Mycobacterium gilvum Illustrates Size-Correlated Relationships between Mycobacteria and Acanthamoeba polyphaga
ABSTRACTMycobacteria 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. gilvumto survive in the trophozoites ofAcanthamoeba polyphagastrain Linc-AP1 by using optical and electron microscopy and culture-based microbial enumerations in the presence of negative controls. We observed that 29% ofA. polyphagacells were infected byM. gilvummycobacteria by 6 h postinfection. SurvivingM. gilvummycobacteria did not multiply and did not kill the amoebal trophozoites during a 5-day coculture. Extensive electron microscopy observations indicated thatM. gilvummeasured 1.4 ± 0.5 μm and failed to findM. gilvumorganisms in the amoebal cysts. Further experimental study of two other rapidly growing mycobacteria,Mycobacterium rhodesiaeandMycobacterium 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 do not significantly grow within and do not kill amoebal trophozoites, in contrast to mycobacteria measuring >2 μm (P< 0.05). The mechanisms underlying such an observation remain to be determined.