Three-dimensional morphology, ultrastructure, and replication of Mycoplasma felis

Long-term survival of Mycoplasma bovis in necrotic lesions and in phagocytic cells as demonstrated by transmission and immunogold electron microscopy in lung tissue from experimentally infected calves

In the lungs of cattle infected with Mycoplasma bovis persistence of the agent for several weeks after infection has been demonstrated by different methods, e.g. isolation of the organism, immunohistochemistry for antigens, and in situ hybridization. The presence of macrophages and neutrophils with cytoplasmic M. bovis antigen suggests that phagocytosis occurs in vivo. It is, however, unknown whether this intracellular immunolabeling detected residual antigen after phagocytosis of M. bovis or surviving organisms in macrophages that use the intracellular survival as a strategy for evasion of the immune response. The aim of this electron microscopic investigation was to study the distribution of M. bovis within caseonecrotic lung lesions and to examine the phagocytes for intracellular presence of the agent. In lung tissue sections from 9 experimentally infected calves originating from two different infection experiments large numbers of mycoplasmas were detected by transmission electron microscopy and by immunogold electron microscopy using M. bovis-specific polyclonal antibodies. M. bovis were found throughout caseonecrotic foci and within the lumen of bronchi containing exudate. The majority of mycoplasmas were located extracellularly within necrotic lung lesions and around neutrophilic granulocytes and macrophages, while fewer organisms were found within the cytoplasm of phagocytes. The results of this study show that there is long-time survival of numerous intact M. bovis in necrotic lung lesions even though large numbers of neutrophils and macrophages are present. These findings show that the phagocytes are not able to eliminate M. bovis from the lungs from necrotic and inflamed lung tissue and indicate that persistence of the agent is possibly due to its capacity to avoid phagocytosis.

Cytoskeleton of mollicutes

Mollicutes are a class of bacteria that lack a peptidoglycan layer but have various cell shapes. They perform chromosome segregation and binary fission in a well-organized manner. Especially, species with polarized cell morphology duplicate their membrane protrusion at a position adjacent to the original one and move the new protrusion laterally to the opposite end pole before cell division. The featured various cell shapes of Mollicutes are supported by cytoskeletal structures composed of proteins. Recent progress in the study of cytoskeletons of walled bacteria and genome sequencing has revealed that the cytoskeletons of Mollicutes are not common with those of other bacteria. Mollicutes have special cytoskeletal proteins and structures that are sometimes not shared even by other mollicute species.

A proposed life cycle model of Spiroplasma mirum based on scanning electron microscopical observations of growth in liquid culture

Cells of Spiroplasma mirum strain SMCA were grown in PPG broth and examined by scanning electron microscopy. Comparison of the results at different time intervals allowed for a model of the life cycle of S. mirum to be proposed. Under favorable growth conditions, helical filament formation was initially observed, followed by the formation of small spherical structures originating from each filament. In old culture, large spherical bodies appeared from entangled helical filaments. From the larger spherical bodies, granular bodies representing the smallest reproductive units were produced to continue the life cycle.

Mycoplasmas isolated from the respiratory tract of horses

Ten mycoplasmas were isolated from 130 nasopharyngeal swabs from thoroughbred horses with acute respiratory disease and three from 198 apparently normal horses. Two mycoplasmas were isolated from 21 tracheal swabs taken at necropsy. These mycoplasmas, together with six isolated from the equine respiratory tract by other workers, were subjected to biochemical and serological tests. Other properties examined in certain representative strains were appearance under the electron microscope, ability to adsorb or agglutinate the erythrocytes of various animal species and the electrophoretic pattern of the cell proteins. On the basis of these test, mycoplasmas from the equine respiratory tract were divided into seven species. Three species belonged to the genus Acholeplasma, members of which do not require sterol for growth, and were identified as A. laidlawii, A. oculi (formerly A. oculusi) originally isolated from the eyes of goats, and a recently named species A. equifoetale, previously isolated from aborted equine fetuses. Of the four sterol-dependent Mycoplasma species, one was indentified as M. pulmonis, a common rodent pathogen. Another cross-reacted serologically with M. felis and should probably be classified as that species. The other two species probably represent new species peculiar to the horse. One of these, represented by the strains N3 and N11, ferments glucose and is serologically distinct from 19 recognized species of glucose-utilizing mycoplasmas and from two species which do not metabolize either glucose or arginine. The other species, represented by four strains, hydrolyses arginine and, because it is serologically distinct from all the named arginine-hydrolysing Mycoplasma species, the name M. equirhinis sp.nov. is proposed for it. Of the seven species, only M. pulmonis and the glucose-utilizing species represented by N3 and N11 were found exclusively in horses with acute respiratory disease. A. oculi was isolated from an apparently normal horse. The other four species were found in normal horses as well as those with respiratory disease, although three out of the four strains of M. equirhinis were from sick horses.

Cell biology of the mycoplasmas

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Osmolar concentration and fixation of mycoplasmas

Broth cultures of Acholeplasma laidlawii were fixed with various concentrations of cacodylate-buffered glutaraldehyde. The shape and ultrastructure of the organisms varied with the osmolar concentration of the fixative. When the fixation mixture was hypertonic to the culture medium, ultrathin sections suggested that the cells had shrunk. Phosphate buffer, sodium chloride, or sucrose at comparable osmolaities had the same effect as sodium cacodylate. Glutaraldehyde itself also contributed to the osmotic effects of the fixation mixture but to a lesser extent than salts or sucrose, to which the cell membrane is impermeable. The osmolar concentration of the fixation mixture seemed of greater importance than pH in determining morphology. The mycoplasma was still susceptible to damage by high concentrations of cacodylate after fixation with 2.5% glutaraldehyde. The best procedure was to fix and wash the organism under conditions isotonic with the growth medium. These conditions were also satisfactory for a filamentous mycoplasma, Mycoplasma orale.

Morphology and ultrastructure of Mycoplasma pneumoniae spherules

During growth in fluid medium, most strains of Mycoplasma pneumoniae produce free-floating granules which become larger with time. We have called these granules "spherules." This study describes the morphological and ultrastructural features of M. pneumoniae strain AP-164 spherules, both free and in association with HeLa cells in cell culture. In thin section, spherules were composed of lobulated cells, connected together by membranes, and ring-shaped cells. The two-dimensional morphology observed varied according to the plane of section and to the age of the culture. In HeLa cell cultures, mycoplasmata were found attached to plasma membranes of cells; in sections, individual mycoplasmata were often aligned in radial apposition to the membranes. Mycoplasmata were not found intracellularly. The three-dimensional morphology of spherules was examined by the critical point method and by scanning electron microscopy. Both methods revealed lobulated forms, ring-shaped forms, and star-shaped forms. Treatment of the spherules with crude porcine pancreatic lipase effectively released large numbers of free organisms. Phosphotungstic acid preparations of these uncentrifuged forms revealed a morphology in agreement with the other methods used. Lobulated ring forms with "beaded" filaments were prominent. In respect to morphology, M. pneumoniae under our conditions resembles that of the type species M. mycoides.