Prolongation of incubation time improves clinical diagnosis of Mycobacterium xenopi infection and allows susceptibility testing of mycobacterial strains against multiple antibiotics

Objectives: Mycobacterium xenopi is a nontuberculous mycobacterium (NTM) whose clinical diagnosis and drug susceptibility studies are frequently hampered by poor in vitro growth. Extending the culture incubation time from 42 days (common-standard) to 56 days could improve the likelihood of diagnosis and provide strains for phenotypic drug susceptibility profiling of this poorly studied but clinically relevant mycobacterium. Methods: Time-to-positivity of mycobacterial cultures incubated for 56 days were analysed and compared. Clinical mycobacteriosis was defined by ATS/IDSA criteria. In vitro susceptibility of M. xenopi isolates was tested by broth microdilution. Results: Of 3852 mycobacteria-positive cultures (26 different mycobacterial species),M. xenopi required by far the longest growth time in culture, exceeding the 42 days commonly used in routine diagnostics in 41.2% of cases versus 4.7% for other NTM and 2.0% for Mycobacterium tuberculosis complex (P