Molecular identification and antimicrobial susceptibility of Mycolicibacterium smegmatis associated with bovine mastitis

Bovine mastitis presents a significant health and economic issue in dairy farming, typically caused by common bacterial pathogens, but atypical organisms such as non-tuberculous mycobacteria (NTM) may occasionally be involved. Among rapidly growing mycobacteria (RGM), Mycolicibacterium smegmatis is usually considered an environmental saprophyte, although sporadic mastitis cases have been described. In this article, a rare case of bovine mastitis in a Simmental cow is presented. The animal showed no systemic clinical signs and failed to respond to intramammary antibiotic therapy. Milk culture revealed acid-fast rods with features consistent with RGM, but MALDI-TOF MS did not provide conclusive identification. Final confirmation was obtained by 16S rRNA gene sequencing, showing 99.73% similarity to M. smegmatis. Antimicrobial susceptibility testing showed resistance to cefoxitin, clarithromycin, and trimethoprim-sulfamethoxazole, while the isolate was sensitive to aminoglycosides, fluoroquinolones, tetracyclines, carbapenems, and linezolid. Rifampicin and azithromycin exhibited high minimum inhibitory concentration (MIC) values consistent with known reduced susceptibility. This case illustrates the diagnostic difficulties associated with atypical mycobacteria, particularly the limitations of conventional phenotypic methods and the importance of molecular approaches. It also underlines the need to consider RGM in cases of mastitis that do not respond to conventional therapy and adds to the limited body of evidence documenting their role in bovine pathology.