Journal of Clinical Microbiology 2012, vol 50(2): 274–280
Trained African giant-pouched rats (Cricetomys gambianus) can detect Mycobacterium tuberculosis and show potential for the
diagnosis of tuberculosis (TB). However, rats’ ability to discriminate between clinical sputum containing other Mycobacterium
spp. and nonmycobacterial species of the respiratory tract is unknown. It is also unknown whether nonmycobacterial species
produce odor similar to M. tuberculosis and thereby cause the detection of smear-negative sputum. Sputum samples from 289
subjects were analyzed by smear microscopy, culture, and rats. Mycobacterium spp. were isolated on Lowenstein-Jensen medium,
and nonmycobacterial species were isolated on four different media. The odor from nonmycobacterial species from
smear- and M. tuberculosis culture-negative sputa detected by >2 rats (“rat positive”) was analyzed by gas chromatographymass
spectrometry and compared to the M. tuberculosis odor. Rats detected 45 of 56 confirmed cases of TB, 4 of 5 suspected cases
of TB, and 63 of 228 TB-negative subjects (sensitivity, 80.4%; specificity, 72.4%; accuracy, 73.9%; positive predictive value,
41.7%; negative predictive value, 93.8%). A total of 37 (78.7%) of 47 mycobacterial isolates were M. tuberculosis complex, with
75.7% from rat-positive sputa. Ten isolates were nontuberculous mycobacteria, one was M. intracellulare, one was M. avium
subsp. hominissuis, and eight were unidentified. Rat-positive sputa with Moraxella catarrhalis, Streptococcus pneumoniae,
Staphylococcus spp., and Enterococcus spp. were associated with TB. Rhodococcus, Nocardia, Streptomyces, Staphylococcus, and
Candida spp. from rat-positive sputa did not produce M. tuberculosis-specific volatiles (methyl nicotinate, methyl para-anisate,
and ortho-phenylanisole). Prevalence of Mycobacterium-related Nocardia and Rhodococcus in smear-negative sputa did not
equal that of smear-negative mycobacteria (44.7%), of which 28.6% were rat positive. These findings and the absence of M.
tuberculosis-specific volatiles in nonmycobacterial species indicate that rats can be trained to specifically detect M. tuberculosis.