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Volume 27, Number 2—February 2021

Spread of Multidrug-Resistant Rhodococcus equi, United States

Sonsiray Álvarez-NarváezComments to Author , Steeve Giguère1, Noah Cohen, Nathan Slovis, and José A. Vázquez-BolandComments to Author 
Author affiliations: University of Georgia, Athens, Georgia, USA (S. Álvarez-Narváez, S. Giguère); Texas A&M University, College Station, Texas, USA (N. Cohen); Hagyard Equine Medical Institute, Lexington, Kentucky, USA (N. Slovis); University of Edinburgh, Edinburgh, Scotland, UK (J.A. Vázquez-Boland)

Main Article


Effect of absence of tetRA(33) determinant from pRErm46 plasmid on R. equi susceptibility to tetracycline and doxycycline, determined on macrolide-resistant isolates collected during 2012–2017*


pRErm46 ΔC1I-tetRA(33)
MIC, μg/mL‡
MIC, μg/mL
Tetracycline Resistant (100)§ 21.33 (8–48)¶ Susceptible (100)§ 1.97 (0.38–3)¶
Doxycycline Susceptible (100) 3.35 (0.75–6)** Susceptible (100) 1.06 (0.25–3)**

*Susceptibility data to other relevant antimicrobials are shown in Appendix Table 2).
†Determined by disk diffusion technique. Isolate percentage shown in parenthesis. Zone diameter susceptibility breakpoints based on Clinical and Laboratory Standards Institute interpretive criteria for Staphylococcus aureus, routinely used for R. equi susceptibility testing in the absence of specific approved criteria for this species (11,16). 
‡Minimal inhibitory concentration determined using Etest strips. Mean value (range in parenthesis).
§p<0.001 by χ2 test.
¶p<0.001 by t-test. 
**p<0.001 by t-test. Presence of TetRA(33) appears to induce a small, statistically significant MIC increase, but MIC remains below the Clinical and Laboratory Standards Institute susceptibility breakpoint for doxycycline (susceptible <4 μg/mL, intermediate 8 μg/mL, resistant >16 μg/mL).

Main Article

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Page updated: January 23, 2021
Page reviewed: January 23, 2021
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