Reply to Brennan, “Biofilms and Mycobacterium tuberculosis”

REPLY

We thank Brennan (1) for his interest in our work. Mycobacteria, including the pathogen Mycobacterium tuberculosis, commonly aggregate and form biofilms in the absence of detergents. Our investigation of the MmpL11 transporter sparked an interest in these multicellular structures, which are readily obtained in vitro and exhibit phenotypic resistance to antibiotics. It is likely that bacteria within the pellicle experience nutrient and oxygen restriction, signals commonly incorporated into in vitro models of dormancy. Therefore, we consider biofilm formation in vitro to be a relevant context to further investigate the cell wall changes that M. tuberculosis undergoes to establish a nonreplicating persistent state. We would highlight that extracellular M. tuberculosis cells are observed in necrotic granulomas of humans and model organisms, where these bacteria adopt the form of a corded or multicellular complex (2, 3). Granulomas are unequivocally a central component of M. tuberculosis latency. We capitalized on a human in vitro granuloma model that uses cells from latently infected individuals to assess the ability of the mmpL11 mutant to withstand immune pressure, which would be present during persistent infection. Combined, we feel that the results of our investigations provide further support that MmpL11 is important in phenotypes associated with latency.