INTRODUCTION

Tuberculosis (TB) is the single most important infectious disease in the world. This disease is caused by Mycobacterium tuberculosis. Although tuberculosis can be cured by antimicrobial therapy and can be prevented by a vaccine [Bacille Calmette-Guerin (BCG)], the total number of cases in the world is still increasing (52). There are 8 millions new cases every year, and over 3 million TB patients will die of this disease (33). After infection, which usually takes place in childhood, the bacteria are capable of persisting in a non-replicating or quiescent state for prolonged periods of time (73). These quiescent bacteria are called “immune persisters” by the authors, because they are induced by, and survive, the immune response (73). Reactivation of persisters, sometimes many years later, is responsible for most cases of active disease (59, 73). About one third of the world's population, two billion people, contain quiescent M. tuberculosis (60) and so this is by far the most common form of the microbe. These carriers provide a huge pool of potential disease, the lifetime risk of a carrier developing active tuberculosis being 5–10% (18). It is not known why many carriers break down into disease, but in some, weakening of the immune system, for example, by co-infection with HIV, is strongly associated with tuberculosis (18).

The main disease-control measures used for tuberculosis are casefinding and chemotherapy (3). However, chemotherapy itself induces persisters which are called “drug persisters” by the authors (30).