Microscopic morphology in smears prepared from MGIT broth medium for rapid presumptive identification of Mycobacterium tuberculosis complex, Mycobacterium avium complex and mycobacterium kansasii

Mycobacterium species has a specific morphology when grown in liquid medium. Mycobacterium tuberculosis complex (MTB) often exhibits serpentine cording, which is different from the dot and crossbarring morphology observed in Mycobacterium avium complex (MAC) and Mycobacterium kansasii (MK), respectively. These characteristic morphologies can be used as a cost-effective method for rapid, presumptive identification of mycobacterial isolates cultured from the MGIT 960 system. By using Kinyoun acid-fast stain, serpentine cording was found in 840 of 904 (92.1%) samples positive for MTB; dot or loose aggregation was observed in 112 of 136 (82.3%) samples positive for MAC; and the cross-barring, ladder-like, morphology was observed in 45 of 56 (80.5%) samples positive for MK. The sensitivity and specificity were 92.9% and 96.4% for MTB; 82.4% and 94.5% for MAC; and 80.4% and 94.6% for MK, respectively. Using growth rate selection to exclude rapid growers, the positive and negative predictive values were 98% and 87.6% for MTB; 78.3% and 98% for MAC; and 78.9% and 99.1% for MK, respectively. Twenty-eight (93.3%) of 30 strains with ball morphology were rapid growers. Microscopic morphology can be used for rapid, presumptive identification of M. tuberculosis complex, M. kansasii, and M. avium complex and act as a guide for appropriate selection of initial probes to reduce costs.