Abstract: Objective To analyze the gene polymorphism of Mycobacterium tuberculosis (MTB) with negative MPT64 antigen in culture filtrate. Methods A total of 1962 mycobacteria clinical isolates were collected from Xi’an Chest Hospital between January 2018 and June 2020 were retrospectively analyzed. The samples were from sputum, bronchoalveolar lavage fluid and pleural effusion, positive in BACTEC MGIT 960 and confirmed by Ziehl-Neelsen anti-acid dyeing method. They were also tested by MPT64 antigen detection (colloidal gold immune-chromatography), p-nitrobenzoic acid/thiophene-2-carboxylic acid hydrazide (PNB/TCH) growth test and mycobacterial species identification (DNA microarray chip method). When results of MPT64 antigen test and the mycobacterial species identification was not inconsistent, GeneXpert MTB/RIF and PNB culture test were performed. MPT64 gene sequencing was performed on 14 strains identified as MTB but negative in MPT64 antigen test. Results Among the 1962 clinical isolates of mycobacteria, 88 (4.5%) were NTM and 1874 (95.5%) were MTB. By MPT64 antigen detection, 87 non-tuberculous mycobacteria (98.9%) and 14 MTB (0.7%) were negative. The results of MPT64 gene sequencing showed that 92.9% (13/14) of the 14 MTB clinical isolates negative in MPT64 antigen detection had mutations in nucleotides 197-259 and amino acid deletions at positions 66-86 of the MPT64 protein gene, and the other one had an IS6110 gene fragment of 1361 bp inserted at position 587. Conclusion The main reason for the false negative detection of the MTB strain in MPT64 antigen test might be the deletion mutation of nucleotides 197-259 of the MPT64 protein gene, and the insertion of IS6110 fragment might also cause the false negative detection.

Key words: Mycobacterium tuberculosis, Antigens, bacterial, Polymorphism, single nucleotide, Bacterial proteins