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

Abstract

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 cross-barring 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.