Brady, M. F., Coronel, J., Gilman, R. H., Moore, D. A. The MODS method for diagnosis of tuberculosis and multidrug resistant tuberculosis. J. Vis. Exp. (18), e845, doi:10.3791/845 (2008).
Patients with active pulmonary tuberculosis (TB) infect 10-15 other persons per year, making diagnosing active TB essential to both curing the patient and preventing new infections. Furthermore, the emergence of multidrug resistant tuberculosis (MDRTB) means that detection of drug resistance is necessary for stopping the spread of drug-resistant strains. The microscopic-observation drug-susceptibility (MODS) assay is a low-cost, low-tech tool for high-performance detection of TB and MDRTB. The MODS assay is based on three principles: 1) mycobacterium tuberculosis (MTB) grows faster in liquid media than on solid media 2) microscopic MTB growth can be detected earlier in liquid media than waiting for the macroscopic appearance of colonies on solid media, and that growth is characteristic of MTB, allowing it to be distinguished from atypical mycobacteria or fungal or bacterial contamination 3) the drugs isoniazid and rifampicin can be incorporated into the MODS assay to allow for simultaneous direct detection of MDRTB, obviating the need for subculture to perform an indirect drug susceptibility test. Competing current diagnostics are hampered by low sensitivity with sputum smear, long delays until diagnosis with solid media culture, prohibitively high cost with existing liquid media culture methods, and the need to do subculture for indirect drug susceptibility testing to detect MDRTB. In contrast, the non-proprietary MODS method has a high sensitivity for TB and MDRTB, is a relatively rapid culture method, provides simultaneous drug susceptibility testing for MDRTB, and is accessible to resource-limited settings at just under $3 for testing for TB and MDRTB.
The MODS assay is targeted at resource-limited settings. For the first time, MODS brings the ability for rapid liquid culture detection of tuberculosis and multidrug resistant tuberculosis to resource-limited settings at just under $3 per test. MODS is a non-proprietary, iterative methodology, and the MODS community is always interested in improvements that other laboratories have managed to make.
A recurrent concern is the biosafety of liquid media culture of tuberculosis because liquids can be spilt or aerosolized. We believe that the MODS assay is more biosafe than any assay that involved indirect drug susceptibility testing because indirect drug susceptibility testing involves the manipulation of highly concentrated solutions of mycobacteria with the attendant risks of spillage and aerosolization; in contrast, MODS simply involves the inoculation of a sputum sample into a plate, after which the plate is sealed within a plastic bag and never again opened. This is supported by data from Korea (Kim 2007) which showed that the occupational risk to laboratory workers who plated out sputum samples without doing drug-susceptibility testing was no greater than that in workers doing sputum smear microscopy; in contrast, those doing drug-susceptibility testing had much higher occupational risk for tuberculosis.
We strongly recommend that none of these procedures be undertaken without proper precautions for laboratory workers. This includes N-95 masks for personal respiratory protection, a Class 2 biological safety cabinet with exhausted air filtered through HEPA filters, and a lock on the laboratory door to stop turbulence of airflow while samples are being manipulated.
Standard operating procedures for processing extrapulmonary samples, a photo library of mycobacterium tubercuclosis and other mycobacteria and bacterial and fungal contamination, a recommended quality assurance strategy, a procedure for accreditation of laboratories to start using MODS, and an FAQ sheet are available at modsperu.org
We would like to acknowledge Sean Fitzwater and Carmen Giannina Luna Colombo for the tuberculosis growth time-lapse video segment. We thank Marty Roper for her thorough and excellent feedback during the editing and co-authoring the User Guide, from which the current protocol was taken mostly verbatim. Production of this video was funded by the NIH/ Fogarty International Center http://www.fic.nih.gov/ David A.J. Moore contributed as a Wellcome Trust Clinical Research Fellow in Tropical Medicine and Reader in Infectious Diseases at Imperial College London (Fellowship award number 078067/Z/05). Mark F. Brady contributed as an NIH/Fogarty International Center Research Fellow.
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