of daptomycin MIC values. (B and C) Bar charts represent the number of isolates for (B) C. striatum and (C) non-C. striatum isolates with daptomycin MIC values before and after overnight daptomycin exposure. The dashed line indicates the susceptible breakpoint for daptomycin (≤1.0 μg/ml is susceptible).

FIG 3

(A to C) Examples of non-C. striatum isolates with (A) uniform susceptibility (MIC, 0.25 μg/ml), (B) uniform nonsusceptibility (MIC, ≥256 μg/ml), and (C) a nonsusceptible subpopulation of colonies to daptomycin (MIC, ≥256 μg/ml). Plates represent MIC testing of isolates following their overnight exposure to daptomycin.

FIG 4

All Corynebacterium isolates exhibited low MIC values to vancomycin (susceptible) and telavancin (indicative of susceptibility) before and after daptomycin exposure. (A) Summary table of vancomycin and telavancin MIC values. (A and B) Bar charts represent the number of isolates tested for (B) vancomycin and (C) telavancin before and after overnight daptomycin exposure. The dashed line indicates the susceptible breakpoint for vancomycin (MIC, ≤ 8.0 μg/ml). There are no telavancin breakpoints available for Corynebacterium species.

FIG 5

Instability of the daptomycin nonsusceptibility phenotype in an isolate of C. macginleyi (COR069-B). Following repeated subculture without selective pressure, the MIC values to daptomycin were tested. (A to D) Representative images are shown for subcultures 1 (A), 2 (B), 3 (C), and 4 (D). Additional subculturing was performed for days 5 to 10 but yielded a single population with an MIC value of 1.0 μg/ml, i.e., susceptible (no resistant subpopulation was noted).

FIG 6

Example of daptomycin-nonsusceptible phenotype variability during testing on three commercially available brands of Mueller-Hinton with blood agar. (A to C) An isolate of C. simulans (COR060-A) was exposed to daptomycin in broth overnight and then tested for daptomycin MIC values on the following three different medium brands: (A) BBL, (B) Hardy, and (C) Remel. A resistant subpopulation of colonies was only observed when tested on BBL medium.