Environmental chlamydiae alter the growth speed and motility of host acanthamoebae.

Symbiosis between living beings is an important driver of evolutionary novelty and ecological diversity; however, understanding the mechanisms underlying obligate mutualism remains a significant challenge. Regarding this, we have previously isolated two different Acanthamoeba strains harboring endosymbiotic bacteria, Protochlamydia (R18 symbiotic amoebae: R18WT) or Neochlamydia (S13 symbiotic amoebae; S13WT). In this study, we treated the symbiotic amoebae R18WT and S13WT with doxycycline (DOX) and rifampicin (RFP), respectively, to establish the aposymbiotic amoebae R18DOX and S13RFP, respectively. Subsequently, we compared the growth speed, motility, phagocytosis, pinocytosis, and morphology of the symbiotic and aposymbiotic amoebae. The growth speed of R18DOX was decreased, although that of S13RFP was increased. A marked change in motility was observed only for R18DOX amoebae. There was no difference in phagocytic and pinocytic activities between the symbiotic and aposymbiotic amoebae. Meanwhile, we observed a significant change in the phalloidin staining pattern and morphological changes in R18DOX (but not S13RFP) aposymbiotic amoebae, indicating a change in actin accumulation upon removal of the Protochlamydia. Infection of C3 (a reference strain) or S13RFP amoebae with Protochlamydia had a harmful effect on the host amoebae, but R18DOX amoebae re-infected with Protochlamydia showed recovery in both growth speed and motility. Taken together, we conclude that endosymbiont environmental chlamydiae alter the growth speed and/or motility of their host Acanthamoeba, possibly implying an close mutual relationship between amoebae and environmental chlamydiae.