Detection of “Non-culturable” <i>Mycobacterium tuberculosis</i> Cells by Culture Methods

Abstract

The specific features of the reactivation of “non-culturable” (NC) Mycobacterium tuberculosis (MTB) cells for the following propagation in liquid medium were defined, as well as the approach for the quantification of these cells by culture methods was suggested. When NC cells obtained in vitro were inoculated into standard Sauton’s liquid medium, a prolonged (up to 20 days) lag-phase is observed, in which no cell propagation is occurred. During the lag-phase, dormant cells secrete substances of unknown nature that inhibit or completely arrest the reactivation and growth of cultures when initial dormant cells concentration is above 107–108 cells/mL. Addition of meat-peptone broth (3.25 g/L) into a standard Sauton’s medium with a 10-fold reduced concentration of glycerol significantly stimulated the reactivation and propagation of the initially dormant cells inoculated at a concentration above the threshold. This modification of the medium composition made it possible to detect up to 103 times more cells in the MTB population by the method of the Most Probable Number (MPN) of cells. Increased number of the detected dormant cells in the population (up to an average of 2.5 orders of magnitude) was also achieved by adding to the reactivation medium recombinant protein RpfB (5 ng/mL), a protein of the Rpf family – resuscitation promoting factor of dormant bacteria. Perhaps the action of a Rpf enzyme is related to the products of its enzymatic activity, since an increased MPN value in the dormant cells population was also observed when the products of mycobacteria peptidoglycan hydrolysis obtained by the coaction of RpfB and endopeptidase RipA were added. The addition of sonicated peptidoglycan fragments at a concentration up to 1 μg/mL had a similar effect. The obtained results may be used as approaches to optimize liquid media composition and culture conditions aiming to identify in clinical samples the pathogen of tuberculosis remain in “non-culturable” state.