Determination of Mycobacterium tuberculosis odour compounds detected by Cricetomys gambianus rats for diagnosis of pulmonary tuberculosis in low-income settings

Abstract

Novel methods for rapid diagnosis of tuberculosis (TB) are urgently needed to complement the widely used smear microscopy in low income settings. Trained TB detection rats offer a promising tool for rapid diagnosis of TB in resource limited settings. The Mycobacterium tuberculosis (Mtb) specific volatile compounds detected by trained Cricetomys gambianus rats in sputum of TB patients are unknown. It is also not known whether rats detect odour signals from other mycobacteria and microorganisms related to Mtb, which cause pulmonary infections resembling TB. In this thesis I investigated Mtb-specific volatile compounds detected by trained rats and whether the target compounds are found in other mycobacterial species and related pulmonary pathogens. The ability of rats to discriminate cultures of different microorganisms, clinical sputa with different Mycobacterium spp., Mtb genotypes and other respiratory tract microorganisms was investigated. Finally, the ability of the rats to discriminate Mtb-specific odour compounds from shared compounds found in Mtb and other microorganisms was determined. Results show that Mtb produce specific volatile compounds which are not produced by other mycobacterial species, mycobacteria-related Nocardia spp. and Rhodococcus spp. and other microorganisms. Trained rats use these Mtb-specific compounds to distinguish TB-positive sputa from TB-negative sputa. Volatile compounds shared by Mtb and other mycobacteria and non-mycobacterial species are not detected by trained rats. A blend of Mtb-specific compounds is detected by rats better than individual compounds. Rats can discriminate cultures of Mtb from those of other microorganisms. Detection of Mtb cultures is influenced by growth stage. Rats detect more frequently TB-positive sputa than negative sputa spiked with Mtb or specific volatiles. Clinical sputa containing Mtb are well discriminated by rats from sputa containing other microorganisms. Sputa containing different Mtb genotypes are also detected. It is concluded that trained rats can reliably diagnose TB. Further studies are needed to determine the optimal combinations/ratios of Mtb-specific volatile compounds to yield similarly higher responses of rats as those of detection of typical TB-positive sputa. Future studies should assess detection of sputa with frequent nontuberculous mycobacterial species. Further investigations including recovery of dormant Mtb in sputum may give insights on actual cause of detection of false positive sputa currently judged by microscopy and conventional Mtb culture in which dormant Mtb cannot grow.