Background- Spiroplasmas are helical, motile mollicutes. The control of their shape and their motility mechanism are only partially understood. The proteins potentially involved in Spiroplasma citri morphogenesis and motility are the main component of the axial internal filament (Fibril), and the five actin-like MreBs. Unlike spiroplasmas, Mycoplasma capricolum subsp. capricolum (Mcc) is non-motile and pleiomorphic although being phylogenetically close to S. citri.Method – All genes encoding the 5 MreBs and fibril were introduced in Mcc chromosome, and their expression was checked by proteomics. Motility and global shape were analyzed using dark-field microscopy, and the cytoskeleton was further imaged by cryoelectron microsocopy.Results – Mcc transformants bearing the seven genes (mreB1-5, SPICI01A_046 and fib) grew either as fully or partially helical cells, or as straight and branched filamentous ones. Using cryoelectron microscopy, the different shapes could be associated to different modes of interaction between the cytoskeleton and the plasma membrane, and expression of cytoskeletal proteins was associated with an increase of the cell body rigidity. Helices showed propagation of kinks and motility movements, which were however not sufficient to efficiently move the cells in one direction in the culture medium, and which did not increase with viscosity. Both helicity and motility showed dependency on pH and temperature. Conclusion – Controlling the shape of Mycoplasma cells enable us not only to investigate the roles of Fib and MreBs in motility and helicity, but also to better? comprehend the adaptive value of Mollicutes’ shapes.< Réduire