Assessment of trimethoprim-sulfamethoxazole susceptibility testing methods for fastidious Haemophilus spp

Objectives: To compare the determinants of trimethoprim-sulfamethoxazole resistance with established susceptibility values for fastidious Haemophilus spp., to provide recommendations for optimal trimethoprim-sulfamethoxazole measurement. Methods: We collected 50 strains each of Haemophilus influenzae and Haemophilus parainfluenzae at Bellvitge University Hospital. Trimethoprim-sulfamethoxazole susceptibility was tested by microdilution, E-test and disc diffusion using both Mueller–Hinton fastidious (MH-F) medium and Haemophilus test medium (HTM) following EUCAST and CLSI criteria, respectively. Mutations in folA, folP and additional determinants of resistance were identified in whole-genome-sequenced isolates. Results: Strains presented generally higher rates of trimethoprim-sulfamethoxazole resistance when grown on HTM than on MH-F, independent of the methodology used (average MIC 2.6-fold higher in H. influenzae and 1.2-fold higher in H. parainfluenzae). The main resistance-related determinants were as follows: I95L and F154S/V in folA; 3- and 15-bp insertions and substitutions in folP; acquisition of sul genes; and FolA overproduction potentially linked to mutations in -35 and -10 promoter motifs. Of note, 2 of 19 H. influenzae strains (10.5%) and 9 of 33 H. parainfluenzae strains (27.3%) with mutations and assigned as resistant by microdilution were inaccurately considered susceptible by disc diffusion. This misinterpretation was resolved by raising the clinical resistance breakpoint of the EUCAST guidelines to ≤30 mm. Conclusions: Given the routine use of disc diffusion, a significant number of strains could potentially be miscategorized as susceptible to trimethoprim-sulfamethoxazole despite having resistance-related mutations. A simple modification to the current clinical resistance breakpoint given by the EUCAST guideline for MH-F ensures correct interpretation and correlation with the reference standard method of microdilution.

Bibliographic Details

Main Authors:	Sierra, Yanik, Tubau, Fe, González-Díaz, Aida, Carrera-Salinas, A., Moleres, Javier, Bajanca-Lavado, Paula, Garmendia, Juncal, Domínguez, M. Ángeles, Ardanuy, Carmen, Martí, Sara
Format:	artículo biblioteca
Published:	John Wiley & Sons 2019-12-04
Subjects:	Antimicrobial susceptibility testing methods, Clinical resistance breakpoint, EUCAST, Haemophilus influenzae, Haemophilus parainfluenzae, Resistance-related determinants, Trimethoprim-sulfamethoxazole,
Online Access:	http://hdl.handle.net/10261/203901
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