Use of microscopic morphology in smears prepared from radiometric cultures for presumptive identification of Mycobacterium tuberculosis complex, Mycobacterium avium complex, Mycobacterium kansasii, and Mycobacterium xenopi.

The aim of this study was to assess the feasibility of a method for presumptive identification of mycobacteria, based on the morphology in smears prepared from radiometric Bactec-positive cultures (Becton Dickinson, USA) and to select the appropriate DNA probe (AccuProbe; Gen Probe, USA). The smear morphology of acid-fast bacilli was evaluated in 468 positive cultures from clinical samples: 313 Mycobacterium tuberculosis complex, 67 Mycobacterium avium complex, 32 Mycobacterium kansasii, 49 Mycobacterium xenopi, and seven Mycobacterium gordonae. The sensitivity and specificity for various morphological patterns were as follows: cord formation for Mycobacterium tuberculosis complex 90% and 100%, respectively; striped bacilli for Mycobacterium kansasii, 66% and 99%; sea urchin for Mycobacterium xenopi, 96% and 99%; short bacilli for Mycobacterium avium complex, 61% and 99%; fine-striped bacilli associated with Mycobacterium avium complex from blood samples, 33% and 98%. This criterion was applied in the selection of a suitable DNA probe for the identification of 178 cultures. The correct probe was selected in 98%, 97%, and 72% of cultures, respectively, for Mycobacterium tuberculosis complex, Mycobacterium avium complex, and Mycobacterium kansasii. The observation of acid-fast bacilli morphology in radiometric cultures is a rapid and cost-efficient method for presumptive identification of common clinical isolates of mycobacteria.