Mycobacterium gilvum Illustrates Size-Correlated Relationships between Mycobacteria and Acanthamoeba polyphaga

Mycobacteria are isolated from soil and water environments, where free-living amoebae live. Free-living amoebae are bactericidal, yet some rapidly growing mycobacteria are amoeba-resistant organisms that survive in the amoebal trophozoites and cysts. Such a capacity has not been studied for the environmental rapidly growing organismMycobacterium gilvum . We investigated the ability ofM. gilvum to survive in the trophozoites ofAcanthamoeba polyphaga strain Linc-AP1 by using optical and electron microscopy and culture-based microbial enumerations in the presence of negative controls. We observed that 29% ofA. polyphaga cells were infected byM. gilvum mycobacteria by 6 h postinfection. SurvivingM. gilvum mycobacteria did not multiply and did not kill the amoebal trophozoites during a 5-day coculture. Extensive electron microscopy observations indicated thatM. gilvum measured 1.4 ± 0.5 μm and failed to findM. gilvum organisms in the amoebal cysts. Further experimental study of two other rapidly growing mycobacteria,Mycobacterium rhodesiae andMycobacterium thermoresistibile , indicated that both measured <2 μm and exhibited the same amoeba-mycobacterium relationships asM. gilvum . In general, we observed that mycobacteria measuring 2 μm (P < 0.05). The mechanisms underlying such an observation remain to be determined.