The main lesion in pulmonary tuberculosis, the
pulmonary cavity, contains a large number of
mycobacteria (about 108 colony forming units). Of
these, a large bacillary population is located in the
thin liquid caseous layer that covers the inner part
of the cavitary wall. Here, the bacilli are
extracellular which multiply actively because of
the availability of oxygen and nutritive
substances. There are at least 2 other bacillary
populations, one inside macrophages and another
inside solid caseous foci; both these populations
are limited in size because environmental
conditions are unfavourable for their growth.1
Among the organisms in these 3 populations,
which are normally drug sensitive, drug
resistant mutants develop at a mean frequency of
about 10-6
