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Ethambutol resistance has mostly been related to mutations in the embB gene. The objective of the present study was to characterize the embB gene in a collection of ethambutol-resistant and ethambutol-susceptible isolates of Mycobacterium tuberculosis complex (MTBC) from Barcelona, and to develop a DNA microarray for the rapid detection of embB mutations in our area. Fifty-three ethambutol-resistant and 702 ethambutol-susceptible isolates of MTBC were sequenced in internal 9821495 bp fragments of the embB gene. In addition, a low-cost, low-density array was designed to include the embB codons identified as being most frequently mutated in our area (LD-EMB array). The global prevalence of embB mutations found among the ethambutol-resistant isolates was 77.4 (41/53). Substitutions in embB306 were the most common [53.7 (22/41)], followed by substitutions in embB406 [26.8 (11/41)]. The presence of mutations in embB406 was related to higher levels of ethambutol resistance and to multidrug resistance. Among unrelated isolates (from 24-locus MIRU-VNTR genotyping), the percentage of embB-mutated isolates was 72.9 (27/37)u59.3 (16/27) in embB306 and 25.9 (7/27) in embB406. None of the ethambutol-susceptible isolates studied showed a mutation in codon 306 or 406. The LD-EMB array showed 100 sensitivity and specificity in identifying the main embB substitutions in our area. Mutations at codons 306 and 406 of embB have a relevant role in resistance to ethambutol in our area. The LD-EMB array developed in this study would appear to be a good molecular test for rapid detection of ethambutol resistance.

Objectives: We analysed the ability of Mycobacterium tuberculosis clinical isolates to penetrate and grow inside murine macrophages as a surrogate of fitness. Methods: Thirty-five drug-resistant and 10 drug-susceptible M. tuberculosis isolates were studied in a murine macrophage model from the J774.2 cell line in a 6 day protocol, performing semi-quantitative counts in Middlebrook 7H11 medium. The mycobacterial penetration index (MPI) after infection and the mycobacterial growth ratio (MGR) inside the macrophages were determined to evaluate the fitness of isolates. Results: Isolates with the katG S315T mutation and multidrug-resistant (MDR) isolates had a significantly lower MGR compared with drug-susceptible isolates. The MPI of the isolates with the katG S315T mutation showed a significant decrease compared with the MPI of those without this mutation. A trend to significantly lower values was also observed on comparing the MPI of the MDR isolates with that of the drug-susceptible isolates and the isolates resistant to isoniazid. Conclusions: The isoniazid-resistant and MDR isolates with mutations in the katG gene showed decreased multiplication inside murine macrophages, suggesting a lower fitness of M. tuberculosis with these resistance patterns.