LETTER TO THE EDITOR Classification of bacterial cell wall hydrolysases and their potentials as novel alternatives to antibiotics – a response to the letter of Biziulevicius and Kazlauskaite Albert Parisien and Christopher Q. Lan Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, Canada Dear Dr Gilmour, Thank you very much for passing us the letter of Biziu- levicius and Kazlauskaite regarding our paper published in the Journal of Applied Microbiology (Parisien et al. 2008). We appreciate very much their interest in our work and the opportunity to classify and further discuss some important issues on this intriguing topic. Classification of bacterial cell wall hydrolases (BCWH) according to their sources The first point raised in the letter was that by proposing the new classification of BCWH based on the source of enzymes, we were trying to replace the generally accepted classification proposed by Strominger and Ghuysen (1967). This was a misunderstanding. The newly proposed classification was simply intended to provide a different angle of discussion, which overlaps with instead of substi- tutes for the existing one because they are based on differ- ent criteria. The classification of Strominger and Ghuysen (1967) was based on the mechanism of the enzymatic clea- vage of peptidoglycan, which classifies BCWH into three groups: (i) carbohydrases (also known as glycosidases), which split the polysaccharide chain of peptidoglycan; (ii) endopeptidases (or peptidases, lytic proteases), which break the peptide interbridge; and (iii) acetylmuramyl- l-alanine amidases (or amidases), which cleave the linkage between the polysaccharide chains and the peptide inter- bridges. This classification has been generally accepted with some minor modifications. For instance, the scope of N-acetylmuramidases (also called N-acetyl-b-d-muramid- ases or muramidases), which are a subgroup of glycosidases that attack the N-acetylmuramic acid fi N-acetylglucos- amine b-1-4 glycosidic bond, has been extended to include ‘lysozymes’ and transglycosylases while the latter were not discussed by Strominger and Ghuysen (1967). Peptido- glycan peptidases were also more precisely classified into two subgroups, endopeptidases and carboxypeptidases (Vollmer et al. 2008). Other BCWH include phospho- choline esterase (e.g. pneumococcal Pce) that releases phosphocholine residues from the cell wall teichoic acid (Vollmer and Tomasz 2001). BCWH from different sources may share similar struc- tural features and bacteriolytic mechanisms. Although the mechanism-based classification was proposed primarily according to the BCWH reported in animals, plants and cellular micro-organisms, however, most of these enzy- matic activities (mechanisms) have been reported in virolysins (Loessner 2005). Nevertheless, lytic enzymes share similar structural features and bacteriolytic Correspondence Christopher Q. Lan, Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON K1N 6N5, Canada. E-mail: Christopher.Lan@Uottawa.ca 2008 / 0935: received 2 June 2008, revised 2 June 2008 and accepted 12 August 2008 doi:10.1111/j.1365-2672.2008.04040.x Abstract This is a response to the letter of Biziulevicius and Kazlauskaite, who made a few valuable comments on our recent paper published on the Journal of Applied Microbiology regarding the classification and potential clinic applica- tions as alternative antibiotics of bacterial cell wall hydrolases (BCWH). We confirm that it is of significant practical and theoretical relevance to categorize BCWH according to their sources owing to their strikingly different anti- bacterial specificities, which are of critical importance to their potential clinic applications. Among the three groups of lytic enzymes, i.e. lysozymes, micro- lysins and virolysins, virolysins seem to be the most promising alternative antibiotics as a result of a few unique characteristics of this group of lytic enzymes. Journal of Applied Microbiology ISSN 1364-5072 1754 Journal compilation ª 2009 The Society for Applied Microbiology, Journal of Applied Microbiology 106 (2009) 1754–1759 ª 2009 The Authors