Despite the growing interest for microbial transglutaminases (TGases), and the large number of genome sequencing data, there is no deep investigation about structural properties within this family of enzymes in bacteria. We performed a classification of microbial TGases, starting from large-scale analysis of all protein sequences annotated as TGase (more than 8000) in database PFAM. We developed a reiterative procedure based on the construction of several phylogenetic trees and manual selection, and detected five main groups of microbial TGases. Searches for sequence motifs evidenced strong conservation in regions containing potential catalytic residues for some groups. Protein structure modelling has been possible for three of the five groups. Analyses of motifs, structural topologies and potential catalytic sites suggest possible interpretations for function similarities and divergences among groups. (C) 2018 Elsevier Inc. All rights reserved.