Genotypic study of isolated resistance to isoniazid in the Mycobacterium tuberculosis complex in a Moroccan hospital

Introduction. Despite the introduction 40 years ago of effective and low-cost treatment for tuberculosis (TB), morbidity and mortality from this disease remain substantial worldwide. According to the WHO, TB is once again the leading cause of death worldwide from a single infectious agent. In 2023, TB caused ~1.25 million deaths, surpassing COVID-19. In Morocco, the number of new TB cases rose from 30,897 in 2017 to 35,000 in 2019, highlighting a concerning upward trend that underscores the persistent challenge TB poses to the country's public health system. The incidence of multidrug-resistant (MDR) or rifampicin (RIF)-resistant TB was estimated at 1.7 per 100,000 inhabitants. Isoniazid (INH) is a cornerstone of first-line TB treatment, and resistance to it, even in the absence of RIF resistance, is associated with delayed treatment response, higher rates of treatment failure or relapse and increased risk of progression to MDR-TB if not promptly identified and appropriately managed. Moreover, current diagnostic algorithms in many settings, including Morocco, may miss INH monoresistance due to their reliance on rapid molecular tests that primarily detect RIF resistance, further emphasizing the emerging threat of drug-resistant TB. Despite this, national data on INH monoresistance remain scarce. Given the increasing burden of TB and the critical importance of early detection of drug resistance, it is essential to better understand patterns of resistance beyond RIF. It is within this context that we conducted the present study, which aims to investigate INH resistance in TB cases (pulmonary or extrapulmonary, new or previously treated) over a period of 3 years. Materials and methods. This is a retrospective study conducted at the Bacteriology Department of Mohammed V Military Instruction Hospital over a period of 3 years. Data were collected via the laboratory information system. Clinical samples underwent treatment using both conventional bacteriological methods and molecular techniques. The study of resistance to major anti-TB drugs was performed using the reverse hybridization technique, specifically the HAIN method (GenoType^® MTBDR plus by Hain Lifescience). Statistical analysis was performed using IBM SPSS Statistics 19 and Microsoft Excel 2019. Results. The study involved 464 patients treated for pulmonary and extrapulmonary TB, including both new cases and those previously treated with positive cultures. The mean age of the patients was 42.2 years, with a range from 8 to 88 years. There was a predominance of males at 74%, with a sex ratio of 2.8. Pulmonary sputum samples accounted for 84.8% of the cases, whereas extrapulmonary samples represented only 15.2%, and the positivity rates for direct examination and culture across all samples were 74% and 100%, respectively. INH resistance had a prevalence of 9% (43 out of 464). Genetic mutations observed indicated that 63% of the clinical isolates resistant to INH had mutations in the katG gene, while 37% had mutations in the inhA gene. Conclusion. The increasing prevalence of Mycobacterium tuberculosis complex strains resistant to one or more first-line anti-TB drugs highlights the urgent need for targeted and ongoing epidemiological surveillance. In this study, we found that INH resistance affected 9% of TB cases over the 3-year period, underscoring a significant yet under-recognized threat to TB control efforts in Morocco. Molecular analysis revealed that the majority of resistant strains carried mutations in the katG gene, with a smaller proportion exhibiting mutations in the inhA promoter region. These findings emphasize the importance of incorporating molecular diagnostics capable of detecting INH resistance even in the absence of RIF resistance into routine TB surveillance programmes. Strengthening diagnostic capacity and updating treatment protocols accordingly will be essential to curb the spread of INH-resistant TB and prevent the emergence of MDR forms.