Morphological changes induced during fixation of Mycoplasma mycoides for electron microscopy

Only spherical mycoplasmas 0.6-0.9 mum diam were observed by darkfield microscopy in single cell suspensions prepared from exponential broth cultures of Mycoplasma mycoides var mycoides and Mycoplasma mycoides var capri. Similar cells were seen in pseudoreplicas by electron microscopy and they are considered characteristic of the morphology of M. mycoides. When the mycoplasmas were fixed by the addition of 10% formalin to the suspensions or washed, centrifuged cells were fixed by glutaraldehyde, filamentous and ring forms were observed in electron micrographs. These are not considered typical of the morphology of M. mycoides but are artifacts produced during the preparation of the mycoplasmas for electron microscopy.