Intracellular fate of Mycobacterium avium subspecies paratuberculosis in monocytes from normal and infected, interferon-responsive cows as determinedby a radiometric method

The ability of Mycobacterium avium subsp, paratuberculosis to survive in bo vine monocytes was studied using radiometric (BACTEC) culture, standard pla te counting and microscopic counting of acid-fast stained monocyte monolaye rs, Results of microscopic counts sharply contrasted with results of viable counts determined both by plate counting and radiometric counting. We obse rved an early phase (the first 6 d after in vitro infection) of intracellul ar bacillary growth, followed by a later phase of mycobacteriostasis or kil ling (up to 12 d after in vitro infection) in monocytes from non-infected c ows. The data suggest that multiplication and death of M. avium subsp. para tuberculosis occur simultaneously in bovine monocytes infected in vitro. Us ing the BACTEC method, we compared the ability of bovine monocytes from nor mal cows and cows infected with M, avium subsp, paratuberculosis and showin g evidence of a strong Th1-like cellular immune response to ingest and inhi bit the intracellular growth of M, avium subsp, paratuberculosis, There was a trend toward greater phagocytosis and faster killing of Mycobacterium av ium subsp, paratuberculosis by monocytes from the infected, immune responde r cows. However, the observed numbers of viable M, avium subsp, paratubercu losis at each time after monocyte infection were not significantly differen t between normal and infected cows.