Mycobacterium avium grown in Acanthamoeba castellanii is protected from the effects of antimicrobials

Mycobacterium avium is a common cause of systemic bacterial infection in pa tients with AIDS. Infection with M. avium has been linked to bacterial colo nization of domestic mater supplies and commonly occurs through the gastroi ntestinal tract. Acanthamoeba castellanii, a waterborne protozoan, mag serv e as an environmental host for M. avium. It has been shown that growth of M . avium in amoebae enhances invasion and intracellular replication of the b acteria in human macrophages and intestinal epithelial cell line HT-29 as w ell as in mice. We determined that growth of M. avium within A. castellanii influenced susceptibility to rifabutin, azithromycin, and clarithromycin. No significant activity against M. avium was seen with rifabutin, azithromy cin, and clarithromycin when used to treat monolayers on both day 1 and day 4 after infection. When tested in a macrophage-like cell line (U937), all compounds showed significant anti-M. avium activity. Growth of M. avium in amoebae appears to reduce the effectiveness of the antimicrobials. These fi ndings may have significant implications for prophylaxis of M. avium infect ion in AIDS.