Symbiont- and Bacterial Cell Size-dependent Backpacking and Grazing of <i>Acanthamoeba</i>

On solid media, Acanthamoeba harboring the endosymbiotic bacterium (Neochlamydia) carries live Escherichia coli on its cell surface without phagocytosing this bacterium, and defends against infection by Legionella pneumophila of a small size in an endosymbiotic bacterium-dependent manner. This implies the presence of an unknown protective mechanism. Therefore, we exami-ned the physical properties of the carried bacteria using transposon insertion mutants that had lost the "backpack" on solid media. A mutant was selected from a library in which the nhaA gene, encoding a Na⁺/H⁺ antiporter, was disrupted. The knockout mutant was longer than the parental strain and was gradually consumed by symbiotic amoebae. Similarly, the NhaA-knockout mutant strain was longer and lacked the backpack. The complementation of nhaA restored bacterial cells to their normal size, and the backpack phenomenon reappeared. Using E. coli elongated by a treatment with mitomycin C, the backpack was not evident, and enlarged bacteria were consumed by symbiotic amoebae. Therefore, symbiotic amoebae protected themselves from intruders by not engulfing small bacteria in an endosymbiosis-dependent manner. The present results propose a novel countermeasure by phagocytic cells against intruders that involves the recognition of bacterial sizes and is dependent on endosymbiosis.