C. difficile is distantly related to.most of the clostridia studied previously. Based on 16S rRNA sequencing the genus Clostridium was divided into 14 clusters and it was proposed that the genus be divided into five new genera and eleven new combinations. C. difficile was assigned to cluster XI, which in itself is taxonomically heterogeneous. Most of the clostridia species previously studied for their lipid composition were placed in cluster I. Clostridium innocuum, which like C. difficile has no PE and has lipids unique to that species [22], was assigned to the narrow cluster XVI [23].