Fate of tubercle bacilli in early experimental infection of the mouse

1. The question of a possible eclipse phase during the first week of experimental mouse tuberculosis has been examined.2. Using fluorescence microscopy to examine sections and to perform counts of stainable bacilli for comparison with counts of culturable bacilli in the lungs it was found that bacilli multiplied very slowly during the first 5 days, and rapidly from about the sixth day onwards.3. At all times there was close agreement between stainable and culturable bacilli in the lungs.4. During the lag phase, the bacilli occurred as widely scattered, often weakly acid-fast cells that were difficult to defect by Ziehl–Neelsen staining.5. Active removal of bacilli by phagocytes is believed not to be important at this stage.6. After 5 or 6 days, clusters of bacilli, rapidly increasing in number and size and strongly acid fast, were readily detected in the lungs.