Bacteria produce several peptidoglycan hydrolases (PGHs). There are at least five types of PGHs: endo-β-N-acetylglucosaminidase, endo-β-N-acetylmuraminidase, N-acetylmuramyl-L-alanine-amidase, endopeptidase and transglycosylase; each known for the specific bond they split in the cell wall (1,19). They are ubiquitous in bacteria and show functions in restructuring the cell wall where they have many important roles in cell wall metabolism including nicking peptidoglycan for insertion of a new peptidoglycan monomer, cell division, cell separation, and cell wall turnover (1,19,26). Many PGHs are termed bacteriolytic because the action of the enzyme often leads to bacterial cell lysis. Some enzymes are able to disintegrate their own peptidoglycan saccules when the cells are placed in unfavorable conditions; and such enzymes are called autolysins.Streptococci are Gram-positive bacteria where individual cells are connected with each other forming a chain morphology. Among the streptococci, S. pneumoniae, the leading cause of pneumonia, blood stream infections and otitis media, is the most extensively studied concerning the biochemistry and physiological functions of the PGHs. S. pneumoniae produces at least three types of PGHs: LytA amidase (16), LytB endo-β-N-acetylglucosaminidase (4, 7) and LytC lysozyme (6). Their roles in cell separation and viru-

Zymographic analysis was performed to know the bacteriolytic enzyme profiles of 4% SDS extracts of oral streptococci, Streptococcus mutans, S. sobrinus, S. sanguis, S. mitis and S. salivarius. We investigated the five strains in each species and found that the profile was very similar among strains of the same species except for S. salivarius (the profile was classified into two types). On the other hand, the profile was considerably different among species. Two major bacteriolytic enzymes of S. mutans showing molecular mass of 80 and 100 kDa were found using SDS‐boiled S. mutans or S. sobrinus cells as substrate. These bacteriolytic activities were less apparent in the gel containing S. mitis or S. salivarius, and also not detectable in the gel containing S. sanguis. S. sobrinus extract showed only one bacteriolytic band (78 kDa) as strong activity using S. sobrinus cells as substrate. S. sanguis extract showed no bacteriolytic bands using any streptococcal cells. Extracts of either S. mitis or S. salivarius showed weak activity by using respective strains as substrate.

Our study demonstrates that Aml has an ability to lyse planktonic and biofilm cells of clinically isolated mutans streptococci in the presence of Triton X-100. These results suggest the possibility of using Aml as an alternative or additional approach for caries prevention.