The emergence of mycobacteria resistant to currently available antimicrobial 
agents has become an important problem in modern medicine. Mycobacterium avium 
and M. tuberculosis are intracellular pathogens that replicate and survive 
within the mononuclear phagocytes. TM4 is a lytic mycobacteriophage that kills 
both extracellular M. avium and M. tuberculosis. When delivered by M. smegmatis 
transiently infected with TM4, it kills both M. avium and M. tuberculosis within 
RAW 264.7 macrophages. To evaluate the treatment of M. avium infection with 
phage in vivo, C57 BL/6 mice were infected with M. avium 109 and, 7 days later, 
treated either once or twice with TM4 phage (7.9 x 10(10) PFU/ml), M. smegmatis 
(4 x 10(8) cFU/ml), or M. smegmatis with TM4 phage delivered intravenously 
(i.v.). Treatment with TM4 phage alone or M. smegmatis without TM4 did not show 
a significant decrease in number of intracellular bacteria in the spleen 
compared with untreated control. In contrast, administration of M. smegmatis-TM4 
resulted in a significant decrease in the number of M. avium in the spleen. 
However, 23% of bacteria recovered from treated mice were resistant to TM4. 
These in vivo studies confirmed the in vitro findings that an avirulent 
mycobacterium can be used as a carrier to deliver antimycobacterial phage 
intracellularly.