Antimicrobial Susceptibilities of Abiotrophia defectiva, Granulicatella adiacens, and Granulicatella elegans

Abstract

Nutritionally variant streptococci (NVS) are fastidious Gram-positive cocci comprised of the species Abiotrophia defectiva, Granulicatella adiacens, and Granulicatella elegans. NVS are an important cause of bacteremia and infective endocarditis (IE) associated with significant morbidity and mortality. Antimicrobial susceptibility testing (AST) was performed for 14 antimicrobials using the broth microdilution MIC method described in the Clinical and Laboratory Standards Institute (CLSI) M45 guideline. A total of 132 clinical NVS blood isolates collected from 2008 to 2014 were tested. Species level identification of NVS isolates was achieved by 16S rRNA gene sequencing and/or matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Ninety isolates were identified as G. adiacens, 37 as A. defectiva, and 5 as G. elegans. All isolates were susceptible to vancomycin (MIC90 = 1 μg/ml), and none displayed high-level resistance to aminoglycosides. G. adiacens was considerably more susceptible to penicillin than A. defectiva (38.9% versus 10.8% of isolates susceptible) but was less susceptible to cephalosporins than was A. defectiva (43.3% versus 100% of isolates susceptible to ceftriaxone). Several isolates were resistant to levofloxacin (6%), erythromycin (51%), and clindamycin (10%). The MIC90 for daptomycin was ≥ 4 μg/ml for G. adiacens and A. defectiva. G. elegans isolates were 100% susceptible to all antimicrobials tested, with the exception of erythromycin, to which only 20% were susceptible. This study provides antimicrobial susceptibility data for a recent collection of NVS and demonstrates important NVS species-related differences with respect to susceptibility to penicillin, cephalosporins, carbapenems, and daptomycin. Species-level identification of NVS organisms when susceptibility testing is not readily available may aid in treatment decisions.

FIG 1

(A to D) A. defectiva, G. adiacens, and G. elegans MIC distributions for penicillin (A), cefotaxime (B), ceftriaxone (C), and ceftaroline (D). CLSI breakpoints (dashed lines) are shown. S, susceptible; I, intermediate; R, resistant. (E) Scatter plot comparison of ceftriaxone (y axis) and penicillin (x axis) MIC values for A. defectiva (top) and G. adiacens (bottom). The number of isolates with the given combination of MIC values is indicated in each square. Ceftriaxone and penicillin breakpoints (dashed lines) are plotted on either axis. The colored areas indicate where both MIC values are susceptible (green), intermediate (yellow), or resistant (red).

FIG 2

(A and B) A. defectiva, G. adiacens, and G. elegans MIC distributions for meropenem (A) and imipenem (B). CLSI breakpoints (dashed lines) are shown. S, susceptible; I, intermediate; R, resistant. (C) Scatter plot comparison of meropenem (y axis) and imipenem (x axis) MIC values for A. defectiva (top) and G. adiacens (bottom). The number of isolates with the given combination of MIC values is indicated in each square. Meropenem and imipenem breakpoints (dashed lines) are plotted on either axis. The colored areas indicate where both MIC values are susceptible (green), intermediate (yellow), or resistant (red).

FIG 3

A. defectiva, G. adiacens, and G. elegans MIC distributions for vancomycin (A), linezolid (B), and daptomycin (C). The vancomycin CLSI breakpoint (dashed line) is shown. S, susceptible.

FIG 4

(A to C) A. defectiva, G. adiacens, and G. elegans MIC distributions for levofloxacin (A), erythromycin (B), and clindamycin (C). CLSI breakpoints (dashed lines) are shown. S, susceptible; I, intermediate; R, resistant. (D) Scatter plot comparison of erythromycin (y axis) and clindamycin (x axis) MIC values for all NVS (left), A. defectiva (left middle), G. adiacens (right middle), and G. elegans (right). The number of isolates with the given combination of MIC values is indicated in each square. Erythromycin and clindamycin breakpoints (dashed lines) are plotted on either axis. The colored areas indicate where both MIC values are susceptible (green), intermediate (yellow), or resistant (red).