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

Abstract

Mycobacterium avium is a common cause of systemic bacterial infection in patients with AIDS. Infection with M. avium has been linked to bacterial colonization of domestic water supplies and commonly occurs through the gastrointestinal tract. Acanthamoeba castellanii, a waterborne protozoan, may serve 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 bacteria in human macrophages and intestinal epithelial cell line HT-29 as well 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, azithromycin, 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 findings may have significant implications for prophylaxis of M. avium infection in AIDS.

FIG. 1

Activities of rifabutin (RIF), clarithromycin (CLM), and azithromycin (AZM) against intracellular M. avium were examined according to two different protocols. (A) A. castellanii and macrophages (U937) were infected as described in the text, and treatment was initiated 4 h after infection; asterisks show that P was <0.05 for the comparison between the number of bacteria in the untreated control and the number of organisms within macrophages following all three antibiotic treatments. No statistically significant difference was observed among the Acanthamoeba groups. (B) A. castellanii was infected with M. avium, and 4 days after infection, treatment with antimicrobials was initiated and continued for 4 more days. No statistically significant difference was observed between the untreated control and the groups treated with antimicrobials.

FIG. 2

Activities of rifabutin (RIF), clarithromycin (CLM), and azithromycin (AZM) against S. aureus within A. castellanii. P was <0.05 for the comparisons between the number of bacteria in untreated control and the number of bacteria within Acanthamoeba treated with the three antibiotics.

FIG. 3

Effect of antibiotics against intra-amoeba M. avium at a 10-fold increase in concentration. Acanthamoeba was infected with M. avium and after 4 h treated with antibiotics for 4 days. P was >0.05 for the comparison between the number of bacteria within untreated control (4-day control) and the number of bacteria within Acanthamoeba treated with antibiotics. BL, baseline, time zero; control, untreated. Rif, rifabutin; Clm, clarithromycin; Azm, azithromycin.