A Peptidoglycan Amidase Mutant of Burkholderia insecticola Adapts an L-form-like Shape in the Gut Symbiotic Organ of the Bean Bug Riptortus pedestris

Bacterial cell shapes may be altered by the cell cycle, nutrient availability, environmental stress, and interactions with other organisms. The bean bug Riptortus pedestris possesses a symbiotic bacterium, Burkholderia insecticola, in its midgut crypts. This symbiont is a typical rod-shaped bacterium under in vitro culture conditions, but changes to a spherical shape inside the gut symbiotic organ of the host insect, suggesting the induction of morphological alterations in B. insecticola by host factors. The present study revealed that a deletion mutant of a peptidoglycan amidase gene (amiC), showing a filamentous chain form in vitro, adapted a swollen L-form-like cell shape in midgut crypts. Spatiotemporal observations of the ΔamiC mutant in midgut crypts revealed the induction of swollen cells, particularly prior to the molting of insects. To elucidate the mechanisms underlying in vivo-specific morphological alterations, the symbiont was cultured under 13 different conditions and its cell shape was examined. Swollen cells, similar to symbiont cells in midgut crypts, were induced when the mutant was treated with fosfomycin, an inhibitor of peptidoglycan precursor biosynthesis. Collectively, these results strongly suggest that the Burkholderia symbiont in midgut crypts is under the control of the host insect via a cell wall-attacking agent.