A novel protein secretion system has been recently discovered in Porphyromonas gingivalis that is responsible for the secretion of C-terminal domain (CTD) proteins such as the gingipains. Objectives: The aim of this study was to relate the identification of CTD proteins from the cells of a particular species with the presence of an electron dense surface layer surrounding those cells. Methods: Potential CTD proteins were first identified by bioinformatics using hidden markov models (hmm) generated from CTD sequences previously identified from membrane fractions of P. gingivalis and Tannerella forsythia. Membrane preparations from other bacterial species were then prepared and analysed by gel electrophoresis and mass spectrometry enabling the identification of CTD proteins by reference to the hmm results. Bacteria were examined for the presence of surface layers by cryo-electron microscopy (cryo-EM). Results: Several hundred potential CTD proteins were found using our hmm model, all restricted to bacterial species within the bacteroidetes/chlorobi phyla. Species found to contain CTD proteins included Prevotella intermedia which is another oral pathogen, Parabacteroides distasonis from the human gut, and Cytophaga hutchinsonii, which digests cellulose and resides in the soil. P. intermedia and P. distasonis were found to produce large amounts of membrane-associated CTD proteins, and this was found to correlate to the presence of an electron dense surface layer (EDSL) when the cells were viewed by cryo-EM. While 20 different CTD proteins were identified from C. hutchinsonii, they were not very abundant. Therefore it was not surprising that an EDSL could not be seen for this species. Conclusion: The CoAtSS is very widespead in the bacteroidetes phylum and is characterised by the attachment of substrates to the outer membrane. In many species the abundance of these proteins is sufficient to form a protein surface layer or coat that is clearly visible by EM.