Evolution of mycobacterial infections in a university hospital in the north of Portugal

Mycobacteria are divided into two major groups: the Mycobacterium tuberculosis Complex (MTC), responsible for tuberculosis (TB), and nontuberculous mycobacteria (NTM), environmental organisms whose clinical importance has been increasingly recognized. Although TB incidence has declined in many countries, the growing recovery of NTM from clinical specimens poses new diagnostic and clinical challenges.

This retrospective cross-sectional study assessed trends in MTC and NTM isolates over a six-year period (2017–2022) at a university hospital in northern Portugal. Laboratory records from all patients investigated for mycobacterial infection were reviewed. Demographic data, microbiological findings, and species identification results were collected and analysed using Microsoft Excel®.

Clinical specimens from multiple anatomical sites were processed according to routine laboratory procedures, including Auramine–Rhodamine smear microscopy, culture in MGIT® and Lowenstein–Jensen media, molecular identification (GeneXpert® and/or Allplex™), and first-line drug susceptibility testing (DST) for MTC isolates. TB was defined by compatible clinical and radiological findings together with MTC isolation in culture.

Among 548 individuals with positive mycobacterial cultures, 365 (66.6%) had MTC and 183 (33.4%) had NTM. Patients with TB were predominantly male (69.3%), and pulmonary disease accounted for 79.5% of cases, while disseminated TB represented 9.9%. Smear-positive pulmonary TB was detected in 55.5% of respiratory specimens. DST was performed in 327 MTC isolates, of which 78.9% were fully susceptible, 18.3% were monoresistant, 2.4% were polyresistant, and 0.3% were multidrug-resistant.

Over the study period, MTC cases tended to decrease, whereas NTM isolations increased, although neither trend reached statistical significance (linear regression: p = 0.106 and p = 0.225, respectively). The decline in TB alongside the increasing recovery of NTM highlights the importance of accurate laboratory identification and careful interpretation of NTM isolates, particularly in distinguishing contamination, colonisation, and true infection. Ongoing epidemiological surveillance and rapid diagnostic methods remain essential for appropriate patient management.