An electron microscopic study of clinical and laboratory-derived strains of teicoplanin-resistant Staphylococcus haemolyticus

Staphylococcal resistance to glycopeptides (which involves more teicoplanin than vancomycin) is uncommon and largely confined to Staphylococcus haemolyticus, an emerging nosocomial pathogen with a tendency to develop antibiotic resistance. In this study, six S. haemolyticus strains, including two isogenic pairs of teicoplanin-susceptible/-resistant strains and two resistant clinical isolates, were used in a morphologic and morphometric electron microscope investigation. Cells from both clinical and laboratory-derived teicoplanin-resistant strains exhibited abnormally roughened, irregular outlines when observed by transmission electron microscopy. However, no significant differences in cell wall thickness resulted from morphometric analysis when the susceptible/resistant cells of the two isogenic pairs were compared. By scanning electron microscopy, an abnormally roughened, blistered surface was associated with teicoplanin-resistant cocci. A certain variability was noted between strains, not clearly related to the resistance level. In freeze-fracture investigations, a higher number per square micrometer of intramembrane particles, more significant in the E than in the P membrane fracture face, was observed in the laboratory-derived resistant clones as compared to susceptible parent strains. Further studies are needed to understand the cause-effect relation between these ultrastructural alterations and staphylococcal resistance to teicoplanin (but not to vancomycin).