LAUSR

In 2015, the World Health Organization (WHO) included a health target in the newly adopted Sustainable Development Goals, striving to end the tuberculosis (TB) epidemic by the year 2030. In low-income countries, TB continues to be one of the most prevalent infectious diseases with leading mortality rates in the pediatric population. A key reason for these unacceptable high morbidity and mortality rates is the poor sensitivity of available diagnostic tests to detect TB. In these countries, pediatric TB is primarily diagnosed based on clinical signs and smear microscopy, consequently resulting in a large proportion of undiagnosed patients. This illustrates the need for new strategies towards tuberculosis control, including development of new diagnostic modalities. In the interesting article of Mgode et al., the potential of olfactory sense of trained African giant pouched rats (Cricetomys ansorgei) was evaluated as a complementary method in the diagnostic workup of pediatric TB. They showed that implementation of trained rats could increase the detection rate of pediatric TB by 68%, compared to a single-test protocol using smear microscopy. Given the severe health implications of TB, such an increase in detection rate is of considerable clinical importance. Interestingly, already around the year 400 BC, the ancient Greek Hippocrates described the potential of the scent of sputum as diagnostic tool in TB. Over the past years, an increasing number of studies demonstrated the potential of scent molecules, or more specifically volatile organic compounds (VOCs), as a diagnostic biomarker for a wide variety of clinical conditions, including metabolic, infectious, and inflammatory diseases and malignancies. VOCs are carbon-based chemicals, volatile at ambient temperatures, and source of the majority of surrounding odors. Being produced during metabolic processes and excreted through all conceivable bodily excrements (e.g., sputum, feces, sweat, urine), they may serve as an ideal clinical biomarker for several pathophysiological processes. VOCs derived from a particular bodily substrate may either have an endogenous (local or systemic) or exogenous origin. In order to discriminate diseased from healthy state, VOCs derived from different substrates usually provide different outcomes. For example, in intestinal diseases such as inflammatory bowel disease, the discriminative accuracy of fecal VOCs is superior over that of exhaled breath and urinary volatiles. In the study by Mgode et al., sputum was chosen as a substrate of interest, which contains VOCs from both endogenous and exogenous sources. Endogenous VOCs in the sputum include volatiles originating from local pulmonary (patho)physiological processes, whereas systemic VOCs are produced during metabolic processes elsewhere in the body before being transported by the bloodstream towards the alveoli and eventually the sputum. Examples of exogenous sources include resident pulmonary microbes, inhaled VOCs, and medicinal metabolites. In TB patients, sputum presumably contains disease-specific exogenous VOCs derived from the TB-causing pathogens residing in the lungs. Mycobacterium tuberculosis, the most commonly isolated causative agent, is transmitted through aerosol droplets into the alveoli where they enter and proliferate in alveolar innate immune cells. Subsequently, infected cells may form a typical nodular granulomatous structure. In case of uncontrolled growth, this may eventually result in lymphatic and blood vessel invasion, allowing for disease dissemination. Therefore, in addition to exogenous microbial VOCs, the sputum of TB patients may also contain local VOCs derived from pulmonary pathophysiological processes and systemic VOCs derived from other infected tissues. Presumably, presence of TB-specific VOCs with both endogenous and exogenous origins allowed for the differentiation between TB and non-TB cases by giant pouched rats. However, a relatively high false-positive detection rate of approximately 25% by rats was demonstrated in previous studies. Other mycobacterial spp. and non-mycobacterial spp. presumably produce similar VOC profiles, or may provoke comparable hostspecific metabolic reactions, consequently impeding accurate identification of M. tuberculosis in sputum samples. The essential role of both host-specific and pathogen specific VOCs in the identification of TB cases by C. ansorgei is well illustrated in a study in which trained rats were presented TB-negative sputum samples spiked with M. tuberculosis culture isolates and natural TB sputum samples. Rats were able to discriminate the latter samples from controls with higher accuracy than the prepared samples, indicating that the inflammatory host response contributes a TBspecific VOC profile. Several key sputum volatiles, differentiating TB cases from non-TB cases by trained rats, were identified using gas chromatography-mass spectrometry (GC-MS). After isolation of these VOCs they were presented to trained rats. Interestingly, only a particular blend of TB-specific volatiles, provided in measured concentrations, allowed for an adequate reaction from these rats. This illustrates the challenge to unravel and identify the volatiles of interest, evoking the desired detection response in rats. Multiple other studies have attempted to identify TB-specific VOCs using chemical analytical techniques such as GC-MS in a