Ultrastructure of Clostridium difficile colonies

•First report on Clostridium difficile colony ultrastructure.•In young cultures great intracolony variability is present.•Invasive C. difficile cells were detected.

The ultrastructural colony architecture of six Clostridium difficile strains belonging to four different ribotypes (001, 027, 078/126 and 036) was studied by scanning electron microscopy (SEM). In 2-day-old colonies several microareas differing in cell length and organization could be differentiated. Some cells at colony edges were extremely long and exhibited invasiveness into the agar. The ultrastructure of 5-day-old colonies is more homogenous and characterized by presence of sporulating cells, spores and extracellular matrix.