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Is M. saskatchewanense misidentified as M. intracellulare using the DNA STRIP technology-based method?

F Bisognin(1,3) F Messina(2) V Ferraro(1) O Butera(2) F Sorella(3) E Girardi(2) A Cannas(2) P Dal Monte(1,3)

1:IRCCS Azienda Ospedaliero-Universitaria di Bologna; 2:IRCCS National Institute for Infectious Diseases L. Spallanzani; 3:Alma Mater Studiorum, University of Bologna

M. saskatchewanense is a recently identified species of pigmented and slowly growing Non-Tuberculous Mycobacteria (NTM), positive to Mycobacterium avium complex (MAC) by AccuProbe system. MAC organisms have been frequently isolated from water samples, collected from different medical devices.

This study aims to assess if DNA STRIP technology misidentified M. saskatchewanense with mycobacteria belonging to Mycobacterium avium complex.

From August 2022 to February 2023, 346 medical device water samples were processed by the referral centre for the detection of mycobacteria in the Emilia-Romagna region, Italy.

Twenty-one MGIT cultures were positive for NTM. GenoType Mycobacterium CM (CM, Bruker) was used as first step for strain identification, and all were found to be components of MAC. A further procedure with GenoType NTM-DR (NTM-DR, Bruker) was performed to discriminate among the different species. Subcultures from positive MGIT were made on Middlebrook 7H11 (BD) to obtain a microbial growth for: (i) phenotypical confirmation by MALDI-TOF mass spectrometry and (ii) molecular confirmation by WGS (MiSeq Illumina) at INMI “L. Spallanzani”, Rome.

All positive cultures were identified as M. intracellulare by CM and NTM-DR assays but, in contrast, colony morphology showed a bright yellow scotochromogenic growth. MALDI-TOF analyses identified the strains as M. saskatchewanense with a high score, and the identification were confirmed by WGS analysis. Sequences are available under the BioProject accession number: PRJNA937079.

MALDI-TOF analysis, supported  by colony morphology and confirmed by WGS, identified NTM as M. saskatchewanense while NTM-DR, the most common genotypic methods used to discriminate strains belonging to MAC complex, misidentified it as M. intracellulare species.

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